Search

An in vitro technique for the measurement of calcium uptake into the maternal-facing fetal chorionic membrane (apical trophoblast) was used to study the relationship between calcium uptake and stage of pregnancy in the sheep. The effects on calcium uptake of varying calcium concentration and temperature of the incubation medium, of adding calcium channel blockers or heavy metals (lanthanum and nickel) or calcium ionophore/agonist were also studied. The data indicate a saturable calcium uptake process, plateauing after 15 min incubation. This uptake remained constant throughout the last third of gestation until a significant fall in uptake was noted during the final week prior to parturition. This uptake was not due to extracellular cellular diffusion since there was no significant uptake of tritiated inulin over the same period in each case. Calcium uptake in this system was also shown to be a temperature dependent process which was abolished at temperatures of 0-4 degrees C. A decrease in calcium concentration to 0.12 mM in the incubation medium also caused a corresponding decrease in calcium uptake to 21 per cent of control (1.2 mM). The addition of the heavy metals lanthanum and nickel also significantly reduced calcium uptake as did the calcium channel blockers verapamil, metoprolol and diltiazem. The calcium channel ionophore A23187 increased calcium uptake into the material facing chorion. Although the interplacentomal chorion may not be representative of the whole of the placental unit, it clearly contains a specific calcium uptake process under local physiological control. The blocking of calcium uptake by the specific I-type calcium channel blocker verapamil may indicate the presence of I-type channels of unusually low sensitivity since the concentration needed to block them was much higher than would be required for excitable I-type channels in isolated cells.

The calcium requirement of the ovine fetus increases progressively throughout pregnancy. The 9-kDa calcium binding protein (calbindin-D9k; 9CBP) is considered to be a reliable marker for epithelia mediating calcium transport. This quantitative in situ hybridization study shows that the levels of 9CBP mRNA show a pregnancy stage-related increase which correlates with fetal calcium demand only in maternal endometrial gland and fetal interplacentomal trophoblast epithelia. Levels of 9CBP mRNA in the placentome, which has by far the greater area of maternofetal contact, show no changes during pregnancy. mRNA for the CaATPase enzyme, a second requirement for calcium transport, is shown to be present in epithelia in interplacentomal and placentomal regions but shows no change in concentration as pregnancy progresses. Results with the 9CBP and CaATPase mRNAs confirm our recent immunocytochemical results with ruminant placenta and indicate the basis for a cellular calcium transport system analogous to that in the enterocyte. The interplacentomal trophoblast system appears to be eminently suitable for investigations of details of the cellular mechanism and control of epithelial calcium transport.

Magnesium is absorbed mainly from the reticulo-rumen and there are a number of factors reducing its absorption. The chief of these is the increased potential difference across the rumen epithelium caused by increased intraruminal potassium concentration. A significant amount of magnesium leaves the extracellular fluid each day as saliva. As only a portion of it is reabsorbed the rest is lost through the endogenous faecal excretion of magnesium. Thus, during impaired magnesium absorption, saliva could play an important role in the aetiology of hypomagnesaemia especially during dietary sodium depletion and the resultant increase in the potassium content of the saliva.

Parathyroid hormone-related protein (PTHrP(1-34)) was infused into five sheep, each fitted with a large rumen cannula. After infusion, significant increases were observed in the total and ionized calcium concentrations in plasma but not in saliva. In contrast, significant decreases in the plasma concentrations of phosphate and potassium and corresponding increases in their salivary concentrations and clearance rates were observed. The salivary concentration of endogenous PTH1P(1-34) was significantly greater than that in plasma sampled simultaneously, but during the infusion of PTHrP(1-34) both plasma and salivary concentrations of PTHrP(1-34) increased.

In late-pregnant goats, daily removal of secretion from one mammary gland stimulated an early secretion of fluid by that gland in five of the six animals studied. This early secretion of fluid was accompanied by an early increase of parathyroid hormone-related protein (PTHrP) 1-86 concentration in the secretion of that gland alone. When lactation was established, glands emptied thrice daily secreted PTHrP (1-86) in greater quantities and concentrations than glands emptied once daily concurrently in the same animals. Toward the end of lactation, abrupt cessation of emptying one gland caused levels of PTHrP (1-86) to decline in the fluid in that gland but not the contralateral, emptied gland. We conclude that, in the goat, PTHrP (1-86) secretion into milk is linked to autocrine factor(s) that stimulate milk secretion when fluid is removed from the gland.

