The prostacyclin analogue carbacyclin inhibits Ca²⁺-activated K⁺ current in aortic baroreceptor neurones of rats

1. Previous studies indicate that prostacyclin (PGI₂,) increases the activity of baroreceptor afferent fibres. The purpose of this study was to test the hypothesis that PGI₂ inhibits Ca²⁺-activated K⁺ current (I(K(Ca))) in isolated baroreceptor neurones in culture. 2. Rat aortic baroreceptor neurones in the nodose ganglia were labelled in vivo by applying a fluorescent dye (DiI) to the aortic arch 1-2 weeks before dissociation of the neurones. Outward K⁺ currents in baroreceptor neurones evoked by depolarizing voltage steps from a holding potential of -40 mV were recorded using the whole-cell patch-clamp technique. 3. Exposure of baroreceptor neurones to the stable PGI₂ analogue carbacyclin significantly inhibited the steady-state K⁺ current in a dose-dependent and reversible manner. The inhibition of K⁺ current was not caused indirectly by changes in cytosolic Ca²⁺ concentration. The Ca²⁺-activated K⁺ channel blocker charybdotoxin (ChTX, 10^-7 M) also inhibited the K⁺ current. In the presence of ChTX or in the absence of Ca²⁺, carbacyclin failed to inhibit the residual K⁺ current. Furthermore, in the presence of high concentrations of carbacyclin, ChTX did not cause further reduction of K⁺ current. 4. Carbacyclin-induced inhibition of I(K(Ca)) was mimicked by 8-bromo-cAMP and by activation of G-protein with GTPγS. The inhibitory effect of carbacyclin on I(K(Ca)) was abolished by GDPβS, which blocks G-protein activation, and by a selective inhibitor of cAMP-dependent protein kinase, PKI_5-24. 5. The results demonstrate that carbacyclin inhibits ChTX-sensitive I(K(Ca)) in isolated aortic baroreceptor neurones by a G-protein-coupled activation of cAMP-dependent protein kinase. This mechanism may contribute to the PGI₂-induced increase in baroreceptor activity demonstrated previously.