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Some key advances occurred last year in understanding mechanisms involved in nutrient absorption. A novel "prechylomicron transport vesicle" was identified; its movement to the Golgi is the rate-limiting step for triacylglycerol absorption. A scavenger receptor (type BI) in the brush border membrane appears to facilitate cholesterol uptake. Several studies define mechanisms for gastrointestinal peptide hormone stimulation of glucose uptake. An oligopeptide transporter, PepT1, is transcriptionally upregulated by certain dietary amino acids and dipeptides. Surprisingly, both insulin and fasting double the maximum velocity for dipeptide uptake (via PepT1), but they act by different mechanisms. Three transporters, SMVT (sodium-dependent multivitamin transporter for biotin and pantothenate), SVCT (for vitamin C), and CaT1 (for Ca uptake from the lumen) have been cloned and are active when expressed in various cells. Additional studies provide insights on Ca absorption and vitamin D action in aging, estrogen deficiency, and adaptation to a low Ca diet. Nramp2, also called DMT1 (divalent metal ion transporter), seems to be a major regulator of transferrin-independent, nonheme iron uptake. Finally, the protein HFE associates with the transferrin receptor and is part of an iron-sensing mechanism that regulates iron absorption. It is defective in hereditary hemochromatosis. HFE and Nramp2 (DMT1) genes are reciprocally regulated.

Lipophilic drugs are carried by chylomicrons secreted by the small intestine and transported in lymph. The intent of this review is to update the reader on the digestion, uptake, and transport of dietary lipids and how these processes impact the absorption of lipophilic drugs by the gut. The digestion of lipids in the gastric and intestinal lumen is discussed as well as the role of bile salts in the solubilization of lipid digestion products for uptake by the gut. Both passive and active uptake of lipid digestion products is reviewed. Also examined is how intestinal lipid transporters located at the brush border membrane may play a role in the uptake of lipids by the enterocytes. The intracellular trafficking and the resynthesis of complex lipids from lipid digestion products are explored. Finally, the formation and secretion of chylomicrons and their potential clinical disorders are described.

Although receptors for somatostatin are found in bone cells, the effect of somatostatin analogs on calcium metabolism is unknown. The authors studied, in a metabolic ward, the effect of octreotide (a long-acting somatostatin analog) and a placebo in two 6-day calcium balance periods in 8 children with Duchenne muscular dystrophy. As expected, octreotide (2 μg/kg, subcutaneously, every 8 hours) reduced serum growth hormone and somatomedin (IGF-1) to levels found in growth hormone deflciency. Octreotide enhanced calcium retention by 30% (96 mg daily [P < 0.04]) in 7 boys for whom complete data (diet, urine, and fecal calcium) were available. In 6 children with urinary calcium excretion (UCa) greater than 50 mg daily, octreotide markedly lowered UCa, from 114 ± 23 mg daily to 61 ± 9 mg daily (P < 0.03). Calcium retention occurred in patients with or without initial hypercalciuria, but the higher the basal UCa, the greater was the inhibition by octreotide (r = 0.79; P < 0.03). Inactive, nonambulatory patients had a more pronounced response of UCa to octreotide (P < 0.02). Octreotide caused a mild, nonsignificant reduction in fecal calcium, with no major changes in serum calcium, phosphorus, parathyroid hormone, urinary excretion of sodium and potassium, or in creatinine clearance. Based on the current observations and the presence of receptors for somatostatin in bone cells, this hormone may have, at least on a short-term basis, an anabolic effect on calcium, perhaps favoring its deposition in bone.