The concentrations of plasma parathyroid hormone-like bioactivity and parathyroid hormone-related protein (1-86) (PTHrP) immunoreactivity were both higher in fetal pigs than in their mothers during the last 3 weeks of gestation. Both activities changed inversely with alterations in the plasma ionized calcium concentration. The data suggest that PTHrP may have a role in calcium homeostasis in the fetal pig, similar to its postulated role in sheep in the stimulation of calcium transport across the placenta.

Not only parathyroid hormone (PTH) but also parathyroid hormone-related protein (PTHrP) may play a role in calcium and phosphate homeostasis in ruminants. In five trained sheep, each with a large rumen cannula, the isolated rumen wash technique was used to measure electrolyte absorption rates from the rumen. After two control periods of measurement, 20 micrograms PTH(1-34) (n = 4), or PTHrP(1-34) (n = 5), was injected intravenously as a loading dose, followed by an infusion of 0.67 micrograms min-1 over 2 h. Both PTH(1-34) and PTHrP(1-34) significantly increased the absorption rates of calcium and phosphate from the reticulo-rumen. The increases in the absorption rates of magnesium, sodium and potassium observed were not significant. The same technique was also used to demonstrate that increasing the intraruminal calcium concentration from 1 to 4 mmol l(-1) caused corresponding increases in the net rate of absorption of both calcium and inorganic phosphate from the reticulo-rumen. Rumen epithelium was taken from four sheep and mounted in Ussing chambers so that fluxes of calcium could be measured in both directions using 45Ca. It was found that the addition of 100 ng ml-1 PTH(1-34) or PTHrP(1-34) to the serosal side increased the net calcium flux rates across the ruminal epithelium. It is concluded that both PTH and PTHrP can influence calcium and phosphate homeostasis in sheep not only by their recognized actions on bone and kidney but also on the absorption of these ions from the forestomachs.

Under normal dietary conditions in which the phosphorus (P) intake of ruminant animals is adequate, the absorption of phosphate (Pi) ions takes place from both the fore-stomachs and the small intestine. The rate of Pi absorption from the latter appears to be at least three times greater than that from the former. When absorption of Pi ions from the reticulorumen is severely reduced, as in dietary P deficiency, the absorption of both calcium and magnesium ions from the reticulorumen becomes impaired. Thus, dietary P deficiency may play a role in the aetiology of acute grass tetany.

The effects of quinidine on divalent mineral absorption were studied using normal and high potassium intraruminal buffers with the in vivo isolated washed rumen technique. A concentration of 1 mmol litre-1 quinidine in the intraruminal buffer decreased the absorption rates of Na+ and C1- from the rumen but had no significant effect on the absorption rates of Mg2+, Ca2+ and K+. Prolonged oral administration of quinidine had no significant effect on the absorption of any of these ions from either a normal or a high potassium ruminal solution.

Five intact and four thyroparathyroidectomized (TPTX) lactating dairy cows were fitted with polyvinyl catheters implanted into the left carotid artery and the left mammary vein (for blood sampling) and into the right external jugular vein (for injections), and with a Foley catheter inserted into the urinary bladder. In these nine cows, daily morning and evening milking induced a prompt and transient rise in mammary venous blood plasma parathyroid hormone-related peptide (PTHrP)(1-34) concentrations, followed by an increase in both urinary phosphate concentration and phosphate renal clearance. Such a milking-induced phosphaturia was not observed when (tyr)34-bPTH(7-34)-NH2 (5 pmol/kg body wt) was infused during milking into the TPTX cows. These results indicate that PTHrP released systematically by the mammary gland during milking might be responsible for the observed phosphaturia.

