In vitro effects of parathyroid hormone on kidney cortical slices: cAMP responses and concomitant inhibition of the Na+ gradient-dependent uptake of phosphate by brush border membrane vesicles isolated from the renal slices

Effect of calbindin D 28K on sodium transport by the luminal membrane of the rabbit nephron

We previously reported that in the rabbit, the vitamin D-dependent calcium binding protein 28K (CaBP 28K) increases calcium (Ca2+) transport in the distal tubule by opening a high affinity Ca2+ channel in the luminal membrane. Since Na+ and Ca2+ transports are interdependent in this membrane, we questioned whether the calbindin has any influence on Na+ transport. Luminal membranes from rabbit proximal and distal tubules were purified and 22Na uptake by the membrane vesicles was measured using the rapid filtration technique. The vesicles were loaded with 280 mM mannitol and 20 mM Tris-Hepes pH 7.4, with either 3 microM CaBP or the carrier. Incubation medium contained 1 mM 22NaCl, 278 mM mannitol, and 20 mM Tris-Hepes pH 7.4. The presence of 3 microM CaBP 28K in the distal luminal membrane vesicles increased the 0.5 mM Ca2+ uptake from 0.91 +/- 0.21 to 1.84 +/- 0.33 pmol/microg/10 s (P < 0.01) and decreased 1 mM Na+ uptake from 0.62 +/- 0.15 to 0.27 +/- 0.08 pmol/microg/10 s (P < 0.05). A similar decrease of Na+ uptake was observed in proximal luminal membrane experiments. The effect on Na+ uptake by the distal membrane was dose-dependent with a IC50 of 4.5 microM. Addition of 2 mM Ca2+ to the incubation medium decreased 1 mM Na + uptake from 0.62 +/- 0.15 to 0.49 +/- 0.12 pmol/microg/10 s (P < 0.05), but did not influence the effect of CaBP 28K on Na+ uptake. Experiments performed in the presence and absence of ethyl isopropyl amiloride (EIPA) suggest that the effect of calbindin involves the Na+/H+ exchanger activity.

Stimulation by thyroid hormone of phosphate transport in primary cultured renal cells

The regulation by thyroid hormone of phosphate transport in primary cultured chick renal cells was examined. The more physiologically active L-analogs of triiodothyronine and thyroxine, but not the D-analogs of the hormones, stimulated the Na+-dependent phosphate uptake system. Na+-independent phosphate uptake and Na+-dependent uptakes of alpha-methylglucoside and L-proline were unaffected. The increase in Na+-dependent phosphate uptake was concentration dependent, exhibited an induction period, and was blocked by inhibitors of RNA and protein synthesis. The stimulation of phosphate uptake by triiodothyronine was due to an increased Vmax rather than to an altered affinity for phosphate. These findings demonstrate that thyroid hormone acts directly on renal cells to modulate phosphate transport and suggest that the renal cell system may serve as a model to examine the mechanism by which thyroid hormone controls gene expression and regulates plasma membrane transport function.

Alpha-2-adrenergic modulation of the parathyroid hormone-inhibition of phosphate uptake in cultured renal (OK) cells

Parathyroid hormone enhances the formation of cAMP and decreases the Na+-dependent uptake of phosphate in cultured renal cells derived from the American opossum (OK cells). Epinephrine, acting as an alpha 2-adrenergic agonist, inhibits the PTH-induced synthesis of cAMP by a pertussis toxin-sensitive mechanism and blunts the inhibition of phosphate transport by PTH. Na+-dependent alpha-methylglucoside and Na+ uptakes by the cells are unaffected by PTH and epinephrine. These findings suggest that alpha 2-adrenergic agonists may selectively modulate PTH-sensitive phosphate transport in the renal proximal tubule.

Possible role of calcium mediators in parathyroid hormone action on phosphate transport in rabbit renal brush border membrane

The possibility of the involvement of intracellular calcium in the action of parathyroid hormone on phosphate transport in renal brush border membrane was examined. Preincubation of rabbit renal proximal tubules with parathyroid hormone or 8-bromo-cAMP induced a significant inhibition on phosphate uptake by the brush border membrane vesicles isolated therefrom. The addition of intracellular Ca antagonists, trifluoperazine or W-7, to the preincubation medium, alone was without effect on phosphate uptake by the brush border membrane vesicles, but abolished the inhibitory effects of parathyroid hormone and 8-bromo-cAMP.

Phosphate uptake by kidney epithelial (LLC-PK1) cells

Phosphate uptake by the cultured kidney epithelial cell (LLC-PK1) was studied. The uptake was Na+ dependent, saturable with respect to phosphate and Na+, and energy dependent. The characteristics of the cell uptake system resembled the properties of phosphate transport in the kidney. Parathyroid hormone, dibutyryl cyclic AMP, and forskolin decreased Na+-dependent phosphate uptake. These agonists did not affect Na+-dependent alpha-methylglucoside uptake. Vasopressin and isoproterenol, which do not affect renal phosphate transport, did not inhibit phosphate uptake by the cell. These findings suggest that the cultured cell system may be a useful experimental model for studies of renal phosphate transport and its regulation.

