The presence of parathyroid hormone-related protein (PTHrP) in human kidney vasculature and the signal transduction pathways stimulated during PTHrP-induced vasodilation of the rabbit kidney were investigated. Immunostaining of human kidney revealed the abundant presence of PTHrP in media and intima of all microvessels as well as in macula densa. In isolated perfused rabbit kidney preconstricted with noradrenaline, 10(-5) M Rp-cAMPS, a direct inhibitor of protein kinase A, produced comparable inhibition of 2.5 x 10(-7) M forskolin- and 10(-7) M PTHrP-induced vasorelaxations. Renal vasorelaxation and renal microvessel adenylyl cyclase stimulation underwent comparable desensitization following exposure to PTHrP. Nitric oxide (NO)-synthase inhibition by L-NAME (10(-4) M), NO scavenging by an imidazolineoxyl N-oxide (10(-4) M) and guanylyl cyclase inhibition by methylene blue (10(-4) M) decreased PTHrP-induced vasorelaxation by 27 to 53%, abolished bradykinin-induced vasorelaxation and did not affect forskolin-induced vasorelaxation. The effects of Rp-cAMPS and L-NAME were not additive on PTHrP-induced vasorelaxation. Damaging endothelium by treating the kidney with either anti-factor VIII-related antibody and complement, gossypol or detergent, did not affect PTHrP- or forskolin-induced vasorelaxations but reduced bradykinin-induced vasorelaxation by 53 to 92%. Conversely, endothelial damage did not alter the inhibitory action of L-NAME on PTHrP-induced vasorelaxation. In conclusion, PTHrP is present throughout the human renovascular tree and juxtaglomerular apparatus. Activation of both adenylyl cyclase/protein kinase A and NO-synthase/guanylyl cyclase pathways are directly linked to the renodilatory action of PTHrP in a way that does not require an intact endothelium in the isolated rabbit kidney.

Previous studies showed that PTHrP exhibits renal vasodilating, arteriolar cAMP stimulating and receptor binding properties. The present experiments were designed to study whether PTHrP may influence renin secretion. Rat kidneys were isolated and single-pass perfused at constant flow and stabilized pressure. Exposures to PTHrP or PTH stimulated a dose-dependent renin release reaching similar Vmax. The affinity (0.1 nM) and threshold concentration (0.01 nM) for PTHrP were about 10 times lower than for PTH. Compared to 10 microM isoproterenol, the maximum renin responses to PTHrP were similar but of shorter duration. The PTHrP dose-response curve was not affected by 10 microM indomethacin. Administered simultaneously, PTHrP and PTH displayed no additive effects. PTHrP-induced renin release as well as the role of extracellular calcium were further studied in nonfiltering kidneys, which were perfused at a constant flow and stable pressure in a closed circuit. Basal renin release was inversely related with perfusate calcium and was depressed by the calcium ionophore BAY-K8644. PTHrP (100 nM) induced a 1.6-fold increase of basal renin release in normocalcic perfusate. Removing calcium abolished renin responses. PTHrP reversed the inhibiting effects of hypercalcic media or BAY-K8644 on basal renin release. The results support calcium-mediated renin stimulating properties for PTHrP, via PTH receptors, independently from baroreceptors, macula densa and prostaglandins.

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The present study was designed to characterize the interaction of calcium and PTH in the control of renin release in isolated rat kidneys perfused in a closed circuit at constant flow. Kidneys were rendered nonfiltering using low perfusion pressures (70 mm Hg) and a hyperoncotic perfusate (100 g/liter BSA). Under these conditions, differences in perfusion pressure were less than 9 mm Hg between control and PTH-treated kidneys over the 50 min of perfusion. In the absence of PTH, renin release was inversely correlated with ionized calcium (Ca2+) concentration, with the highest release of renin noted with 1 mM EGTA and no added calcium. Also, verapamil treatment markedly elevated renin release, even in the presence of 2 mM Ca2+. In contrast, renin secretion was strongly depressed by 20 nM BAY-K8644 in the perfusate. In medium containing normal calcium concentrations (1 mM Ca2+), rat PTH(1-34) induced a 2-fold greater renin accumulation than in the control, non-PTH-treated kidneys. Isoproterenol induced a 5-fold stimulation under the same conditions. In the 0 Ca2+/1 mM EGTA perfusion, PTH did not elevate renin secretion. Renin release in response to PTH in 2 mM Ca2+ was similar to that observed in the 1 mM Ca2+ perfusion. PTH also reversed the effects of BAY-K8644 to suppress renin release. In verapamil-treated kidneys, PTH failed to stimulate renin release. These results indicate that PTH stimulates renin release by a process independent of the baroreceptors and macula densa. The Ca2+ modulation of PTH-induced renin release is consistent with the reported ability of PTH to block calcium channels and relax vascular smooth muscle.

1. We have previously reported that pharmacological concentrations (125nmol/l) of parathyroid hormone may stimulate renin release in the stable recirculating and non-filtering isolated rat kidney. 2. In the present study we have attempted to extend these initial observations by examining the concentration-related response of renin release to parathyroid hormone, using the same model of isolated kidney, and determining whether the effect of parathyroid hormone on renin release can be demonstrated by more direct approaches. Thus, the effects of parathyroid hormone on renin secretion were investigated in two other renal preparations: isolated rabbit glomeruli and isolated rat juxtaglomerular cells. 3. In the isolated kidney, rat parathyroid hormone significantly stimulated renin accumulation in the perfusate in a concentration-related manner with a threshold of 1 nmol/l. 4. In both glomeruli and juxtaglomerular cells bovine [Nle8,18,Tyr34]parathyroid hormone-(1-34)amide effectively and repeatedly stimulated renin release. These results imply that there is a direct stimulatory effect of parathyroid hormone on renin release. 5. We also examined the effect of [Nle8,18,Tyr34]parathyroid hormone-(1-34)amide during extracellular calcium buffering in the glomeruli. [Nle8,18,Tyr34]parathyroid hormone-(1-34)amide was uneffective in calcium-free medium. Increasing the extracellular ionized calcium concentration to 2.5 mmol/l increased the extent of stimulation in accordance with the reported ability of parathyroid hormone to block calcium channels and relax vascular smooth muscle cells. 6. These results provide further support for the role of parathyroid hormone as a direct mediator of renin secretion; moreover, the renin-stimulating action of parathyroid hormone may be mediated through the inhibition of calcium influx.