ARACHIDONIC-ACID POTENTIATES THE FEEDBACK RESPONSE OF MESANGIAL BKCA CHANNELS TO ANGIOTENSIN-II

The influence of arachidonic acid (AA) on the feedback regulation of m esangial contraction by large Ca2+-activated K+ channels (BKCa) was de termined through single-channel analysis using the patch clamp method. The mesangial BKCa is a low-gain negative feedback inhibitor of contr action that is activated in response to agonist-induced Ca2+ transient s and membrane depolarization. AA activated BKCa in cell-attached patc hes in a dose-dependent manner with a maximal effect at 400 nM and a h alf-maximal response at 49 nM. In inside-out patches, AA directly acti vated BKCa with a maximal effect at 400 nM. BKCa was activated signifi cantly in response to addition of 100 nM ANG II in the presence but no t the absence of AA. Since it was shown previously that fatty acids st imulated both soluble and membrane-bound guanylyl cyclase, we determin ed whether AA activated BKCa by interfering with cGMP-mediated signal transduction pathways. It was previously shown that 10 mu M cGMP, via cGMP-dependent protein kinase, activated BKCa in a biphasic manner wit h an early increase in probability of a channel existing in an open st ate (P-o) and a subsequent inactivation mediated by protein phosphatas e 2A (PP2A). We found that 10 mu M dibutyryl-cGMP enhanced BKCa activi ty in an additive manner with saturating concentrations (400 nM) of AA . Moreover, the inactivation phase mediated by PP2A was not abolished. Thus AA do es not affect the phosphorylation/dephosphorylation regula tory cycle for BKCa. It is concluded that AA potentiates the ANG II fe edback response of BKCa by a mechanism that is independent of the phos phorylation cycle.