Relative sensitivity of kidney and bone to the amino-terminal fragment b-PTH (1-30) of native bovine parathyroid hormone: Implications for assessment of bioactivity of parathyroid hormone fragments in vivo and in vitro

Adenylate cyclase activation in renal cortical membranes is widely used to assess the potency of parathyroid hormone (PTH) and hormonal fragments. Recent studies, however, suggest that the structural requirements for biological activity of PTH in bone may differ from those in kidney. In isolated perfused canine bone, cyclic AMP production is markedly increased by syn-b-PTH (1-34) whereas intact b-PTH (1-84) has minimal effect. Furthermore, a potent inhibitor of PTH stimulated adenylate cyclase activity, Nle⁸Nle¹⁸Tyr³⁴ syn-b-PTH (3-34) NH₂, devoid of biological activity in kidney, is an agonist in isolated bone. The present studies examine the effects in bone and kidney of the amino-terminal fragment b-PTH (1-30), prepared by cleavage of intact b-PTH (1-84) by Cathepsin B in vitro. In basolateral canine renal cortical membranes, b-PTH (1-30) was less active than syn-b-PTH (1-34) in its ability to stimulate adenylate cyclase. Half maximal stimulation of adenylate cyclase activity by b-PTH (1-30) was 50 nM compared with 2 nM for syn-b-PTH (1-34). Maximum enzyme activation by b-PTH (1-30) reached only 50% of the activation by syn-b-PTH (1-34). Addition of Gpp(NH)p (1 mM) increased the affinities for both peptides. The relative difference in potency, however, remained unchanged. In contrast, in isolated bone, b-PTH (1-30) and syn-b-PTH (1-34) were equipotent in increasing cyclic AMP production. These data provide further evidence for differences in the structural requirements for PTH activity in bone and kidney and suggest that bioassays of PTH and PTH fragments in kidney may not accurately reflect the effects of PTH in bone.