cAMP activates BKCa channels in pulmonary arterial smooth muscle via cGMP-dependent protein kinase

The signal transduction mechanisms defining the role of cyclic nucleotides in the regulation of pulmonary vascular tone is currently an area of great interest. Normally, signaling mechanisms that elevate cAMP and guanosine-3',5'-cyclic monophosphate ( cGMP) maintain the pulmonary vasculature in a relaxed state. Modulation of the large-conductance, calcium- and voltage-activated potassium (BKCa) channel is important in the regulation of pulmonary arterial pressure, and inhibition ( closing) of the BKCa channel has been implicated in the development of pulmonary hypertension. Accordingly, studies were done to determine the effect of cAMP-elevating agents on BKCa channel activity using patch-clamp studies in pulmonary arterial smooth muscle cells (PASMC) of the fawn-hooded rat (FHR), a recognized animal model of pulmonary hypertension. Forskolin ( 10 muM), a stimulator of adenylate cyclase and an activator of cAMP-dependent protein kinase (PKA), and 8-4-chlorophenylthio (CPT)-cAMP ( 100 muM), a membrane-permeable derivative of cAMP, opened BKCa channels in single FHR PASMC. Treatment of FHR PASMC with 300 nM KT5823, a selective inhibitor of cGMP-dependent protein kinase (PKG) activity inhibited the effect of both forskolin and CPT-cAMP. In contrast, blocking PKA activation with 300 nM KT5720 had no effect on forskolin or CPT-cAMP-stimulated BKCa channel activity. These results indicate that cAMP-dependent vasodilators activate BKCa channels in PASMC of FHR via PKG-dependent and PKA-independent signaling pathways, which suggests cross-activation between cyclic nucleotide-dependent protein kinases in pulmonary arterial smooth muscle and therefore, a unique signaling pathway for cAMP-induced pulmonary vasodilation.