Independent and exclusive modulation of cardiac delayed rectifying K+ current by protein kinase C and protein kinase A

Expression of minK in Xenopus oocytes results in a current similar to the cardiac slow delayed rectifying K+ (I-Ks) current. Modulation of the I-Ks current in cardiac myocytes has been studied extensively because of its role in shaping the cardiac action potential. The human and cat minK cDNA have been cloned, but their regulation by protein kinases has not been characterized. We report hen on the complex modulation of human and cat I-Ks currents by protein kinase C (PKC) and protein kinase A (PKA). Activation of PKC by phorbol ester (100 nmol/L phorbol 12,13-didecanoate [PDD]) produces an increase in I-Ks current that peaks after 20 minutes and then subsequently decreases to approximate to 50% of the control level after 1 hour. PKA activation only produces a sustained increase in I-Ks current. Interestingly, premodulation by PKC prevents I-Ks current modulation by PKA, and PKC has no effect on I-Ks current after potentiation by PKA. This shows that the I-Ks current is modulated by PKC and PKA in a mutually exclusive manner and suggests that multiple interacting phosphorylation sites are involved. Activation of PKC by diacylglycerol analogues only produces a slow decrease in I-Ks current. The biphasic effects of PKC on I-Ks current activated by PDD can also be separated by dose and duration. Low doses of PDD (5 nmol/L) or brief applications (5 minutes) of 100 nmol/L PDD only produces I-Ks current activation. These data suggest that there are at least 2 independent PKC phosphorylation sites in the minK-KvLQT1 channel. Additionally, long-term activation of PKC strongly attenuates the I-Ks current expression even when the corresponding changes in capacitance are taken into account.