cAMP-Dependent Vasodilators Cross-Activate the cGMP-Dependent Protein Kinase to Stimulate BK _Ca Channel Activity in Coronary Artery Smooth Muscle Cells

Abstract

Abstract-cAMP-dependent vasodilators are used to treat a variety of cardiovascular disorders; however, the signal transduction pathways and effector mechanisms stimulated by these agents are not fully understood. In the present study we demonstrate that cAMP-stimulating agents enhance the activity of the large-conductance, calcium-activated potassium (BK Ca ) channel in single myocytes from coronary arteries by "cross-activation" of the cGMP-dependent protein kinase (protein kinase G, PKG). Single-channel patch-clamp data revealed that 10 mol/L isoproterenol, forskolin, or dopamine opens BK Ca channels in coronary myocytes and that this effect is attenuated by inhibitors of PKG (KT5823; Rp-8-pCPT-cGMPS), but not by inhibiting the cAMP-dependent protein kinase (protein kinase A, PKA). In addition, a membrane-permeable analog, CPT-cAMP, also opened BK Ca channels in these myocytes, and this effect was reversed by KT5823. Direct biochemical measurement confirmed that dopamine or forskolin stimulates PKG activity in coronary arteries but does not elevate cGMP. Finally, the stimulatory effect of cAMP on BK Ca channels was reconstituted in a cell-free, inside-out patch by addition of purified PKG activated by either cGMP or cAMP. In contrast, channel gating was unaffected by exposure to the purified catalytic subunit of PKA. In summary, findings from on-cell and cell-free patch-clamp experiments provide direct evidence that cAMP-dependent vasodilators open BK Ca channels in coronary myocytes by cross-activation of PKG (but not via PKA). Biochemical assay confirmed this cross-activation mechanism of cAMP action in these arteries. This signaling pathway is a novel mechanism for regulation of potassium channel activity in vascular smooth muscle and other cells. Key Words: cAMP Ⅲ protein kinase G Ⅲ BK Ca channel Ⅲ coronary Ⅲ cross-activation E arly studies investigating the cellular effects of cyclic nucleotides demonstrated antagonism between cAMP and cGMP in most tissues. 1 An exception to this general rule was vascular smooth muscle (VSM), in which both nucleotides produced the same physiological response, ie, vasodilation; however, the signaling mechanisms by which cAMP or cGMP induced this response were unknown. Most investigations assumed that the vasodilatory response to either nucleotide was mediated via a distinct transduction cascade involving its corresponding nucleotide-activated protein kinase; however, research over the last 10 years has revealed a more complicated signaling process. Findings from the laboratories of Corbin and Lincoln demonstrated that "crossactivation" of the cGMP-dependent protein kinase (protein kinase G, PKG) by cAMP could be a key element in the signal transduction cascade of cAMP-induced vasodilation. 2,3 Subsequent studies have established that cGMP can also exert physiological effects by stimulating cAMP-dependent protein kinase (protein kinase A, PKA) activity. For example, PKA appears to mediate nitric oxide-dependent inhibition of aortic smooth muscle cell proliferation 4 and cGM...

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