In the sheep, goat and pig, radiolabelled parathyroid hormone related protein (PTHRP) and immunoreactive PTHRP(1-34) and (1-86) were rapidly cleared from the circulation. Metabolic clearance rates (MCR) were in the range of 1.25-7.5 ml/min per kg and were slightly slower than that of intact PTH in man (10 ml/min per kg); while the mean MCR of labelled PTHRP(1-86) in fetal sheep and goats was significantly faster than that in their respective mothers (14.4 vs 4.0 ml/min per kg respectively). This may reflect increased metabolism of PTHRP by fetal tissues, e.g. the placenta. Similar rates of clearance of radiolabelled PTHRP(1-141), (1-86) and (1-34) suggest that clearance involves the amino terminus of the molecule.

Synthetic parathyroid hormone fragment PTH(1-34) has been reported recently to inhibit uterine contractions stimulated by a variety of agonists. We have studied the effect in this system of the parathyroid hormone-related protein fragment PTHrP(1-34) which shows 60% homology with PTH over the first thirteen amino acid residues. The effects of two different PTHrP fragments on acetylcholine-stimulated uterine contractions in vitro were studied. Whereas synthetic hPTHrP(75-86 amide) (10(-9)-10(-7) M) was without effect, synthetic hPTHrP(1-34) (10(-9)-10(-7) M) was capable of inhibiting, in a dose-related fashion, uterine muscle contractions precontracted with 10(-6) M-acetylcholine. In a second series of experiments the bovine PTH(3-34) fragment itself was shown to have no stimulatory effect on acetylcholine-stimulated contractions. Also this fragment in an equimolar concentration (10(-7) M) failed to antagonize the effects of PTHrP(1-34) on acetylcholine-stimulated uterine contractions. However, a 100-fold excess molar concentration of bPTH(3-34) (10(-6) M) completely abolished the inhibitory action of hPTHrP(1-34) (10(-8) M) on acetylcholine-stimulated uterine contractions. These results clearly show that the inhibitory action of PTH(1-34) and PTHrP(1-34) on uterine contractions depends on the integrity of the amino-terminal region of the molecule.

Parathyroid hormone-related protein (PTHRP) has been quantified by sensitive specific immunoassays in mammary venous blood and milk from 7 days before to 7 days after parturition in the goat. A significant venous-arterial concentration gradient in plasma PTHRP 1-86 concentrations was demonstrated across the mammary gland, indicating that PTHRP enters the maternal circulation and may have a role in calcium homoeostasis during lactation. Significant and sustained increases in mammary venous and milk PTHRP 1-86 concentrations were found from 1 day before parturition to 7 days afterwards, with peak concentrations of 1.57 +/- 0.58 pmol/l (plasma) and 8.69 +/- 2.95 nmol/l (milk) (mean +/- S.E.M.) occurring on day -1 and the day of parturition respectively. Estimates of the mammary output of PTHRP into plasma in four goats averaged 9% (range 1-25%) of that secreted into milk. Suppression of maternal prolactin concentrations by bromocriptine significantly reduced milk yield and the mammary venous PTHRP concentration, without affecting the concentration of PTHRP in milk. In conclusion, parturition in the goat is associated with a sustained increase in secretion of PTHRP into both plasma and milk; the former may be involved in maternal calcium homoeostasis, whereas the latter may have a role in the neonate.

It has been confirmed that the foetal parathyroid glands are important in development and that thyroparathyroidectomy (TXPTX) of the ovine foetus with thyroxine (T4) replacement leads to hypocalcaemia, retarded skeletal development, depressed calcification and rickets, relative to thyroidectomy plus T4 replacement. Histomorphometric and biochemical (urinary excretion of deoxypyridinoline) indices of bone resorption are also reduced. However, skeletal calcification can be restored to normal by long-term infusion of the TXPTX foetuses with phosphate and calcium sufficient to normalise the plasma Ca2+ x Pi ion product. Nevertheless, depressed resorption, reduced osteoblast numbers and delayed development persisted. The evidence suggests that the abnormally low number of resorption cavities and osteoclasts may result from the reduction in circulatory parathyroid-hormone-related protein consequent upon the removal of the foetal parathyroid glands and that this hypercalcaemic factor has a direct effect upon the process of resorption and primary trabecular remodelling of the foetal skeleton.