In order to delineate the ages of onset and decline in sensitivity to growth hormone (GH) in rats, the ability of ovine GH in vitro to stimulate amino acid uptake and protein synthesis was studied in diaphragm muscle. GH (25 mug/ml) produced a significant, but small (12-17%), stimulation of alpha-aminoisobutyric acid (AIB, 1 mM) uptake in diaphragms from fed intact rats 7-24 days old, but not from 21-day fetuses or from fed rats 4, 30, 50, 100, or 130 days old. When intact rats were fasted for 24 h, both the magnitude and consistency of the responses to GH increased considerably. In fasted rats, GH significantly stimulated AIB transport by 10,29, 77,111, and 58% at ages 4, 7, 11, 15, and 23 days, respectively; GH was not effective at age 3 or 30 days. At age 15 days, 0.5 mug/ml GH stimulated AIB transport by 96%. GH (25 or 0.5 mug/ml) significantly increased leucine incorporation into protein at age 15 days in fasted intact rats. When rats were hypophysectomized at age 28, 47, or 98 days and tested 2-3 days later, the basal rate of protein synthesis fell, but the basal rate of AIB transport did not; however, GH now stimulated both AIB uptake and amino acid incorporation into protein. These results suggest that rat diaphragm muscle first becomes sensitive to GH at about age 4-7 days (AIB transport), but that further development may be necessary for the anabolic action of the hormone (protein synthesis) to be seen. Stimulation of AIB uptake is maximal about 15 days after birth. Thereafter, diaphragm muscles from normal rats becomes progressively more refractory to the acute stimulatory effects of GH on transport processes. Hypphysectomy of rats 30 days old or more leads to the re-establishment of GH-responsiveness by the AIB transport system. This cannot be attributed merely to a lowering of the basal transport rate; rather hypophysectomy appears to result in the loss of an inhibitor of growth hormone's acute stimulatory actions on transport processes. Previous work suggests that this refractory state is produced in normal rats by occasional surges of endogenous growth hormone. The dominance of the refractory state appears about the time of weaning. This may protect the weaned rat, whose diet is more variable than it is during the suckling period, from periodic depletion of amino acids and glucose from the plasma in response to intermittent surges of GH, thereby allowing GH to produce a sustained stimulation of protein synthesis and growth.

Groups of young (30-to 32-day-old), freely behaving female rats were decapitated at hourly intervals over a 24-h period, and the concentrations of GH and insulin in the plasma were measured by RIA. Paired hemidiaphragms from these rats were incubated for 40 min in buffer containing 2-[14C]aminoisobutyric acid (AIB) to measure amino acid transport and [3H]-leucine to assess protein synthesis. One member of each pair of hemidiaphragms was also exposed to rat GH (5 μ/ml). Several episodes of GH secretion were evident, as suggested by accumulations of rats having high or low levels of plasma GH at certain times of the day. Out of 192 rats studied, 46 had values above 50 ng/ml (based on a rat GH reference standard having a potency in the immunoassay of 2.7 times NIAMDD-RP-1). Peaks of GH concentration were observed at 0300 h, 0600–0700 h, and 2200 h. No clear peaks were seen at other times, perhaps due to a loss of synchrony in GH secretion among rats during the middle of the day. Mean plasma insulin levels di...

It has been reported that fatty acids preferentially inhibit serum-stimulated incorporation of sulfate by embryonic chick cartilage, suggesting that they may interfere with the effects of a proposed mediator (serum somatomedin) of the actions of growth hormone (GH). This was studied further in mammalian cartilage. Butyrate and octanoate at concentrations of 0.5 to 5 mM produced a concentration-dependent inhibition of both basal and serum-stimulated sulfate and thymidine incorporation by costal cartilage from hypophysectomized rats. Butyrate also inhibited basal and serum-stimulated sulfate incorporation in cartilage from normal pigs and normal sucklings rats. In all 3 test systems, oleate (0.2--5 mM) bound to serum albumin (4 g/dl) was ineffective. There was no evidence that fatty acids preferentially inhibited the stimulation of sulfate incorporation produced by serum.

Calcium accumulates in muscles of dystrophic hamsters (DH) and patients with Duchenne muscular dystrophy. Various Ca antagonists were beneficial to the cardiomyopathy of DH, but had only minor effects on skeletal muscle. We administered a new Ca antagonist, diltiazem, 25 mg/kg/day orally to normal and dystrophic hamsters from ages 37 to 92 days. We observed a marked reduction in muscle Ca in DH treated with diltiazem: 73% in the heart, 61% in the diaphragm, and 48% in the rectus femoris. Plasma CK was significantly lower (by 37%) in treated DH, while the elevated rate of noncollagen protein synthesis in the diaphragm was not diminished. Histologically, the most important change was a reduction in Ca deposits in the heart. Diltiazem was well-tolerated by all animals and did not modify Ca content in normal hamsters. This study suggests that diltiazem may have therapeutic value in those conditions that are accompanied by excessive accumulation of Ca in tissues, such as muscular dystrophy.

