Stimulation of calcium uptake by parathyroid hormone in renal brush-border membrane vesicles. Relationship to membrane phosphorylation

The effect of parathyroid hormone (PTH) on Ca2+ uptake was studied in brush-border membrane vesicles (BBMV) prepared from the kidneys of dogs administered 4-5 micrograms/kg of bovine PTH 1-84 in vivo. PTH stimulated Ca2+ uptake at 20 s of incubation from control values of 231 +/- 21 to 306 +/- 30 pmol/mg of protein, p less than 0.001. The stimulation of Ca2+ uptake by PTH was not reversed by incubation of the BBMV with the Ca2+ ionophore, despite the fact that Ca2+ uptake was several times greater than the expected uptake at equilibrium, indicating that most of the uptake represented Ca2+ binding to the BBMV. In BBMV from kidneys exposed to PTH, hypotonic lysis or increasing the osmolality of the solution external to the BBMV did not affect Ca2+ uptake. These data also indicated that the largest fraction of Ca2+ uptake in the presence of a chemical potential represented binding of Ca2+ to BBMV. Ca2+ binding was initially to the exterior of the BBMV, then translocated within the membrane and to the interior vesicular face as assessed by chelation of Ca2+ bound to the BBMV by ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Incubation of BBMV from kidneys exposed to PTH with gentamicin, which competes with Ca2+ for anionic phospholipid-binding sites, reversed the stimulatory effects of PTH on Ca2+ uptake. Phosphorylation of BBMV and PTH treatment in vivo had similar effects on BBMV phospholipid composition increasing the levels of anionic phospholipids. Phosphorylation of the BBMV also produced gentamicin-inhibitable increases in membrane Ca2+ binding. Phosphorylation of BBMV from kidneys exposed to PTH was inhibited suggesting a higher state of phosphorylation in vivo. The data demonstrate that PTH administered in vivo stimulated Ca2+ binding in BBMV that was gentamicin inhibitable and associated with an increase in the membrane content of anionic phospholipids.

Increased Na+-H+ exchange in brush border vesicles from dogs with renal failure

In the remnant kidney model of chronic renal failure, absolute reabsorption of Na+ in the proximal tubule of the remaining nephrons is increased over normal. Absolute proximal tubular reabsorption of bicarbonate and proximal tubular H+ excretion per nephron have also been shown to be increased over normal in his model of renal disease. Na+ uptake in membrane vesicles isolated from the brush border membrane of remnant kidneys of dogs with chronic renal failure is increased over uptake in membrane vesicles isolated from kidneys of normal dogs. In the present studies an amiloride-sensitive, electroneutral Na+-H+ exchanger was identified in canine renal brush border membrane vesicles. Na+ uptake in membrane vesicles in the presence of an initial H+ gradient (intravesicular pH less than extravesicular pH) was increased in membrane vesicles isolated from the remnant kidneys of dogs with chronic renal failure over that in membrane vesicles from kidneys of normal dogs. This increase was abolished by amiloride. It is possible that the alterations in Na+ and bicarbonate reabsorption and H+ excretion in the remnant kidney model of chronic renal failure can be explained on the basis of increased activity of the Na+-H+ exchanger in the renal brush border membrane.

Parathyroid hormone stimulation of renal phosphoinositide metabolism is a cyclic nucleotide-independent effect