Pig thyroid glands were surgically isolated in situ and perfused with autologous blood to which was added known concentrations of calcitonin gene-related peptide (alpha CGRP). When thyroids were perfused with measured concentrations of CGRP within the range of 0.6-600 nM, the secretion rate of calcitonin (CT) was stimulated while the release of T3, T4, and somatostatin remained unchanged. Specific binding of 125I-CGRP to pig thyroid plasma membranes was demonstrated, and binding was inhibited by unlabelled CGRP but not by CT or by other peptides unrelated structurally to CGRP. The findings indicate that the pig thyroid gland contains plasma membrane binding sites for CGRP and that CGRP is capable of stimulating the secretion of CT.

The plasma Ca concentration of the fetus is maintained higher than maternal levels by active placental transport. Ca, Mg and PO4 accumulation by the fetus is mainly associated with skeletal growth. The fetal parathyroid glands are essential for maintenance of elevated plasma Ca, which is necessary for the stimulation of fetal osteoblasts and mineralization of cartilage and osteoid. Fetal thyroparathyroidectomy (TxPTx) results in a decreased activity of the placental Ca pump. The presence of a parathyroid hormone-related protein (PTHrP) has been demonstrated in fetal parathyroid glands and placental tissue. Extracts of fetal parathyroid glands and purified PTHrP, as well as recombinant PTHrP (1-84, 1-108 and 1-141), stimulate Ca and Mg but not PO4 transport across the placenta of TxPTx-ized fetuses perfused with autologous blood in the absence of the fetus. Parathyroid hormone (PTH) and the N-terminal region of PTHrP do not stimulate placental Ca and Mg transport. It is concluded that a mid-molecule region of this novel hormone may be required to stimulate placental Ca transfer and contribute to the regulation of fetal Ca homeostasis.

The fetal plasma magnesium concentration exceeds that of the mother but the difference is small compared to that of ionized calcium concentration. Although not fully independent of changes in maternal magnesaemia, fetal magnesaemia showed a high degree of autonomy during both hypermagnesaemic and hypomagnesaemic changes induced in the ewe. As with calcium, the placental gradient is reversed after fetal thyroparathyroidectomy (TXPTX) with thyroxine replacement. During perfusion in situ of the placenta from such TXPTX fetuses isolated from the fetus itself, a stable positive placental gradient of magnesium concentration could be re-established between the perfusing blood and the maternal circulation. As with calcium, this gradient could be increased by fetal calf parathyroid extract, parathyroid hormone-related protein (PTHrP 1-141), PTHrP (1-84) but not by PTHrP (1-34). It was concluded that a mid-molecule portion of PTHrP can stimulate a putative placental pump which is responsible for the gradients of both calcium ions and magnesium across the ovine placenta.

Perfusion in situ of the placenta of intact or previously parathyroidectomized fetal lambs has been used to assess the ability of three mid-molecule fragments of the human parathyroid hormone-related protein (PTHrP) molecule to stimulate the placental transport of calcium and magnesium. PTHrP(67-86 amide) was most effective but some activity was also shown by PTHrP(75-86 amide) and by PTHrP (75-84) in decreasing order. This placental action of PTHrP(67-86 amide) was rapid and could be observed using the placenta from an intact fetus, whereas it was necessary to use the placenta from a previously parathyroidectomized fetus to demonstrate stimulation of placental calcium transport by PTHrP(1-84). PTHrP(67-86 amide) may resemble the molecule that activates the placental calcium pump.

An assay for plasma osteocalcin (BGP) in ruminants is described. Satisfactory cross-reaction was obtained for bovine BGP in several species, namely ovine, cervine, caprine and human. Sample stability at room temperature, after freezing and thawing and after lyophilization and reconstitution is described. Sheep of both sexes have been used throughout and no significant sex effect has been noted. A series of experiments showed a lack of circadian rhythm of plasma BGP concentration in sheep. Plasma BGP values from sheep ranging in age from 0.25 to 7 years fell exponentially from 300 ng/ml at 0.25 years to approximately 50 ng/ml at 1.25 years of age where it remained until the end of the study at 7 years. Long-term administration of adrenocorticotrophic hormone (ACTH) depressed plasma BGP concentrations, although plasma BGP levels were not affected sufficiently by short-term administration of ACTH to increase plasma cortisol concentrations 10-fold during a period of 4 h. It was confirmed that dietary phosphate deficiency depressed plasma BPG concentrations. It is concluded that neither circadian variation nor the short-term stress of handling and bleeding untrained animals need be considered in sampling sheep for measurement of plasma BGP concentration and that the change with age reflected the pattern of development of skeletal maturity in sheep. Values in both growing and mature sheep were at least 6 times higher than in human subjects, which may reflect the 10-fold higher skeletal growth rate in sheep than in humans. This high skeletal activity in sheep may have been selected for in the development of highly productive breeds.

