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Micropuncture and clearance experiments were done on normal dogs to investigate magnesium handling by proximal and distal nephron segments. Tubular fluid electrolytes were analyzed with the electron microprobe. Tubular fluid to ultrafilterable magnesium ratio (TF/UF magnesium) was observed to rise above unity but less than the TF/P insulin ratio generated along the proximal tubule. This is in contrast to the other major cations, the ratios of which remain close to unity as water is abstracted. Tubular fluid obtained from the distal tubule contained less magnesium than the glomerular filtrate (mean TF/UF magnesium of 0.6) indicating the loop of Henle is the major nephron segment reclaiming a significant portion of the filtered load. The faction of filtered load remaining at the distal sampling site was similar to the fraction appearing in the urine (8% vs 7%) indicating very little reabsorption beyond the distal tubule in these normal states.

Tubular magnesium reabsorption was investigated by recollection micropuncture and in vivo microperfusion techniques in acutely thyroparathyroidectomized rats made magnesium deficient by dietary deprivation. Henle's loop which normally reclaims the major portion of filtered magnesium was examined by elevation of intraluminal magnesium concentration. The transport capacity in these conditions was significantly lower in magnesium deficient rats (41%) compared to normal animals (71%) at comparable magnesium delivery rates. Acute infusion of MgCl2 further depressed loop magnesium reabsorption independent of intraluminal magnesium delivery. Parathyroid hormone did not alter magnesium transport capacity in magnesium deficient rats but resulted in enhanced transport in acutely hypermagnesemic deficient rats. Calcium reabsorption followed a similar qualitative pattern as magnesium with respect to loop function and urinary excretion. These results are consistent with a depressed transport capacity for magnesium in the loop of Henle of magnesium deficient rats which is independent of intraluminal magnesium delivery and circulating parathyroid hormone level.

Sodium and calcium are normally reabsorbed in parallel in the renal tubule. Both parathyroid hormone (PTH) and thiazide diuretics may influence this relationship. This study was designed to show whether the dissociation of Na from Ca transport produced by thiazides is dependent upon the presence of PTH. Hydropenic thyroparathyroidectomized (TPTX) dogs were given chlorothiazide alone and together with PTH. Chlorothiazide alone significantly increased fractional excretion of sodium (0.5 +/- 0.3-5.6 +/- 0.3%) and calcium (0.74 +/- 0.18-1.4 +/- 0.24%). However, the Ca/Na excretion ratio fell markedly from 1.57 to 0.24%. Micropuncture revealed this dissociation to occur at the distal tubule. Proximal reabsorption of water, sodium, and calcium were inhibited to an equal extent. However, distal fractional sodium reabsorption fell 10% whereas calcium reabsorption remained unchanged following chlorothiazide administration in TPTX animals. When phosphaturic doses of PTH were administered with chlorothiazide, no significant changes were observed in calcium or sodium reabsorption. It is concluded that PTH plays no role in the dissociation of sodium from calcium reabsorption resulting from acute chlorothiazide administration.

Recollection micropuncture and clearance studies were carried out on thyroparathyroidectomized hamsters to clarify the localization of the effects of parathyroid hormone (PTH) on renal electrolyte transport. The clearance data confirmed that PTH inhibits phosphate and enhances calcium and magnesium reabsorption. These effects appeared to result from actions of the hormone in several parts of the nephron. In the proximal tubule PTH did not affect H2O reabsorption but inhibited phosphate reabsorption ((TF/P)PO4 increased from 0.46 +/- 0.04 to 0.57 +/- 0.03, P less than 0.02) and appeared to enhance calcium and magnesium reabsorption ((TF/UF)Ca decreased from 1.41 +/- 0.07 to 1.25 +/- 0.06, P less than 0.001, and (TF/UF)Mg from 1.66 +/- 0.10 to 1.51 +/- 0.08, P less than 0.05; in control animals (TF/UF)Ca increased from 1.51 +/- 0.10 to 1.65 +/- 0.11, P less than 0.01). PTH further inhibited phosphate reabsorption and enhanced calcium and magnesium reabsorption between the late proximal and early distal sites of puncture. Comparison of fractional deliveries of calcium and magnesium from the late distal tubule with their fractional excretions suggests an additional effect beyond the distal puncture site. The phosphaturic, but not the calcium- and magnesium-retaining, effects of PTH were abolished by a 16-h fast.