The effects of parathyroid hormone (PTH) and cyclic nucleotides on renal phosphoinositide metabolism were studied using cortical tubules isolated from dog kidneys. PTH stimulated the initial rates of 32Pi incorporation into phosphatidylinositol 4'5'-diphosphate, phosphatidylinositol 4'-monophosphate, phosphatidylinositol and phosphatidic acid. PTH also caused a 45-55% increase in the actual tissue levels of these phospholipids by 5 min of incubation. By 30 min of incubation, the levels of 32Pi incorporated were similar in PTH and control flasks, but the actual levels of the phosphoinositides remained elevated, indicating stimulation of their turnover. Additional evidence of increased turnover of phosphatidylinositol was obtained from tubules pre-incubated with myo-[2-3H]inositol. PTH stimulated a rapid short-lived decrement in [3H]phosphatidylinositol while total phosphatidylinositol levels increased, indicating increased turnover rates of phosphatidylinositol. In tubules pre-incubated with [14C]arachidonic acid, indicating utilization of diacylglycerol produced during turnover for resynthesis of phosphatidic acid and phosphoinositides. Cyclic nucleotides and phosphodiesterase inhibition failed to reproduce the effect of PTH on phosphoinositide metabolism. These studies indicate that PTH stimulates renal phosphoinositide metabolism through a mechanism independent of cAMP which results in a net synthesis of the phosphoinositides.

Uptake of Pi in brush border vesicles after release of unilateral ureteral obstruction

After release of complete unilateral ureteral obstruction, a decreased fractional excretion of phosphate (Pi) is observed in the postobstructed kidney compared with the nonobstructed (control) kidney. To determine whether this decrease in the urinary excretion of Pi is due to changes in Na+-dependent Pi transport across the renal brush border membranes of postobstructed and control kidneys, membrane vesicles were prepared from the brush borders of kidneys from dogs that had undergone complete unilateral ureteral obstruction. Alkaline phosphatase activity was decreased in membrane vesicles isolated from postobstructed kidneys. No differences were observed in Na+-dependent Pi transport or in Na+ uptake in membrane vesicles isolated from postobstructed as compared with control kidneys. The in vivo administration of parathyroid hormone decreased Na+-dependent Pi transport in membrane vesicles isolated from postobstructed and control kidneys despite the absence of a phosphaturic response. Our findings suggest that no intrinsic change in the transport characteristics of Pi across the luminal membrane of renal tubular cells occurs with unilateral ureteral obstruction. The findings are consistent with the suggestion that the low fractional excretion of Pi in the postobstructed kidney results from very low filtered loads of Pi on the postobstructed side.

Cyclic AMP-dependent protein phosphorylation in canine renal brush-border membrane vesicles is associated with decreased phosphate transport

It is known that the administration of parathyroid hormone to dogs results in phosphaturia and decreased phosphate transport in brush-border vesicles isolated from the kidneys of those dogs. Parathyroid hormone has been shown to activate adenylate cyclase at the basal-lateral membrane of the renal proximal tubular cell. It has been postulated that parathyroid hormone-induced phosphaturia is effected through phosphorylation of brush-border protein by membrane-bound cAMP-dependent protein kinase. An experimental system was designed such that phosphorylation of brush-border vesicles and Na+-stimulated solute transport could be studied in the same preparations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membrane vesicles revealed cAMP-dependent phosphorylation of 2 protein bands (Mr = 96,000 and 62,000), which was enhanced by exposure of the inside of the membrane vesicles to ATP and cAMP. Cyclic AMP-dependent phosphorylation of brush-border vesicles was accompanied by inhibition of Na+-stimulated Pi but not D-glucose transport or 22Na+ uptake. When renal brush-border vesicles from parathyroidectomized and normal dogs were phosphorylated in vitro in the presence and absence of cAMP, both the cAMP-dependent phosphorylation and inhibition of Na+-stimulated Pi transport were greater in vesicles isolated from kidneys of parathyroidectomized dogs relative to control animals. We conclude that the cAMP-dependent phosphorylation of brush-border membrane-vesicle proteins is associated with specific inhibition of Na+-stimulated Pi transport. The phosphaturic action of parathyroid hormone (PTH) could be mediated through the cAMP-dependent phosphorylation of specific brush-border membrane proteins.

