Calcium/calmodulin-dependent kinase 2 mediates Epac-induced spontaneous transient outward currents in rat vascular...

The Ca²⁺ and redox-sensing enzyme Ca²⁺ /calmodulin-dependent kinase 2 (CaMKII) is a crucial and well-established signalling molecule in the heart and brain. In vascular smooth muscle, which controls blood flow by contracting and relaxing in response to complex Ca²⁺ signals and oxidative stress, surprisingly little is known about the role of CaMKII. The vasodilator-induced second messenger cAMP can relax vascular smooth muscle via its effector, exchange protein directly activated by cAMP (Epac), by activating spontaneous transient outward currents (STOCs) that hyperpolarize the cell membrane and reduce voltage-dependent Ca²⁺ influx. How Epac activates STOCs is unknown. In the present study, we map the pathway by which Epac increases STOC activity in contractile vascular smooth muscle and show that a critical step is the activation of CaMKII. To our knowledge, this is the first report of CaMKII activation triggering cellular activity known to induce vasorelaxation.

Activation of the major cAMP effector, exchange protein directly activated by cAMP (Epac), induces vascular smooth muscle relaxation by increasing the activity of ryanodine (RyR)-sensitive release channels on the peripheral sarcoplasmic reticulum. Resultant Ca²⁺ sparks activate plasma membrane Ca²⁺ -activated K⁺ (BK_Ca ) channels, evoking spontaneous transient outward currents (STOCs) that hyperpolarize the cell and reduce voltage-dependent Ca²⁺ entry. In the present study, we investigate the mechanism by which Epac increases STOC activity. We show that the selective Epac activator 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3', 5-cyclic monophosphate-AM (8-pCPT-AM) induces autophosphorylation (activation) of calcium/calmodulin-dependent kinase 2 (CaMKII) and also that inhibition of CaMKII abolishes 8-pCPT-AM-induced increases in STOC activity. Epac-induced CaMKII activation is probably initiated by inositol 1,4,5-trisphosphate (IP₃ )-mobilized Ca²⁺ : 8-pCPT-AM fails to induce CaMKII activation following intracellular Ca²⁺ store depletion and inhibition of IP₃ receptors blocks both 8-pCPT-AM-mediated CaMKII phosphorylation and STOC activity. 8-pCPT-AM does not directly activate BK_Ca channels, but STOCs cannot be generated by 8-pCPT-AM in the presence of ryanodine. Furthermore, exposure to 8-pCPT-AM significantly slows the initial rate of [Ca²⁺ ]_i rise induced by the RyR activator caffeine without significantly affecting the caffeine-induced Ca²⁺ transient amplitude, a measure of Ca²⁺ store content. We conclude that Epac-mediated STOC activity (i) occurs via activation of CaMKII and (ii) is driven by changes in the underlying behaviour of RyR channels. To our knowledge, this is the first report of CaMKII initiating cellular activity linked to vasorelaxation and suggests novel roles for this Ca²⁺ and redox-sensing enzyme in the regulation of vascular tone and blood flow.