Vasopressin stimulates action potential firing by protein kinase C-dependent inhibition of KCNQ5 in A7r5 rat aortic smooth muscle cells

Abstract

[Arg8]-vasopressin (AVP), at low concentrations (10–500 pM), stimulates oscillations in intracellular Ca2+concentration (Ca2+spikes) in A7r5 rat aortic smooth muscle cells. Our previous studies provided biochemical evidence that protein kinase C (PKC) activation and phosphorylation of voltage-sensitive K+(Kv) channels are crucial steps in this process. In the present study, Kvcurrents ( IKv) and membrane potential were measured using patch clamp techniques. Treatment of A7r5 cells with 100 pM AVP resulted in significant inhibition of IKv. This effect was associated with gradual membrane depolarization, increased membrane resistance, and action potential (AP) generation in the same cells. The AVP-sensitive IKvwas resistant to 4-aminopyridine, iberiotoxin, and glibenclamide but was fully inhibited by the selective KCNQ channel blockers linopirdine (10 μM) and XE-991 (10 μM) and enhanced by the KCNQ channel activator flupirtine (10 μM). BaCl2(100 μM) or linopirdine (5 μM) mimicked the effects of AVP on K+currents, AP generation, and Ca2+spiking. Expression of KCNQ5 was detected by RT-PCR in A7r5 cells and freshly isolated rat aortic smooth muscle. RNA interference directed toward KCNQ5 reduced KCNQ5 protein expression and resulted in a significant decrease in IKvin A7r5 cells. IKvwas also inhibited in response to the PKC activator 4β-phorbol 12-myristate 13-acetate (10 nM), and the inhibition of IKvby AVP was prevented by the PKC inhibitor calphostin C (250 nM). These results suggest that the stimulation of Ca2+spiking by physiological concentrations of AVP involves PKC-dependent inhibition of KCNQ5 channels and increased AP firing in A7r5 cells.