Responses of chick renal cell to parathyroid hormone: effect of vitamin D

The in vitro incubation of chick renal cells with parathyroid hormone (PTH) resulted in the inhibition of Na+-dependent phosphate uptake when the cells were isolated from 1,25-dihydroxycholecalciferol 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]-repleted chicks but not when the cells came from vitamin D-deficient animals. Na+-independent phosphate and Na+-dependent alpha-methylglucoside uptakes were not affected by PTH and the vitamin D status of the bird. The activation of chick renal cell adenylate cyclase by PTH was significantly blunted when the enzyme was from vitamin D-deficient animals relative to the activation of the enzyme from repleted cockerels. This alteration was due to a change in maximum velocity of the system rather than an effect on the affinity for hormone. The response of adenylate cyclase to other hormones, e.g., prostaglandin E2, and activators, e.g., 5' -guanylyl-imidodiphosphate and forskolin, was not affected by the vitamin D status of the animal. PTH had little effect in activating protein kinase in cells from vitamin D-deficient chicks. In cells from vitamin D-sufficient birds, PTH caused a fourfold increase in adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase. Dibutyryl cAMP inhibited Na+-dependent phosphate uptake by cells from 1,25-(OH)2D3-repleted animals, but the cyclic nucleotide had no effect on phosphate uptake in cells from vitamin D-depleted chicks. This finding suggests that the loss of PTH receptor sites known to be concomitant with the secondary hyperparathyroidism associated with vitamin D deficiency is only a partial explanation for the failure of PTH to inhibit phosphate uptake in cells from vitamin D-deficient animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of parathyroid hormone on Na+-dependent phosphate transport and cAMP-dependent 32P phosphorylation in brush border vesicles from isolated perfused canine kidneys

Concentrative uptake of 32Pi induced by the dissipation of a Na+ gradient (overshoot) was demonstrated in brush border membrane vesicles obtained from isolated perfused canine kidneys. Na+-dependent 32Pi transport was decreased in brush border vesicles from isolated kidneys perfused with parathyroid hormone (PTH) for 2 h compared to uptake measured in vesicles from kidneys perfused without PTH. Cyclic AMP-dependent 32P phosphorylation of a 62,000 Mr protein band was demonstrable on autoradiograms of sodium dodecyl sulfate-polyacrylamide gels of membrane suspensions from kidneys perfused +/- PTH. Evidence that perfusion with PTH resulted in cAMP-dependent phosphorylation in isolated kidneys from parathyroidectomized dogs (decreased cAMP-dependent 32P phosphorylation of the 62,000-Mr band in brush border vesicles) was obtained after 2-h perfusion with PTH. Decreased 32P phosphorylation was not observed if membranes were allowed to dephosphorylate prior to 32P phosphorylation in vitro. We conclude that brush border vesicles from isolated perfused canine kidneys can be used to study the action of PTH on Na+-Pi cotransport in brush border membranes and on cAMP-dependent phosphorylation of the membrane. It is strongly suggested that PTH effects changes in Na+-dependent 32Pi transport in isolated brush border vesicles and changes in 32P phosphorylation of vesicles via a direct action on the renal cortical cell rather than as a consequence of extrarenal actions of the hormone.

Inhibition of renal brush border phosphate transport and stimulation of renal gluconeogenesis by cyclic amp and parathyroid hormone

The aims of the study were to determine whether 8-bromo-cyclic AMP (8BcAMP) in vivo mimics the inhibitory action of parathyroid hormone (PTH) on phosphate transport across the brush border membrane (BBM) of the renal proximal tubule, and to examine whether changes in BBM transport are accompanied by changes in the rate of renal gluconeogenesis. Thyroparathyroidectomized dogs were anesthetized and equilibrated, and control urine collections were obtained prior to removing the left kidney. Subsequent intravenous infusion of 8BcAMP at 50 mg/hr for 2 hr increased fractional excretion of phosphate from 4 +/- 1 (controls) to 29 +/- 4% (P less than 0.001) without changing glomerular filtration. In BBM vesicles isolated from the renal cortex, the initial Na+-dependent transport of phosphate was decreased from 747 +/- 135 (controls) to 564 +/- 126 pmoles per mg per 0.25 min after 8BcAMP (P less than 0.025), but Na+-independent phosphate uptake and Na+-dependent L-proline uptake were not changed significantly. Renal gluconeogenesis in the same animals was increased from 2.5 +/- 0.3 (controls) to 5.3 +/- 0.5 mumoles glucose per g tissue per hr after infusion of 8BcAMP (P less than 0.001). Infusion of PTH, like 8BcAMP, inhibited BBM phosphate transport and stimulated renal gluconeogenesis. We conclude that the inhibitory action of cyclic AMP and PTH on BBM phosphate transport is accompanied by stimulation of gluconeogenesis which suggests, indirectly, that changes in gluconeogenesis may be part of the intracellular mechanism for regulating BBM phosphate uptake in response to certain stimuli.