The signaling mechanisms defining the role of protein kinases in pulmonary vascular physiology regulation is an area of great interest.  Normally, signaling mechanisms which elevate cyclic AMP (cAMP) and cyclic GMP (cGMP) maintain the pulmonary vasculature in a relaxed state.  Modulation of large-conductance, calcium- and voltage-activated potassium (BK_Ca) channels is important in the regulation of pulmonary arterial pressure and inhibition of BK_Ca channels is implicated in the development of pulmonary hypertension. Accordingly, studies done in pulmonary arterial smooth muscle cells of the Fawn-Hooded rat, a recognized animal model of pulmonary hypertension, shows that cAMP opens BK_Ca channels. Treatment with KT5823, a selective inhibitor of cGMP-dependent protein kinase (PKG) inhibits the effect of cAMP. In contrast, blocking cAMP-dependent protein kinase (PKA) with KT5720 has no effect indicating that cAMP activates BK_Ca channels via PKG-dependent and PKA-independent signaling pathways which suggests ‘cross-activation’ between cyclic nucleotide-dependent protein kinases in hypertensive pulmonary arterial smooth muscle.  In addition, protein kinase C (PKC) activation inhibits the BK_Ca channel response to cAMP, which is blocked by the specific PKC isozyme inhibitors Gö 6983, and Gö 6976. These studies indicate that specific PKC isozymes inhibit cAMP-induced activation of BK_Ca channels via PKG in hypertensive pulmonary arterial smooth muscle.