P-2 PURINOCEPTOR STIMULATION ATTENUATES PTH INHIBITION OF PHOSPHATE-UPTAKE BY A G-PROTEIN-DEPENDENT MECHANISM

Purinergic P-2 receptors are present on proximal renal tubules, but th eir function is unknown. Because P-2 agonists antagonize vasopressin-s timulated water transport in the distal tubule by inhibiting activatio n of adenylyl cyclase, we postulated that P-2 receptor activation bloc ks parathyroid hormone (PTH) inhibition of phosphate uptake in proxima l tubule by preventing PTH-stimulated adenosine 3',5'-cyclic monophosp hate (cAMP) generation. PTH inhibition of sodium-dependent phosphate u ptake was attenuated by alpha,beta-methylene-ATP (AMP-CPP), a P-2y rec eptor agonist, but not by 2-methyl-thio-ATP, a P-2x receptor agonist, in a dose-dependent manner. AMP-CPP diddose-dependent manner. AMP-CPP did not attenuate inhibition of phosphate uptake produced by direct ac tivation of adenylyl cyclase with forskolin, by addition of the cAMP a nalogue 8-bromo-cAMP, or by inhibition of cAMP phosphodiesterase with RO-20-1724. Additionally, AMP-CPP had no effect on basal or PTH-stimul ated cAMP production. As PTH also stimulates protein kinase C activati on, the effect of AMP-CPP on inhibition of phosphate uptake stimulated by phorbol 12-myristate 13-acetate (PMA) was tested. AMP-CPP had no e ffect on PMA-induced inhibition of phosphate uptake. Pretreatment with pertussis toxin abolished the attenuating effect of AMP-CPP on PTH in hibition of sodium-dependent phosphate uptake. We conclude that activa tion of purinergic P-2 receptors attenuates the inhibitory effect of P TH on sodium-dependent phosphate uptake by a G protein-dependent mecha nism that is independent of cAMP generation, protein kinase A activati on, or protein kinase C activation.