Clearance and micropuncture studies have been performed in dogs to examine the effects of acute and chronic metabolic acidosis and acute alkalosis on tubular sodium and calcium transport. Acute metabolic acidosis, induced by the infusion of hydrochloric acid, decreased proximal fluid reabsorption and increased the fractional delivery of sodium and calcium to the distal tubule, but not to the final urine. In comparison with normal dogs, dogs with chronic metabolic acidosis (induced by feeding ammonium chloride) showed an increase in proximal fluid reabsorption and a dissociation of calcium from sodium reabsorption more distally, leading to an increased delivery of calcium relative to sodium at the distal tubule and in the final urine. The infusion of sodium bicarbonate to correct chronic metabolic acidosis, both in intact and thyroparathyroidectomized (TPTX) dogs, reduced proximal fluid reabsorption and caused a selective enhancement of calcium reabsorption relative to sodium in the more distal nephron, resulting in a reversal of the dissociation observed in acidosis, both at the distal tubule and in the final urine. By contrastin fusion of sodium chloride in parathyroid-intact acidotic dogs did not reduce proximal fluid reabsorption or enhance tubular calcium reabsorption. In nonacidotic dogs, both intact and TPTX, infusion of sodium bicarconate to induce acute alkalosis resulted in selhese data demonstrate the presence of a component of tubular calcium reabsorption situated beyond the proximal tubule, which is inhibited by chronic (but not acute) metabolic acidosis and enhanced by metabolic alkalosis (or bicarbonate infusion) independently of parathyroid hormone.

Magnesium and calcium transport was studied in the superficial proximal tubule, loop of Henle, and superficial distal tubule of the rat by in vivo microperfusion. Magnesium chloride was used to vary plasma and perfusate magnesium concentration. Perfusion with magnesium-free Ringer solution resulted in little magnesium entry into the tubule lumen in either normomagnesemic or hypermagnesemic rats. Magnesium concentration increased with water abstraction along the perfused proximal tubule whether markedly above or below plasma concentrations. Absolute proximal magnesium reabsorption increased with perfusate concentration; however, fractional magnesium and calcium reabsorption decreased in proportion to net sodium reabsorption with elevated extracellular magnesium. Magnesium absorption increased in the loop of Henle proportional to luminal magnesium concentration. Sodium and calcium reabsorption was not affected. Acute elevation of plasma magnesium, however, significantly depressed absolute magnesium reabsorption to a greater degree than calcium whereas sodium was unchanged. The effects of extracellular magnesium onloop reabsorption were reflected in the final urine. Reabsorption of sodium, calcium, and magnesium in the perfused distal tubule was concentration-dependent and not altered by luminal or extracellular magnesium concentration.

Because the rat papilla has parathyroid hormone (PTH)-sensitive adenylate cyclase and because of indirect evidence that PTH may alter collecting duct water and also calcium transport, the effects of PTH on rat papillary collecting duct water and calcium transport have been studied. PTH in concentrations of 50 and 500 ng/ml significantly increased diffusional water permeability by 20 and 38%, respectively, while 5,000 ng/ml had no additional effect. This permeability response was small when compared to a 78% increase in water permeability with a maximal (0.5 ng/ml) concentration of antidiuretic hormone (ADH). The normal increase in water permeability with ADH was depressed in the presence of PTH (500 ng/ml) but was overcome when the ADH concentration was increased from 0.5 to 5 ng/ml. Neither PTH nor ADH altered the permeability of the collecting duct to calcium which was low (0.19 +/- 0.03 micron/s). Increasing either the bath or perfusate calcium concentration from 1 to 5 mM did not alter calcium permeability. These studies suggest that PTH acts as a partial agonist to ADH within the papillary collecting duct and that PTH is unlikely to have a major role in collecting duct calcium transport.

1. Dietary magnesium deficiency is commonly associated with significant potassium depletion although the mechanisms responsible are unknown. Because the kidney has an important role in both magnesium and potassium homeostasis, clearance and micropuncture experiments were performed on thyroparathyroidectomized magnesium-deficient, normal and hypermagnesaemic rats to study the effect of body magnesium status on renal potassium handling. 2. Dietary magnesium restriction that reduced total-body magnesium by 30% did not alter renal potassium excretion despite a 10% reduction in total-body potassium. Graded magnesium infusions increased the fractional excretion of potassium in both magnesium-depleted and normal rats. However, the increase in the dietary depleted group was significantly less than in the control group (5-10 and then 13% compared with 7-19 and then 28% respectively). These changes in urine potassium excretion followed alterations in distal-tubule function. Parathyroid hormone did not alter potassium excretion in any of the experimental groups in contrast with its effect on magnesium excretion. 3. These data support the concept of distal tubular control of renal potassium homeostasis. The body magnesium status appears to exert some control over cellular potassium content and to alter indirectly distal-tubule potassium excretion.

