VASOPRESSIN AND PROTEIN KINASE-A ACTIVATE G-PROTEIN-SENSITIVE EPITHELIAL NA+ CHANNELS

To determine the molecular steps involved in the vasopressin-induced r enal Na+ reabsorption, the patch-clamp technique was utilized to study the role of this hormone in the regulation of apical Na+ channels in renal epithelial A6 cells. Addition of arginine vasopressin (AVP) indu ced and/or enhanced Na+ channel activity within 5 min of addition unde r cell-attached conditions. The AVP-induced channel activity was a ref lection of both an increase in the average apparent channel number (0. 2-1.7) and the percent open time (2-56%). Addition of the phosphodiest erase inhibitor, 3-isobutyl-1-methylxanthine, the adenosine 3',5'-cycl ic monophosphate (cAMP) analogues, 8-(4-chlorophenylthio)-cAMP and 8-b romo-cAMP, or forskolin elicited a comparable effect to that of AVP. T he induced channels had similar properties to Na+ channels previously reported, including a channel conductance of 9 pS, Na+-to-K+ selectivi ty of 3-5:1, and high amiloride sensitivity. The cAMP-dependent protei n kinase A (PKA) in the presence of ATP induced and/or enhanced Na+ ch annel activity in excised inside-out patches with a change in average apparent channel number and percent open probability similar to those observed with either AVP or cAMP analogues in intact cells. Addition o f activated pertussis toxin (100 ng/ml) completely blocked the AVP- or PKA-induced Na+ channel activity in excised inside-out patches, where as incubation of intact cells with the toxin completely prevented the effect of both activators. The data indicate that AVP mediates its eff ect through a cAMP-dependent pathway involving PKA activation whose ta rget is the G protein pathway that regulates apical epithelial Na+ cha nnel activity.