SUBCELLULAR MECHANISM FOR CA2+-DEPENDENT ENHANCEMENT OF DELAYED RECTIFIER K+ CURRENT IN ISOLATED MEMBRANE PATCHES OF GUINEA-PIG VENTRICULAR MYOCYTES

Intracellular Ca2+ augments delayed rectifier K+ current (I-K) in cardiac myocytes, which may play a major modulatory role in repolarization of action potentials. We investigated subcellular mechanisms for Ca2+-induced enhancement of I-K in large-pipette inside-out membrane patches excised from isolated guinea pig ventricular myocytes. When [Ca2+](i) was raised from 10(-8) to 10(-6) mol/L, the amplitude of I-K measured at +80 mV was increased from 12.0 +/- 2.2 to 19.5 +/- 3.3 pA (P < .01). The enhancement of I-K by Ca2+ was dose dependent, with an EC(50) of 3.8 x 10(-8) mol/L. A calmodulin antagonist, W7 (50 mu mol/L), calmidazolium (100 mu mol/L), or HT-74 (20 mu mol/L), added to the intracellular solution abolished enhancement of I-K by Ca2+, whereas the inactive form of the W7 analogue, W5, had no effect on I-K. In the presence of a protein kinase inhibitor with a relatively high specificity for protein kinase C (H7), for protein kinase A (H8 or peptide-type inhibitor PKI), or for calmodulin kinase II (KN-62) or a nonspecific inhibitor of serine/threonine protein kinases (staurosporine), increases in [Ca2+](i) still enhanced I-K Ca2+-induced enhancement of I-K was also observed when Mg2+ and ATP were omitted from the intracellular solution to delete exogenous phosphate donors and when adenylylimidodiphosphate was added to preclude trapped cytoplasmic substrates. Thus, cardiac I-K was enhanced by increases in [Ca2+](i) at a physiological range via a calmodulin-dependent pathway, which did not involve a phosphorylation process.