Chronic nephron loss is compensated by functional adaptations which preserve electrolyte homeostasis. The response to volume expansion and diuretics was tested in dogs. Three phase recollection micropuncture studies were performed to assess the response of the remnant kidney in various stages of renal failure to furosemide administration (10 mg/Kg) and graded volume expansion (3 percent and 10 percent body weight). After the diuretic maneuvers, mean fractional excretion of sodium, potassium and water rose progressively in normal dogs (Stage I), with a greater increase in the remnant kidneys in the presence (Stage II) and absence (Stage III) of the contralateral kidney. Proximal and distal TF/P (Inulin) ratios were depressed after 3 percent volume expansion. However, proximal TF/P (Inulin) was not further lowered after 10 percent volume expansion and furosemide administration, while distal TF/P (Inulin) ratios were progressively depressed. The distal TF/P (Inulin) ratios in Stage III were significantly lower than in Stage II under analogous conditions. Our results suggest that the adaptive increase in the response of sodium transport by the remnant kidney to the diuretic maneuvers occurs in the loop of Henle, both in the azotemic and the non-azotemic stage. Adaptation of potassium excretion, as revealed by distal micropuncture, took place in the azotemic Stage III. Chronic functional adaptation for electrolyte transport occurs even before azotemia in the distal nephron and includes the proximal tubule with azotemia.

In this study, micropuncture experiments were performed to examine the segmental reabsorption of bicarbonate and chloride in the normal dog kidney (stage 1) and in the remnant kidney (stage 3) of dogs with experimental renal failure. The protocol consisted of three phases: (1) 3% extracellular fluid volume expansion, (2) infusion of Ringer's solution containing 90 mM of sodium bicarbonate, and (3) infusion of 150 mM of sodium bicarbonate in Ringer's solution. In the animals with remnant stage 3 kidneys, the ratio of absolute bicarbonate reabsorption/absolute sodium reabsorption was increased compared to dogs with stage 1 kidneys at both the proximal and distal sampling sites. These data suggest that bicarbonate reabsorption was elevated in both the distal and the proximal tubules in experimental chronic renal failure. In contrast to the reabsorption of bicarbonate, chloride reabsorption was depressed in stage 3 kidneys at the late proximal puncture site. This resulted in greater delivery to the distal nephron. The distal segments reabsorbed a constant fraction of delivered chloride, resulting in an increase in fractional chloride excretion in chronic renal failure compared to that in normal animals.

Recollection micropuncture and clearance studies were performed on 21 thyroparathyroidectomized hamsters to characterize the effect of chlorothiazide on tubular sodium, calcium, and magnesium transport. Acute administration of chlorothiazide resulted in a marked natriuresis while urinary calcium excretion fell and magnesium remained unchanged. The fraction of sodium, calcium, and magnesium remaining at the late proximal tubule increased modestly from 65 +/- 4 to 75 +/- 3, 68 +/- 3 to 75 +/- 4, and 78 +/- 4 to 85 +/- 2%, respectively. Distal tubular fluid to ultrafilterable plasma (TF/UF) sodium concentration rose from 0.24 +/- 0.03 to 0.44 +/- 0.04 whereas distal TF/UFCa concentration fell from 0.58 +/- 0.05 to 0.38 +/- 0.06. The fraction of sodium remaining at the distal tubule rose from 4.0 +/- 1.4 to 10.0 +/- 1.4% while that of calcium decreased from 10.2 +/- 1.1 to 7.6 +/- 1.2% following administration of chlorothiazide. No change was observed in distal delivery of magnesium. Thus chlorothiazide acted in the distal tubule to decrease sodium reabsorption, enhance calcium transport, and had little effect on distal magnesium reabsorption. These data are consistent with the distal tubular action of chlorothiazide which is independent of parathyroid hormone.

The effect of acute potassium infusion on renal tubular reabsorption of potassium by magnesium-deficient and pair-fed control rats has been studied by the recollection micropuncture method. During potassium chloride infusion, the amount of potassium remaining in the distal tubule is lower in magnesium-deficient than in pair-fed rats. This could be due to a reduction in potassium secretion or to an enhancement of potassium reabsorption. The present study demonstrates enhanced renal potassium retention in the magnesium-deficient rat, and is in contrast to previous reports of renal potassium wasting in this circumstance.

