Possible role of calcium in parathyroid hormone actions in rabbit renal proximal tubules.

Using a glucose microassay and in vitro isolated renal tubule perfusion technique, we have studied the actions of parathyroid hormone (PTH) on gluconeogenesis (GNG) and fluid (Jv) and phosphate (Jp) transport rates in isolated rabbit renal proximal tubules. In proximal straight tubules (PST), PTH stimulated GNG and inhibited Jv and Jp. In proximal convoluted tubules (PCT), PTH inhibited Jv but failed to affect GNG and Jp. An increase in Ca concentration, however, stimulated GNG and allowed PTH to inhibit Jp in PCT. Addition of the intracellular Ca antagonists trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) abolished the inhibitory effects of PTH on Jv and Jp in both PCT and PST. In conclusion, these studies suggest that Ca-dependent intracellular pathways may be involved in the actions of PTH in rabbit renal proximal tubules. The altered response to PTH in rabbit PCT may be due to alterations in the response of intracellular Ca to the hormone.