Guanyl nucleotide potentiation of parathyroid hormone-stimulated adenylate cyclase in chicken renal plasma membranes: a receptor-independent effect.

We investigated the interaction of guanyl nucleotides with the parathyroid hormone (PTH) receptor-adenylate cyclase system in chicken renal plasma membranes. Micromolar concentrations of guanosine triphosphate and its hydrolysis-resistant analog 5'-guanylimidodiphosphate [Gpp(NH)p] increased both basal and PTH-stimulated adenylate cyclase activity. The enzyme activation produced by the amino-terminal 1--34 peptide of bovine PTH [bPTH-(1--34)] was potentiated by both guanyl nucleotides, although quantitatively greater effects were seen with Gpp(NH)p. The apparent activation constant for bPTH-(1--34) stimulation of adenylate cyclase, 16 nM, was reduced to 3.7 nM in the presence of 1.0 microM Gpp(NH)p. The interaction of guanyl nucleotides with the PTH receptor was evaluated by measurement of specific 125I-labeled bPTH-(1--34) binding to chicken renal plasma membranes in the presence and absence of Gpp(NH)p. There was no effect of the guanyl nucleotide on the rate of binding or dissociation of 125I-labeled bPTH-(1--34) from its renal receptor. Scatchard analysis of steady state PTH binding revealed that 1.0 microM Gpp(NH)p had minimal effect on either the affinity of PTH receptors (Kd increased from 25 nM to 30 nM) or their number (total number of binding sites increased from 8.6 to 9.4 pmol/mg protein). Separate experiments demonstrated a concentration-dependent effect on Gpp(NH)p to decrease the apparent activation constant for bPTH-(1--34) stimulation of adenylate cyclase, with no detectable guanyl nucleotide effect on the affinity of PTH receptors. The results suggest that guanyl nucleotides may enhance the coupling of occupied PTH receptors to adenylate cyclase independent of direct nucleotide effects on renal PTH receptors.