The slow delayed rectifier K+ current is differently regulated under baseline conditions and following β-adrenergic stimulation in canine ventricular cardiomyocytes

In conclusion, many different signaling pathways are involved in regulating I _Ks . Under baseline conditions the regulation is strongly [Ca ²⁺ ] _i -dependent, with PKA and CaM-CaMKII involved, whereas during ß-adrenergic stimulation it is [Ca ²⁺ ] _i -independent and supposes a pivotal role of EPAC-mediated activation of CaMKII. Sympathetic activation robustly increases the slow delayed rectifier K ⁺ current (I _Ks ) in the mammalian ventricular myocardium, however, exact downstream pathways involved in the β-adrenergic regulation of the current are not fully elucidated yet. This study examined the Ca ²⁺ sensitivity of I _Ks and the contribution of the protein kinase A (PKA) and the calcium/calmodulin kinase II (CaMKII) pathways in regulating I _Ks in isolated canine ventricular myocytes. Experiments were carried out under ß-adrenergic receptor activation (10 nM isoproterenol) and under baseline conditions (without isoproterenol). I _Ks was measured as an HMR-1556 sensitive current with the action potential voltage clamp technique under the physiological intracellular calcium homeostasis of the cells. Reducing intracellular Ca ²⁺ concentration ([Ca ²⁺ ] _i ) with 1 µM nisoldipine decreased the peak and mid-plateau densities of I _Ks , reduced the current integral, and increased the time-to-peak value. In contrast, ß-adrenergic receptor activation by isoproterenol resulted in larger I _Ks densities and integral, and shorter time-to-peak value. These effects of isoproterenol on I _Ks were significantly smaller when the CaMKII inhibitor 1 µM KN-93 was present in the cells, but the PKA inhibitor 3 µM H-89 did not exert such effect. Importantly, all effects of isoproterenol on I _Ks have fully developed even in the presence of 1 µM nisoldipine. Under baseline conditions the mid-plateau density of I _Ks , was significantly smaller in the presence of KN-93, H-89 or nisoldipine, while peak I _Ks density and the current integral were significantly smaller only in nisoldipine.