The presence of calcitonin gene-related peptide (CGRP) in both C-cells and nerve fibres around the thyroid blood vessels and follicles suggests that it may play a dual role, one of which may be in hormone secretion. Beta-CGRP injected intracerebroventricularly (icv) (1.875 nmol) caused a significant increase in the calcitonin CT) content of the C-cells in rat thyroid glands. The calcitonin content of the thyroids was determined by a radioimmunoassay using two antibodies and the results were expressed as ng CT/mg of fetal tissue. A significant rise in CT content of the thyroids occurred (5.5 +/- 0.5 n = 6), compared with the controls (3.5 +/- 0.1 n = 6). It is presumed that this change reflects an increase in the rate of secretion of CT in the thyroid glands of those rats so treated. The peptide might act on the central nervous system to stimulate the catecholamine outflow from the thyroid nerves and thereby increase the secretion rate of CT.

Bi-directional transport rates of magnesium across the placenta of the ewe have been measured towards the end of pregnancy. The results indicated that approximately 0.042 and Mg/h/kg fetal body weight passed from ewe to fetus and that about 0.012 mg Mg/h/kg fetal body weight was transported back to the ewe. There was a significant, although small, positive gradient in plasma magnesium concentration from ewe to fetus.

The duodenal and ileal absorption of 32P was studied by an in vivo method in control chicks and in chicks fed for 2 weeks a diet deficient in calcium or phosphorus. The results showed a highly significant increase in duodenal and ileal phosphate absorption in chicks fed a low calcium diet and a smaller, though also significant increase in duodenal but not ileal phosphate absorption in chicks fed a low phosphorus diet. The adaptation phenomenon concerning intestinal phosphorus absorption is presumably due in part to a compensatory increase in 1,25-dihydroxy-vitamin D3 in both deficient diets. However, much higher renal l-hydroxylase activity was found in chicks fed the low calcium diet than in chicks fed the low phosphorus diet. The results indicate that duodenal and ileal absorption of phosphate is a vitamin D mediated process, but the adaptation to a low phosphorus diet tends to be less efficient than that to a low calcium diet, especially in the ileum.

The effects of oral betamethasone (25 micrograms/kg/d), low calcium (1 g/kg), and low phosphorus (2,5 g/kg) diets on weight gain were investigated in 513 growing chicks. The chicks were divided according to initial weight into a lower weight group (less than 100 g each) and a higher group (more than 100 g each). The increases in weight were recorded after periods of 7, 14 and 21 days. The increases in weight in the older chicks (high weight group) were less than in the younger chicks. There was a greater reduction in rate of growth in chicks fed the low phosphorus diet than in those fed the low calcium diet, particularly in the older birds. Betamethasone administration also reduced the rate of growth but this was not additive to the effect of a low calcium diet. In contrast, the greatest reduction in growth rate was observed in the chicks fed the low phosphorus diet and given betamethasone.

The thyroid gland isolated in situ in 4 pigs was perfused with PGI2 given at increasing rates (1.38, 2.75, 13.8, 27.5 and 138 ng/mn). In the all animals a significant dose-dependent reduction in hormonal iodine secretion rate was observed. These results confirm in vitro experiments with PGE1 (Burke et al., 1972).