The effect of phosphate infusion on renal tubular handling of calcium and phosphate was examined in dogs which had been thyroparathyroidectomized (TPTX) immediately prior to the studies. Phosphate infusions in TPTX animals caused a small decrease in total and ultrafilterable plasma calcium, and decreased phosphate reabsorptive capacity in the proximal tubule and loop segment. Infusion of CaCl2 during phosphate loading to offset the fall in plasma calcium prevented the reduction in proximal phosphate reabsorptive capacity. However, between the proximal and distal sampling site, the reduction in phosphate reabsorptive capacity could not be prevented by CaCl2 administration. These data are consistent with the presence of two phosphate transport systems; one in the early proximal tubule, modulated by changes in plasma calcium level, and a second in the loop segment, which is independent of calcium. While the data suggest that the depression of proximal phosphate reabsorption during phosphate infusion may be secondary to the fall in plasma calcium concentration, they do not exclude a direct effect of infused phosphate on proximal phosphate reabsorption that may be antagonized by an opposing direct effect of the calcium infusion.

Micropuncture studies were performed in 26 dogs with a unilateral remnant kidney to examine its response to modest extracellular volume expansion and furosemide administration in the presence (Stage II) and absence (Stage III) of an intact contralateral kidney. During hydropenia in 15 Stage II dogs, proximal and distal transport of sodium and potassium was comparable to that of normal dogs (Stage I). Following 3% volume expansion, fractional proximal reabsorption was reduced similarly in Stages I and II. Although a slightly greater reduction in fractional loop reabsorption of sodium in Stage II after volume expansion was not significant, it was significantly greater with furosemide administration. In 11 Stage III dogs, proximal fractional reabsorption was depressed during hydropenia, and the loop sodium response to both volume expansion and furosemide administration was exaggerated. In contrast, greater increase in distal potassium secretion was demonstrated mainly in Stage III but not in Stage II remnant kidneys both before and after the diuretic maneuvers. The observations of exaggerated sodium response to furosemide by the remnant kidney in both Stages II and III but greater potassium response only in Stage III suggest that independent factors are responsible for these adaptations when functioning renal mass is reduced.

Micropuncture studies were performed on 27 thyroparathyroidectomized dogs to determine the segmental reabsorption of magnesium before and after graded magnesium infusion. Overall kidney reabsorption, as determined by the difference between filtered magnesium and urinary excretion, initially increased with elevation of plasma magnesium up to 3.5 meq/liter. Further elevation of plasma magnesium resulted in the appearance in the urine of all additional increments in filtered magnesium. Thus, renal magnesium reabsorption followed a pattern characteristic of a maximal transport rate (Tm). Evaluation of the proximal tubule by micropuncture demonstrated that net reabsorption of magnesium rose proportional to the increase in filtered magnesium and accounted for a constant fractional reabsorption of 14%. In contrast, magnesium reabsorption in Henle's loop initially increased with low delivery rates but peaked and fell with high plasma magnesium concentrations. Little magnesium reabsorption was observed between the distal collection site and final urine. Accordingly, the overall urinary excretion pattern was a summation of the different effects occurring in the proximal tubule and Henle's loop. Thus, renal magnesium reabsorption is not characterized by a Tm process but is a composite of distinct transport properties of the proximal tubule and the loop of Henle.

Micropuncture and clearance studies were performed on normal untreated and polyuric lithium chloride treated rats (10-12 days). A persistent hypernatremic state quickly developed in the polyuric lithium treated rats during hydropenia resulting from an increased urinary loss of water over sodium chloride, as the fractional excretion of sodium remained at control levels. Superficial proximal tubule and loop of Henle fluid reabsorption was depressed by 8 and 17%, respectively, in lithium-treated rats during this period. By contrast, water reabsorption in the distal tubule and collecting system was significantly increased in the lithium animals, being 27% of the filtered load compared with 20% in normal rats. These results suggest that the urinary-concentrating defect induced by lithium treatment is due primarily to a depression of proximal tubule and possibly loop of Henle function, and that water reabsorption within the distal nephron may in fact be augmented: thus it is unlikely that the action of antidiuretic hormone is significantly impaired. Marked phosphaturia and hypocalciuria were also noted in the lithium-treated rats.

