INDEPENDENT AND EXCLUSIVE MODULATION OF CARDIAC DELAYED RECTIFYING K-KINASE-C AND PROTEIN-KINASE-A( CURRENT BY PROTEIN)

Expression of minK in Xenopus oocytes results in a current similar to the cardiac slow delayed rectifying K+ (I-Ks) current. Modulation of t he I-Ks current in cardiac myocytes has been studied extensively becau se of its role in shaping the cardiac action potential. The human and cat minK cDNA have been cloned, but their regulation by protein kinase s 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 k inase A (PKA). Activation of PKC by phorbol ester (100 nmol/L phorbol 12,13-didecanoate [PDD]) produces an increase in I-Ks current that pea ks 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 PK C 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 sugges ts that multiple interacting phosphorylation sites are involved. Activ ation of PKC by diacylglycerol analogues only produces a slow decrease in I-Ks current. The biphasic effects of PKC on I-Ks current activate d 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 pr oduces I-Ks current activation. These data suggest that there are at l east 2 independent PKC phosphorylation sites in the minK-KvLQT1 channe l. Additionally, long-term activation of PKC strongly attenuates the I -Ks current expression even when the corresponding changes in capacita nce are taken into account.