The purpose of this study was to determine whether total serum calcium, parathyroid gland structure, and/or levels of parathyroid hormone, 1,25 and 24,25 DHCC, are altered in pigs with dental and skeletal fluorosis. Eight experimental animals receiving 2 mg F-/kg b.w. per day from age 8-14 months were compared with eight controls. Concentrations of plasma fluoride and total plasma calcium were assessed at intervals throughout the experiment and during a 48 hour period at day 110-111 of the experiment. At the same time, concentrations of immunoreactive parathyroid hormone were measured using a homologous labeled antibody for porcine hormone, and a radioimmunoassay was used to assess concentrations of 1,25 DHCC and 24,25 DHCC. Parathyroid tissue volumes were assessed at the end of the experiment by quantitative histology using volumetry and point counting. Plasma fluoride increased from 0.0007 +/- 0.0001 mmol/liter to 0.0127 +/- 0.002 mmol/liter in pigs receiving fluoride. In spite of this increase, total plasma calcium remained the same throughout the experiment. Volumes of parathyroid tissue, and levels of circulating parathyroid hormone 1,25 DHCC and 24,25 DHCC, were not significantly changed. It was therefore concluded that disturbance of calcium homeostasis is not an obligatory finding in dental and skeletal fluorosis and consequently does not play an essential role in the pathogenesis of these hard tissue lesions.

The effect of Ca2+ ion concentration on the 25 hydroxylation of tritiated cholecalciferol (3HD3) was investigated using homogenates of ovine liver from vitamin D replete sheep. A significant decrease in the production of 25 hydroxycholecalciferol (25OHD3) was observed when the concentration of Ca2+ in the homogenate was raised above 0.68 mmol/l by the addition of calcium gluconate. Similarly, a final concentration of 37 mumol EGTA/1 (equivalent to a Ca2+ concentration of 26.5 nmol/l) was associated with a 50% reduction of 25OHD3 production. That is, a broad bell-shaped relationship was observed between the production of 25OHD3 and the Ca2+ concentration in the homogenate. These changes in the rate of production of 25OHD3 were reproduced with hepatocytes from vitamin D replete rats, prepared by collagenase perfusion, using the drugs dantrolene sodium (DaNa) to reduce (ED50 = 57 mmol/l) and veratridine to increase (ED50 = 550 mmol/l) the intracellular Ca2+ concentration. Hepatocytes from vitamin D replete rats also showed a reduction in 25 hydroxylation of D3 (ED50 = 6 ng/ml) in response to the addition of 1-25 dihydroxycholecalciferol (1-25 (OH)2D3). The calmodulin antagonists; W7, compound 48/80, trifluoperazine (TFP) and calmidazolium (R24571) were all found to effect a dose response inhibition of the 25 hydroxylation of cholecalciferol by homogenates of ovine liver. R24571 had a similar inhibitory effect (ED50 = 70 mumol/l) upon the 25 hydroxylase enzyme of rat hepatocytes. It is concluded that the 25 hydroxylation of cholecalciferol in liver of vitamin D replete rats and sheep is calcium sensitive and is reduced in the presence of increased concentrations of 1,25(OH)2D3. Calmodulin may also be involved in the regulation of hepatocyte 25-hydroxylase activity by Ca2+.

1 alpha, 25-Dihydroxycholecalciferol (1,25-(OH)2D3) has been measured in human serum by radioimmunoassay. The assay uses a high titre antiserum raised in sheep against 1,25-(OH)2D3-25-hemisuccinate, conjugated to bovine serum albumin. The sensitivity of the assay is 10 pg/tube. Other hydroxylated forms of vitamin D3 cross react with the antiserum and are therefore removed from serum extracts by chromatography on Sephadex LH 20 followed by high pressure liquid chromatography. The mean (+/- SEM) serum 1,25-(OH)2D3 concentration for a group of healthy adult subjects was 41 +/- 2.5 pg/ml. None was detected in anephric patients and the concentration was low or undetectable in patients with chronic renal failure.

Placental perfusions were carried out in six ewes during the last two weeks of gestation. Perfusions were carried out using autologous fetal blood and the flow rates adjusted to give a perfusion pressure of 50--70 mmHg. Perfusion plasma calcium concentrations rose steadily throughout the perfusions achieving a mean increase of 1.65 mmol/1 above initial concentration within 100 minutes. A further three ewes in the last two weeks of gestation were thyroparathyroidectomized and normal plasma calcium concentration maintained by an intravenous infusion of calcium borogluconate. After three days, placental perfusions were carried out as before. The mean perfusion plasma calcium concentration achieved by those three ewes in a period of 100 minutes showed an increase of 1.25 mmol/1. It is concluded that the presence of the fetus is not necessary for the continued active transfer of calcium across the placenta from mother to fetus. The reduced rate of accumulation of calcium in the perfusate in TXPTX ewes is attributed to a decline in 1,25-DHCC concentrations in both maternal and fetal circulations. The implications of these results in relation to fetal calcium homeostasis and the placental transfer of calcium are discussed.