Decreased luminal membrane transport of phosphate in chronic renal failure

To examine the effects of the hyperparathyroidism that accompanies chronic renal failure (CRF) on Pi transport across the renal proximal tubular luminal membrane, brush border membrane vesicles (BBMV) were prepared from the viable portion of the kidney of dogs with CRF or normal renal function. Initial rates of Na+ gradient-dependent Pi transport were decreased in BBMV from dogs with CRF compared with normal dogs. Initial rates of 22Na+ uptake in BBMV from dogs with CRF, however, were increased, suggesting a more rapid dissipation of the Na+ gradient in these vesicles. Initial rates of Pi transport measured under Na+-equilibrated conditions were decreased in BBMV from dogs with CRF. Parathyroidectomy of dogs with CRF abolished this decrease in Pi uptake. We conclude that the adaptations in Pi excretion associated with CRF are accompanied by changes in Na+-dependent Pi transport in BBMV and, therefore, possibly across the luminal membrane of the renal tubule. Moreover, the activity of this transport system can be altered by parathyroidectomy of dogs prior to isolation of BBMV.

Parathyroid hormone inhibition of phosphate transport in renal brush border vesicles from phosphate-depleted dogs

Dietary phosphate (Pi) restriction increases renal Pi reabsorption and induces resistance to the phosphaturic action of parathyroid hormone. Na+-gradient-stimulated Pi transport in membrane vesicles isolated from the renal brush border of experimental animals has been shown to parallel changes in renal Pi reabsorption induced by dietary Pi restriction and in vivo administration of parathyroid hormone. Dietary Pi restriction has been shown to markedly inhibit the phosphaturic response to parathyroid hormone in rats and dogs. Parathyroid hormone has been reported not to decrease the Na+-gradient-stimulated transport of Pi in brush border membrane vesicles isolated from dietary Pi restricted rats unless the rats were administered an acute Pi load prior to killing, however, thyroparathyroidectomy of rats fed a low Pi diet has been reported to increase Na+-gradient-stimulated Pi transport. Using the dietary Pi restricted dog, we demonstrated no significant decrease in renal reabsorption of Pi in response to parathyroid hormone administration. However, significant decreases in Pi transport in brush border membrane vesicles isolated from the kidneys of dietary Pi restricted dogs were observed in response to in vivo parathyroid hormone administration. These data demonstrate that the resistance to the phosphaturic action of parathyroid hormone observed in vivo does not include resistance to the inhibitory effect of parathyroid hormone on Pi transport in brush border membrane vesicles. Thus, the data suggest that parathyroid hormone continues to alter Pi transport characteristics of the brush border membrane in states of Pi depletion despite the resistance to parathyroid hormone seen in vivo.

Peripheral metabolism of intact parathyroid hormone. Role of liver and kidney and the effect of chronic renal failure

The plasma disappearance rate (metabolic clearance rate) of administered intact parathyroid hormone (intact PTH) was analyzed in awake dogs with indwelling hepatic and renal vein catheters. The metabolic clearance rate (MCR) of intact PTH was found to be very rapid, 21.6 +/- 3.1 ml/min per kg in 11 normal dogs. The liver accounted for the greatest fraction of the MCR of intact PTH (61 +/- 4%) by virtue of an arterial minus venous (a - v) difference across the liver of 45 +/- 3%. The renal uptake of intact PTH accounted for 31 +/- 3% of the MCR of intact PTH. The renal a - v difference for intact PTH of 29 +/- 2% was significantly greater than the filtration fraction indicating renal uptake of intact PTH at sites independent of glomerular filtration. Together, the hepatic and renal clearances of intact PTH accounted for all but a small fraction of the MCR of intact PTH. The MCR of intact PTH, rendered biologically inactive by oxidation, was markedly decreased to 8.8 +/- 1 ml/min per kg. The a - v difference of oxidized intact PTH was reduced both in the liver and kidney. These data suggested that the high uptake rates of intact PTH are dependent, at least in part, upon sites recognizing only biologically active PTH. Chronic renal failure (CRF) decreased the MCR of intact PTH to 11.3 +/- 1.3 ml/min per kg (n = 10). Both the hepatic and renal a - v differences of intact PTH were reduced in dogs with CRF. This resulted in reductions in the hepatic and renal clearances of intact PTH. These studies identify the liver as a major extrarenal site of PTH metabolism affected by CRF. They suggest that CRF impairs the function of the major uptake sites involved in intact PTH metabolism.