In contrast to its traditional role as a catabolic agent, epinephrine added to diaphragm muscle from hypophysectomized (hypox) rats stimulated three different processes that reflect accumulation of substances in peripheral tissue: uptake of an amino acid, α-aminoisobutyric acid (AIB), uptake of a monosaccharide, 3-O-methyl-glucose, and the incorporation of leucine and tyrosine into protein. In buffer containing 1 mg/ml ascorbate to retard oxidation of catecholamines, the minimal effective concentrations were 10-7 M for AIB transport and 5 × 10-9 M for tyrosine incorporation. Norepinephrine was less potent, but it, too, stimulated AIB transport and tyrosine incorporation. One catabolic process, glycogenolysis, was also enhanced in hypox rat diaphragms, which initially responded between 10-8-10-7 M epinephrine. On the other hand, in diaphragms from intact rats, it was necessary to raise the concentration of epinephrine to 5 × 10-4 M to stimulate AIB transport, but tyrosine incorporation was never increased....

This study explored further the hypothesis that intestinal cells have two pathways for producing large triacylglycerol-rich lipoprotein particles. The hydrophobic surfactant Pluronic L-81 (L-81) inhibits formation of chylomicrons (containing triacylglycerol synthesized from dietary fatty acids and monoacylglycerol, through the monoacylglycerol pathway), but not formation of very-low-density lipoproteins. L-81 does not inhibit lymphatic lipid transport during infusion of egg phosphatidylcholine, whose fatty acid is processed through the α-glycerol phosphate pathway and is transported in lymph in very-low-density lipoproteins. Thus, the first part of this study tested whether L-81 cannot inhibit the α-glycerol phosphate pathway, and thus L-81 can only affect chylomicron lipid secretion. Intestinal lymph fistula rats were infused with a lipid emulsion containing [1- 14 C]oleic acid, but no monoacylglycerol, to ensure that the oleic acid will be channeled to the α-glycerol phosphate pathway. Experimental rats received 1 mg/h of L-81 in their emulsion whereas control rats lacked L-81. Lymphatic triacylglycerol output, measured both chemically and radioactively, was markedly suppressed in the experimental rats as compared to the controls. Thus, these data indicate that the reason why lipid transport was unaffected by L-81 when egg phosphatidylcholine was infused was not because of the pathway used for the resynthesis of triacylglycerol from phosphatidylcholine. In the second part of this study, we measured the appearance time for chylomicron (in control rats) and for very-low-density lipoprotein (in L-81-treated rats). The appearance time is defined as the time between placement of radioactive fatty acid into the intestinal lumen and the appearance of radioactive lipid in the central lacteal. The average appearance time for the control rats was 10.8 min, which was significantly shorter than the 16.2 min in the L-81-treated experimental rats. This difference in appearance time further supports the hypothesis that chylomicron and very-low-density lipoprotein are packaged separately in the enterocytes and only the formation of chylomicron is inhibited by L-81.