The results indicate that oral administration of verapamil for 2 weeks to the chick is followed by an increase in the efficiency of the duodenal absorption of calcium. In these chicks both a decrease in serum calcium level and an increase in the activity of renal 1 alpha-hydroxylase were observed. The increased calcium absorption following prolonged treatment with verapamil resembles that induced by a low calcium diet. The mechanism of both responses presumably involves an increased production of 1,25-dihydroxyvitamin D3. Both verapamil- and low calcium diet-induced adaptations are capable of overcoming the inhibitory action of betamethasone on intestinal calcium absorption. No effects on calcium absorption were noted if verapamil was administered intraperitoneally which suggests that verapamil exerts its action directly on the intestinal mucosa.

The aim of the study was to investigate the effect of aluminium on duodenal calcium absorption, the impairment of which can represent a pathogenic factor in the development of aluminium bone lesions. The authors investigated in the chick the effect of Al(OH)3 administered orally and of AlCl3 administered subcutaneously on the duodenal absorption of 47Ca, on serum concentration of calcium, phosphorous, aluminium and 1,25-dihydroxycholecalciferol and on bone morphology. Oral administration of Al(OH)3 for 8, 15, and 22 days was without any significant change. Subcutaneous administration of Al/Cl/3 for 8 day was associated with a significant increase in serum aluminium and 1,25-dihydroxycholecalciferol levels and with a significant decrease of the duodenal absorption of 47Ca. Decreased intestinal absorption of calcium may play a pathogenic role in the development of aluminium osteopathy. Increased serum 1,25-dihydroxycholecalciferol reflects presumably a compensatory mechanism of the lowered calcium absorption.

Thyroxine (T4) injected subcutaneously into 3-week-old chicks (30 mug/kg body weight daily for 8 days) reduced the absorption of calcium from in vivo ligated loops of duodenum and the duodenal calcium binding protein (CaBP) activity measured by the Chelex ion exchange method. Chicks fed either a commercial diet or an artificial diet containing 0.5% or 1.0% calcium showed similar response to treatment with T4. A significant effect of T4 on either calcium absorption of CaBP activity could not be demonstrated in chicks fed a high calcium diet (2.0%). The possible mechanisms of action of T4 on calcium absorption are discussed.

Metabolism trials were conducted with two sheep to determine the daily endogenous faecal magnesium excretion and the true magnesium availability of grass which had either a high potassium or a high sodium content. The experiment was conducted as a simple cross-over design and the isotope dilution technique using 28Mg was employed. The sheep ate sodium fertilised grass containing 0-73 per cent Na and 1-55 per cent K and potassium fertilised grass containing 0-15 per cent Na and 3-12 per cent K in the dry matter. The daily endogenous faecal magnesium excretion was not found to be significantly different for each sheep on both grasses; this excretion was approximately 13 per cent of the total faecal magnesium output. The true availability of magnesium was greater in the sodium fertilised grass than in the potassium fertilised grass. It was concluded that the daily endogenous faecal magnesium excretion was not affected by altering the herbage levels of sodium and potassium.

Duodenal absorption of calcium, renal 25-hydroxycholecalciferol-1-hydroxylase activity, percentage bone ash, growth rate and plasma levels of Ca and P were all measured in chicks which had been treated with the synthetic glucocorticoid, betamethasone, at the therapeutic dose level of 25 microgram/kg daily. It was found that treatment for 2-3 weeks resulted in a significant reduction in Ca absorption rate, renal 1-hydroxylase activity and growth rate but an increase in bone ash and plasma Ca. If betamethasone was administered for 10 days and then stopped 4-13 days before these parameters were measured, a compensatory increase was found in 1-hydroxylase activity while the Ca absorption rate attained control values in all but the 4 day group in which it was still significantly depressed. It was concluded that a therapeutic dose level of betamethasone depresses both duodenal Ca absorption and renal 1-hydroxylase activity although there may be no direct causal relationship between these two parameters. Recovery was complete in 5-7 days after cessation of treatment.