Regulation of canine renal vesicle Pi transport by growth hormone and parathyroid hormone

Renal phosphate (Pi) reabsorption is increased by growth hormone (GH) and decreased by parathyroid hormone (PTH). Na+-stimulated Pi transport across the brush border membrane of the proximal tubule is the initial step in the process of Pi reabsorption. To determine whether changes in Pi reabsorption induced by GH or PTH are accompanied by changes in brush border membrane Na+-gradient-stimulated Pi transport, we examined the effect of in vivo GH and PTH administration and thyroparathyroidectomy on Pi transport by isolated brush border membrane vesicles prepared from canine kidney. In experiments in which the effect of PTH administration was examined, the same animal provided the control kidney (before PTH administration) and the experimental kidney (after PTH administration). The Na+-gradient Pi overshoot in vesicles isolated from normal, GH-treated and thyroparathyroidectomized dogs was increased after in vivo PTH administration. GH administration and thyroparathyroidectomy increased the height of the overshoot compared to normal. PTH administration decreased the apparent V value by 44% in vesicles from normal animals. The apparent V value was increased, compared to normal, by GH (34%) and thyroparathyroidectomy (57%). PTH administration decreased the apparent V in both the latter groups. GH administration to thyroparathyroidectomized dogs further increased the apparent V. Changes in the apparent V paralleled changes in Pi reabsorption in vivo induced by experimental manipulations. We conclude that changes in renal Pi reabsorption induced by GH were like those induced by PTH, accompanied by changes in the Na+-stimulated Pi transport system in the renal brush border membrane, and that the effect of PTH on vesicular Pi transport in GH-treated dogs did not differ from the effect on vesicles from normal animals.

Effect of intact parathyroid hormone on hepatic glucose release in the dog

The liver has been shown to remove parathyroid hormone (PTH) from its arterial circulation by a mechanism that is selective for the intact form of the peptide (PTH 1-84). The present studies demonstrate that PTH has biologic effects on the liver in vivo. Bovine PTH 1-84 stimulated hepatic glucose release in dogs with indwelling hepatic vein catheters from basal values of 31+/-8 to 68+/-9 mg/min per kg after bolus injections of PTH. The effect on hepatic glucose release was apparent by 5 min and persisted for the 80 min of observation. The NH(2)-terminal PTH fragment (syn b-PTH 1-34) had no effect. Bovine PTH 1-84 administered in doses designed to produce circulating levels of immunoreactive PTH similar to the endogenous levels observed in uremic dogs also increased the incorporation of (14)C from infused [(14)C]alanine into glucose, and increased estimated hepatic uptake of both chemical and [(14)C]alanine, while increasing hepatic glucose release. Thus, administration of "physiologic levels" of b-PTH 1-84 stimulated hepatic glucose release in part through increased gluconeogenesis in vivo, whereas syn b-PTH 1-34 had no demonstrable effect. Circulating levels of insulin rose after PTH administration, an increase which presumably represents a secondary response to the rise in glucose release. These results suggest that the liver is a target organ of PTH, and that PTH might potentially alter carbohydrate metabolism during hypersecretion. They also suggest that hepatic uptake of PTH may be related in part to production of a specific biologic effect rather than just simple peptide degradation.

The benefits of evaluation of the patient with recurrent or multiple calcium stones

Metabolic evaluation of patients with multiple or recurrent calcium stones has been fruitful. The great majority of cases has demonstrated abnormalities contributing to stone formation. Hypercalciuria on the basis of increased intestinal absorption of calcium was a common finding. The evaluation led to improved guide lines for therapy.