Cumulative evidence indicates that there is an increased accumulation of calcium in dystrophic muscle and that this may have a pathophysiological role in the progression of the dystrophic process. The accumulation may be related to a defect of the plasma membrane. Because parathyroid hormone (PTH) stimulates calcium influx into the cytosol, the chronic effects of surgical ablation of the parathyroid glands on muscle Ca, Mg, protein synthesis, and histology, as well as plasma creatine phosphokinase (CPK), Ca, and Mg, were studied in normal and dystrophic (BIO 14.6) hamsters. Thyroparathyroidectomized (TPTX) hamsters receiving replacement doses of l-thyroxine were killed at age 90 d, 55 d after TPTX. In intact dystrophic hamsters, the Ca content in the heart was 20 times higher than in normal animals and was reduced by half in TPTX dystrophic hamsters. Similar results were observed in diaphragm and rectus femoris. No abnormalities in Mg content were observed in intact or TPTX dystrophic hamsters. Ether-extractable fat of the heart and diaphragm was reduced in dystrophic hamsters and was not modified by TPTX. Protein synthesis was enhanced in the diaphragm of dystrophic hamsters but was not changed by TPTX. The concentration of CPK in plasma was elevated in dystrophic hamsters and fell significantly after TPTX. In the latter animals, microscopic examination of the heart showed lesser signs of dystrophy, particularly in the degree of fibrosis. To determine the degree of dystrophy at the age when TPTX was performed, identical analyses were made in 35-d-old hamsters. Definitive histological signs of dystrophy were observed, and although the Ca content in heart, diaphragm, and rectus femoris was elevated, the values were lower than in 90-d-old intact and TPTX dystrophic hamsters. This indicates that chronic TPTX in dystrophic hamsters reduces, but does not arrest, the dystrophic process. In normal hamsters, a 50% reduction in plasma Ca concentration was observed 6 h after TPTX; 55 d after TPTX, however, plasma Ca was within normal limits in both normal and dystrophic hamsters. No parathyroid tissue was observed in serial sections of the trachea and adjacent tissues in TPTX animals. This suggests that in chronically TPTX hamsters fed a standard laboratory diet, plasma Ca can be maintained by mechanisms independent of parathyroid function. The data indicate that in dystrophic hamsters TPTX causes a marked reduction in: (a) muscle Ca accumulation, (b) levels of plasma CPK and, (c) intensity of histological changes in the heart. These changes were independent of the levels of plasma Ca and were not observed in normal hamsters. We conclude that PTH accentuates the dystrophic process, probably by enhancing the already increased Ca flux into muscle (apparently caused by defective sarcolemma). We postulate that normal secretion of PTH may have a deleterious effect in congenital or acquired conditions associated with altered plasma membranes.

When bovine growth hormone is cleaved with cyanogen bromide, several fragments are produced: an N-terminal pentapeptide (fragment E); a 109-residue peptide (fragment A) which is adjacent to the N-terminal pentapeptide and is connected to a 30-residue peptide (fragment B) by a disulfide bridge; a 25-residue peptide (fragment C) which probably lies between fragments A and B in the intact molecule; and a C-terminal dodecapeptide (fragment D). In the present study, several of these fragments were tested for biological activities typical of native bovine growth hormone. Fragment AB (fragments A and B linked by a disulfide bond) was found to stimulate the incorporation of leucine-14C into the protein of diaphragms isolated from hypophysectomized rats, which had received 2 injections of the fragment over a 25-hr period. In addition, fragment AB added in vitro stimulated the conversion of 14C-glucose into CO2 and fatty acids by isolated adipose tissue of hypophysectomized rats. The ability to stimulate glucose ut...

Highly purified porcine (PGH) and human (HGH) growth hormones were reduced,S-aminoethylated and cleaved with cyanogen bromide. The two largest fragments resulting from the cleavage of PGH were separated from the reaction mixture and purified. These were porcine fragment A (about 112 residues) beginning at the fifth residue from the N-terminus and fragment B (55 residues), which follows fragment A in the sequence of the PGH molecule. The largest fragment resulting from the cleavage of HGH, human fragment A (111 residues), was also prepared in purified form. Human fragment A and porcine fragment A represent roughly equivalent portions of their respective hormone molecules. These purified fragments were then tested for biological activity. Porcine fragment A, when injected intraperitoneally into hypophysectomized rats, stimulated the in vivo incorporation of 14C-leucine into the protein of the diaphragm and liver. Also, both porcine fragment A and human fragment A stimulated the in vitro incorporation of leu...

Fifty years ago, Houssay showed that pituitary ablation ameliorates the hyperglycemia of pancreatectomized dogs.1 This approach was later applied to human diseases, such as diabetes mellitus and cancer. A conclusion of those studies is that under certain circumstances, hormones secreted in normal amounts may be deleterious in human disease. In this presentation we will discuss the role that normally functioning parathyroid glands may have in muscular dystrophy.