Micropuncture experiments were performed in thyroparathyroidectomized dogs to examine the influence of changes in filtered calcium load on segmental tubular calcium reabsorption. Filtered calcium load was changed either by reducing glomerular filtration rate (GFR) by aortic clamping (group I) or by progressive calcium infusion (group II) to increase plasma ultrafilterable calcium concentration (UFCa). The results suggest that fractional proximal calcium reabsorption responds similarly to altered filtered load, whether produced by changes in GFR or UFCa. In contrast, fractional reabsorption by the loop segment is progressively reduced as UFCa is increased but is relatively unchanged by alterations in filtered load secondary to altered GFR. These data indicate a specific parathyroid hormone-independent reciprocal effect of UFCa on calcium reabsorption in the loop segment, which may be an important determinant of urinary calcium excretion.

The acute effects of intravenous infusions of phosphate and parathyroid hormone (PTH) upon the renal tubular handling of sodium, potassium, calcium, magnesium, and phosphate were examined in phosphate-depleted dogs using recollection micropuncture techniques. Hypercalciuria in phosphate depletion results from an impairment of calcium reabsorption between proximal and distal sampling sites, which can be partially corrected by the acute administration of PTH or by phosphate infusion. Magnesium reabsorption was normal in phosphate-depleted dogs but increased in parallel with calcium in the distal tubule following PTH and phosphate infusion. Phosphate was avidly reabsorbed in the phosphate-depleted dog so that excretion was very low even during the infusion of PTH or of neutral phosphate. Only with the infusion of both PTH and phosphate was a normal phosphaturic response observed.

Free-low micropunction experiments were performed in mongrel dogs and in Dalmatian coach hounds infused with urate to obtain Purate levels of 0.15-0.21 mM before and during the infusion of pyrazinioc acid (PZA). In the absence of PZA, mongrel dogs excreted approximately 50% and Dalmatians 140% of filtered loads of urate. In mongrel dogs net reabsorption occurred only in the proximal convoluted tubules. PZA enhanced net proximal reabsorption and revealed the occurrence of proximal secretion, whereas fractional urate excretion in the urine decreased only slightly. In Dalmation dogs urate fluxes across walls of proximal convoluted tubules resulted in either net reabsorption or net secretion, with no mean change. Net urate secretion occurred between superficial late-proximal and early-distal tubules, and considerably decreased fractional excretion of urate. The renal handling of PZA was similar in mongrel and in Dalmatian dogs.

Fractional reabsorption of water, sodium, and potassium at proximal and distal tubular sites within the nephron was studied by recollection-micropuncture experiments on dogs undergoing hypertonic mannitol diuresis. After an initial control hydropenic phase, 16% mannitol in modified Ringer's solution was administered intravenously, resulting in marked increases in fractional excretion of water (28.7%), sodium (12.6%), and potassium (63.9%). Inulin clearance decreased significantly from 35.1 to 25.2 ml/min. Analysis of paired micropuncture data revealed a significant decrease in tubule fluid to plasma (TF:P) inulin ratios in both the proximal tubule (1.63-1.45) and distal tubule (5.38-1.94). There was also a significant decrease in proximal TF:P sodium ratios (0.99-0.93) and potassium ratios (1.05-0.98). Distal TF:P sodium ratios, in contrast, rose significantly (0.38-0.59), while TF:P potassium ratios tended towards unity whether initially greater or less than one. Fractional reabsorption of sodium and water decreased by 5% and 10% respectively in the proximal tubule, but to a lesser extent than the resulting increases in fractional urinary excretion. The nonreabsorbed fraction, however, had increased sharply at the point of distal puncture for water (32%), sodium (26%), and potassium (26%), indicating a large inhibitory effect within the loop of Henle in addition to the smaller proximal effects.

Proximal and distal tubule micropuncture studies were performed to examine the response to graded extracellular volume (ECV) expansion in 10 normal dogs (stage I), 11 dogs with a unilateral remnant kidney (stage II), and 7 dogs with a remnant kidney after removal of the contralateral kidney (stage III). Before ECV expansion in stage III, there was a suggestive reduction in proximal tubule as well as loop fractional reabsorption of sodium. After ECV expansion to 3% body weight proximal tubule reabsorption was depressed in all groups of animals, while little further inhibition was observed in this segment with additional expansion to 10% body weight. In contrast, the fraction of filtered sodium remaining in the distal tubule rose progressively in all three groups after graded ECV expansion, suggesting that the graded natriuretic response found in the final urine was largely due to a similar response in the loop of Henle rather than that in the proximal tubule. The distal tubule response of the remnant kidney in both stages II and III was greater than that in stage I. These data indicate that although enhanced sodium excretion per nephron in chronic renal failure may be related to uremia, its exaggerated response to ECV expansion is due, at least in part, to certain as yet unidentified intrarenal factors consequent to reduction in functioning renal mass.