Effects of glucose and parathyroid hormone on the renal handling of myoinositol by isolated perfused dog kidneys

The effects of glucose and parathyroid hormone (PTH) on the transport and metabolism of myoinositol (MI) and [2-(3)H]MI were studied in isolated perfused dog kidneys. Studies during perfusion of kidneys with normal and elevated glucose concentrations demonstrated that under normal conditions the isolated kidney reabsorbed 94.7+/-0.2% of the filtered MI, and the renal production of (3)H-metabolities of MI was 117.9+/-6% of the filtered MI load. This indicated that entry of MI into tubular cells by reabsorption was not the sole pathway for entry into the pool of MI within the kidney undergoing catabolism. High glucose perfusate decreased MI reabsorption to 68.6+/-4.7% and thus decreased delivery of [2-(3)H]MI into the catabolic pool from the reabsorptive pathway. In the high glucose experiments, the rate of [2-(3)H]MI catabolism exceeded [2-(3)H]MI reabsorption by the same fraction as in normal glucose experiments, which indicates that high glucose did not affect nonreabsorptive access of MI to the catabolic site. In contrast to the effects of glucose, PTH administration resulted in an increase in perfusate MI concentration and a decrease in the perfusate [2-(3)H]MI specific activity. Concomitantly, urinary MI and [2-(3)H]MI concentrations were increased, again with a decrease in [2-(3)H]MI specific activity. These results indicate that PTH caused a release of MI into the urine (not the same as decreased MI reabsorption, which would not affect urinary [(3)H]MI specific activity) and into the perfusate of the isolated kidneys. These effects on MI release were about coincidental with the increase in urinary cyclic 3',5'-AMP after PTH and preceded the peak phosphaturic effect of PTH. There was no detectable effect of PTH on MI synthesis from glucose as a source of the MI released into the urine and perfusate. However, PTH temporarily halted accumulation of tritiated MI catabolites. There was no effect of inactivated PTH on urinary cyclic 3',5'-AMP or on MI transport, which indicates that the PTH effect on MI handling was a specific hormonal effect. These studies clarify the renal metabolism of MI, and they demonstrate heretofore unknown effects of PTH on the renal handling and metabolism of MI. The effects of PTH on renal MI metabolism have important implications in renal carbohydrate metabolism and phospholipid turnover.

Selective uptake of the synthetic amino terminal fragment of bovine parathyroid hormone by isolated perfused bone

Studies from our laboratory have shown that the metabolic clearance rate of carboxy terminal immunoreactive parathyroid hormone (i-PTH) can be accounted for by extraction of i-PTH by liver and kidney. In contrast, there was no demonstrable hepatic uptake of the synthetic amino terminal bovine PTH fragment (syn b-PTH 1-34) and the kidney accounted for only 45% of the metabolic clearance rate of amino terminal i-PTH. This suggested that another major site, presumably bone, played a role in the metabolism of syn b-PTH 1-34. Extraction of i-PTH by isolated perfused bone was studied during infusion of purified bovine PTH (b-PTH) 1-84 or syn b-PTH 1-34. In five studies during infusion of syn b-PTH 1-34 the arteriovenous difference for i-PTH across bone was 36%. In contrast, no significant uptake of carboxy terminal i-PTH was observed in nine studies during infusion of b-PTH 1-84. In addition, when H(2)O(2)-oxidized (biologically inactive) syn b-PTH 1-34 was used no arteriovenous difference was observed. These findings correlated with the ability of these PTH preparations to stimulate cyclic AMP production by the perfused bone. Syn b-PTH 1-34 increased cyclic AMP production at perfusate PTH concentrations of 1-5 ng/ml, whereas b-PTH 1-84 evoked only a minimal response at concentrations of 10-20 ng/ml. We conclude that bone is a major site of metabolism of the amino terminal PTH fragment, syn b-PTH 1-34. In addition, these data suggest that cleavage of the intact hormone, with the production of amino terminal PTH fragments by peripheral organs (liver and kidney), may play a major role in the regulation of PTH effects on bone.