Dissociation of parathyroid hormone and cyclic-3 ', 5 ' amp effects on Na-Pi uptake by cells isolated from proximal straight tubules of rat kidney

Studies in respiratory alkalotic or short-term phosphate deprived rats rais ed the possibility that in straight portion of proximal tubules (PST) cAMP might be not a mediator of PTH in inhibition of phosphate reabsorption. The present experiments directly compared the sensitivity of Na-dependent phos phate [P-32] (Na-Pi) uptake to PTH or cAMP by PCT or PST cells freshly prep ared from outer cortex and outer stripe of outer medulla of rat kidney. The purity of the cells was examined by activity of enzymes specific for PST i .e. glutamine synthetase, gamma-glutamyl transpeptidase and creatine kinase , a marker enzyme for medullary thick ascending limb (MTAL) and distal conv oluted tubule. Similar inhibition of Na-Pi uptake by 1-34 bPTH by PST and P CT cells was observed: -33.0 and -30.0 % (ns), respectively. In contrast, d ibutyryl cAMP decreased Na-Pi uptake only by PCT but not by PST cells: -31. 0 and -3.6% (p < 0.02), respectively. The 3-isobutyl-1-methylxanthine (IBMX ), a phosphodiesterase inhibitor, resulted in slight stimulation of Na-Pi u ptake by PST but strong inhibition by PCT cells: 7.8 vs -26.0% (p < 0.001). In contrast to PCT in PST cells cAMP seems to play a minor role as a media tor of inhibition of Na-Pi uptake by PTH.