Arachidonic acid potentiates the feedback response of mesangial BKCa channels to angiotensin II.

The influence of arachidonic acid (AA) on the feedback regulation of mesangial contraction by large Ca(2+)-activated K+ channels (BKCa) was determined through single-channel analysis using the patch clamp method. The mesangial BKCa is a low-gain negative feedback inhibitor of contraction that is activated in response to agonist-induced Ca2+ transients and membrane depolarization. AA activated BKCa in cell-attached patches in a dose-dependent manner with a maximal effect at 400 nM and a half-maximal response at 49 nM. In inside-out patches, AA directly activated BKCa with a maximal effect at 400 nM. BKCa was activated significantly in response to addition of 100 nM ANG II in the presence but not the absence of AA. Since it was shown previously that fatty acids stimulated both soluble and membrane-bound guanylyl cyclase, we determined whether AA activated BKCa by interfering with cGMP-mediated signal transduction pathways. It was previously shown that 10 microM cGMP, via cGMP-dependent protein kinase, activated BKCa in a biphasic manner with an early increase in probability of a channel existing in an open state (Po) and a subsequent inactivation mediated by protein phosphatase 2A (PP2A). We found that 10 microM dibutyryl-cGMP enhanced BKCa activity in an additive manner with saturating concentrations (400 nM) of AA. Moreover, the inactivation phase mediated by PP2A was not abolished. Thus AA does not affect the phosphorylation/dephosphorylation regulatory cycle for BKCa. It is concluded that AA potentiates the ANG II feedback response of BKCa by a mechanism that is independent of the phosphorylation cycle.