Rats were administered streptozocin (STZ; 50 or 75 mg/kg i.v., tail vein) or vehicle. Approximately 2 wk later, venous and arterial catheters were implanted for subsequent (24 h later) vasopressin, electrolyte, and hemodynamic measurements. STZ-induced diabetic (STZ-D) rats demonstrated a dose-dependent increase in the plasma glucose concentration, plasma osmolality, and plasma vasopressin concentration. Mean arterial blood pressure (MABP) was unchanged, but heart rate was reduced. Diabetes-prone BB rats, maintained on or withdrawn from insulin treatment for 24–48 h, and diabetes-resistant rats were instrumented and studied as above. Spontaneous-diabetes–prone rats demonstrated increases in plasma glucose concentration and plasma osmolality similar to STZ-D rats but had significantly greater plasma vasopressin concentrations. The significant decrease in MABP observed in these animals probably contributed to the enhanced vasopressin response. We conclude that both osmotic and cardiovascular parameters play important roles in vasopressin secretion in diabetic rats.

We conducted a double-blind trial with the calcium antagonist, diltiazem (8 mg/kg/d), for 24 to 32 months in 22 boys with Duchenne muscular dystrophy, who were paired by functional activity and age. No adverse clinical or ECG effects of diltiazem were detected. In eight matched pairs, completing 28 months, manual muscle testing scores fell somewhat less in the diltiazem group (from 5.5 to 4.6) than in the placebo group (from 5.3 to 4.2), although the difference between groups was not significant (p = 0.06). The 95% confidence interval for the difference in slopes of regression lines obtained from trimonthly manual muscle tests on all subjects was markedly asymmetric in favor of the diltiazem group, but this difference was also not significant. There was less deterioration of functional activity of lower extremities in the diltiazem-treated group, when beginning and end values were analyzed (p = 0.03). However, the difference in slopes of regression lines obtained from trimonthly determinations was nonsignificant. Similarly, the beginning versus end comparisons of systolic and diastolic blood pressure showed a significantly (p less than 0.05) smaller elevation of blood pressure in the diltiazem-treated group, but no difference was observed when the slopes of all values were analyzed. All other clinical and laboratory variables were unaffected by diltiazem treatment. The findings in manual muscle tests and functional activity suggest a beneficial trend with chronic diltiazem treatment in DMD.

The ability of GH in vitro to stimulate leucine incorporation into protein and the uptake of 2-aminoisobutyric acid (AIB) and 3-O-methyl-glucose (3-OMG) was studied in diaphragm muscle from dwarf (dw/dw) mice. Ovine GH (25 mug/ml) significantly enhanced the rate of protein synthesis in muscle from untreated dwarfs. In contrast, GH usually failed to stimulate transport, although occasionally a small increase in AIB uptake was seen. Insulin (50 muU/ml) readily stimulated both AIB transport and protein synthesis in muscle from untreated dwarfs. Dwarfs were treated for 4 days with thyroxine (T4) (0.3-5 mug/day) or 3,5,3'-triiodo thyronine (T3) (0.1 mug/day). This enhanced the responsiveness of the transport systems to GH and seemed to increase the basal transport rates for AIB and 3-OMG. After the T4-treatment 0.75 mug/ml GH stimulated AIB transport in mice aged 15 weeks; 25 mug/ml GH stimulated AIB transport in dwarfs aged 5 weeks-1 yr. and leucine incorporation into protein in dwarfs aged 5 weeks-2 yr. This last result suggests that response to GH does not cease merely due to increasing age. Synergism by T3/T4 was not permanent, and the maximum response to GH occurred 1 week after the last injection of T4. The possible mechanisms whereby thyroid hormones enhance responsiveness of the transport systems to GH are discussed.

Bovine growth hormone (bGH) covalently bound to Sepharose was tested for biological activity in vitro on diaphragm muscle and adipose tissue from hypophysectomized rats. Only under special conditions did bGH—Sepharose seem to stimulate a—aminoisobutyric acid (AIB) transport, the incorporation of leucine into protein, or glucose utilization. Exposure of adipose tissue to GH—Sepharose, like exposure to free GH, also appeared to make the tissue refractory to subsequent stimulation by free bGH. It is doubtful, however, that GH produced these changes while still bound to the Sepharose, since material with GH—like activity was found in the medium after removal of the bGH—Sepharose by centrifugation. This active material appeared only when the incubation medium contained tissue or had been previously exposed to tissue. These findings are consistent with the hypothesis that the tissues release some substance which in turn releases active material, perhaps GH itself, from the GH—Sepharose complex. Endocrinology 92...