The Ca2+-sensing receptor in the rabbit cortical thick ascending limb (CTAL) is functionally not coupled to phospholipase C

The recently cloned rabbit kidney Ca2+-sensing receptor (RabCaR) was functi onally characterized in microperfused rabbit cortical thick ascending limb (CTAL) segments. Reverse transcriptase polymerase chain reaction (RT-PCR) c onfirmed that this nephron segment contains mRNAs coding for the RabCaR. El evation of the extracellular Ca2+ concentration ([Ca2+](e)) from 1 to 5 mmo l l(-1) induced an increase in the fluorescence emission ratio (R), thus re flecting an increase in intracellular Ca2+ activity ([Ca2+](i)). This incre ase was inhibited by verapamil, nifedipine and SKF 96365, and potentiated b y a previous application of Bay K 8644. Neither verapamil nor Bay K 8644 mo dified the resting [Ca2+](i). This suggests that the basolateral Ca2+ influ x induced by a high [Ca2+](e) occurs via verapamil- and dihydropyridine-sen sitive Ca2+ channels, which are not open under resting conditions. In contr ast to that evoked by antidiuretic hormone (ADH), the [Ca2+](i) increase in duced by a high [Ca2+](e) did not result from an accumulation of inositol p hosphates. Neomycin, Gd3+, Mg2+, commonly used agonists of the Ca2+-sensing receptor, did not increase the [Ca2+](i). In the presence of verapamil, AD H still produced a transient [Ca2+](i) increase that was not observed in th e presence of an increased [Ca2+](e). These results suggest that the RabCaR in rabbit CTAL cells is not functionally coupled to phospholipase C. In co nclusion, the high [Ca2+](e)-induced [Ca2+](i) increase involves verapamil- and dihydropyridine-sensitive Ca2+ channels and is independent of phosphoi nositide metabolism. Whether these channels are activated by the RabCaR rem ains to be elucidated.