Structure-activity relationship of parathyroid hormone: relative sensitivity of rabbit renal microvessel and tubule adenylate cyclases to oxidized PTH and PTH inhibitors

Abstract

It has been shown previously that secondary structural changes of bPTH-(1-34) (synthetic amino-terminal (1-34) fragment of bovine parathyroid hormone), obtained by oxidation of the methionines 8 and 18, abolished its hypotensive but not its hypercalcemic action. Hence, it has been postulated that the various physiological effects of the hormone are mediated by different receptors that require different regions or configurations of the peptide. To further examine this hypothesis the relative sensitivity of the PTH-responsive adenylate cyclase of microvessels and tubules isolated from rabbit kidney cortex, to oxidized PTH and PTH inhibitors, was examined. In the presence of GTP, bPTH-(1-34) stimulated both microvessel and tubule adenylate cyclase in a dose-dependent fashion and with analogous affinities (ED50 = 52 nM in the microvessels and 85 nM in the tubules). Hydrogen peroxide treatment of bPTH-(1-34) resulted in the loss of the adenylate cyclase stimulating potency in the microvessels while there was substantial enzyme activation (ED50 = 900 nM) in the tubules. Oxidized PTH inhibited the untreated PTH-stimulated adenylate cyclase, suggesting that oxidized PTH still retains an affinity for vascular receptor sites. Similar treatment of the sulfur-free PTH analog [Nle8,18, Tyr34]bPTH-(1-34)NH2, where methionines have been replaced by norleucine, had little or no effect in both fractions. In the microvessels the synthetic PTH antagonist analogs [Nle8,18, Tyr34]bPTH-(3-34)NH2 and [Tyr34]bPTH-(7-34)NH2, strongly inhibited the adenylate cyclase responses to bPTH-(1-34). No inhibition was seen in the tubules with the same molar ratios of inhibitor to native PTH. Together, these results suggest strongly that the differences in the adenylate cyclase response to various PTH fragments most likely represent a difference in the structural requirements for PTH actions between microvessels and tubules.