The mammalian fetus is maintained hypercalcaemic relative to its mother by the action of a calcium pump believed to be located at the basement membranes of the epithelial cells of the fetal chorion. It has recently been demonstrated that the activity of this putative pump is stimulated by a new fetal hormone, parathyroid hormone-related protein, described originally as the product of a human BEN cell line which was derived from a lung tumour associated with hypercalcaemia of malignancy. Whereas the circulating level of immunoreactive parathyroid hormone in the fetus is very low, in keeping with the hypercalcaemia, the plasma concentrations of bioactive parathyroid hormone and parathyroid hormone-related protein can be measured using a sensitive cytochemical bioassay and the separate concentrations assessed by pre-incubation with appropriate antisera. The total plasma concentration of both hormones is inversely related to the prevailing calcium ion concentration but the set point of parathyroid hormone-related protein is probably higher than that for parathyroid hormone. Probably as a result of the hypercalcaemia, the circulating concentration of calcitonin is also higher than in maternal plasma and may serve to limit bone resorption to favour net bone accretion as part of the overall growth of the fetus. Vitamin D and its most active metabolite, 1,25(OH)2D, can pass across the placenta in either direction, in contrast to most peptide hormones. In addition to the supply of some 1,25(OH)2D by the mother to her fetus, the fetal placenta and fetal kidneys can all synthesize 1,25(OH)2D. The relative concentrations circulating in maternal and fetal plasma pools vary with the species, presumably as a result of differing importance of the three sources of supply to the fetus and the relative concentrations of vitamin D-binding protein circulating in mother and fetus. The importance of parathyroid hormone-related protein derived from fetal parathyroid glands has been clearly demonstrated in the fetal sheep. Such animals develop rickets following the removal of their parathyroid glands, despite the demonstration of this substance in fetal placental membranes. However, the relative importance of the parathyroid glands versus the placenta and its membranes as the principal source of parathyroid hormone-related protein remains to be elucidated and may vary with species.

The effect of oral administration of betamethasone in single daily doses (25 microgram kg-1 d-1) and in single alternate-day doses (50 microgram kg-1 every other day) on the duodenal absorption of calcium, on the jejunal absorption of phosphate and on weight gain has been studied in chicks using the in vivo ligated loop technique. The daily administration of betamethasone for three weeks markedly reduced the absorption of calcium and phosphate as well as the growth rate. Alternate-day treatment lacked all these adverse effects of betamethasone and the results did not differ significantly from those in the control chicks.

Acute studies have been carried out with pregnant sows and their fetuses during the last 2 weeks of gestation. In blood samples obtained simultaneously it was shown that the concentrations of ionized calcium, calcitonin and parathyroid hormone (PTH) in fetal plasma were all greater than the corresponding values in maternal plasma. In contrast, the fetal concentrations of 24,25-dihydroxy vitamin D3 were lower than maternal and those of 1,25-dihydroxy vitamin D3 were not significantly different. In one experiment the circulatory level of fetal PTH was shown to be higher than in the sow by both an immunoradiometric assay and a very sensitive cytochemical assay. The latter showed the fetal: maternal ratio to be approximately three. This inappropriately high level of fetal PTH was unaffected by short term hypocalcaemic and hypercalcaemic changes in the fetal circulation. Although part of this fetal PTH may have been secreted in response to the action of beta-catecholamines released as a result of the experimental conditions, the transplacental gradient of PTH was also demonstrated in the chronically catheterized fetus. A short term period of either hypercalcaemia or hypocalcaemia induced in the mother was without consistent effect upon plasma calcium concentration in the fetus. However, an increase in ionized calcium concentration in the fetal plasma was found to follow the cessation of a period of maternal hypocalcaemia. It can be seen that the porcine fetus possesses the important elements of a calcium homeostatic mechanism, namely parathyroid hormone, calcitonin and the biologically active metabolites of vitamin D. Although the degree of independence from the mother is somewhat less than in the sheep, it is concluded that the porcine fetus is largely autonomous with respect to calcium homeostasis.