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Recent studies have described regional differences in the electrophysiology and pharmacology of ventricular myocardium in canine, feline, rat, guinea pig, and human hearts. In this study, we use standard microelectrode and whole-cell patch-clamp techniques to examine the characteristics of the action potential and the delayed rectifier K+ current (IK) in epicardial, M region (deep subepicardial to midmyocardial), and endocardial cells isolated from the canine left ventricle. Cells from the M region displayed much longer action potential durations (APDs) at slow rates. At a basic cycle length of 4 s, APD measured at 90% repolarization was 358 +/- 16 (mean +/- SEM), 262 +/- 12, and 287 +/- 11 ms in cells from the M region, epicardium, and endocardium, respectively. Steady state APD-rate relations were steeper in cells from the M region. In complete Tyrode's solution, IK was smaller in myocytes from the M region when compared with those isolated from the epicardium or endocardium. Further characterization of IK was conducted in a Na(+)-, K(+)-, and Ca(2+)-free bath solution to isolate the slowly activating component of the delayed rectifier (IKs) from the rapidly activating component (IKr). IKs was significantly smaller in M cells than in epicardial and endocardial cells. With repolarization to -20 mV, IKs tail current density was 1.99 +/- 0.30 pA/pF (mean +/- SEM) in epicardial cells, 1.83 +/- 0.18 pA/pF in endocardial cells, and 0.92 +/- 0.14 pA/pF in M cells. Voltage dependence and time course of activation and deactivation of IKs were similar in the three cell types. The relative contribution of IKr and IKs among the three cell types was examined by using 6 mmol/L [K+]o Tyrode's solution with and without E-4031, a highly selective blocker of IKr. An E-4031-sensitive current was observed in the presence but not in the absence of extracellular K+. This rapidly activating component showed characteristics similar to those of IKr as described in rabbit and cat ventricular cells. Deactivation of IKr was significantly slower than that of IKs. IKr (E-4031-sensitive component) tail current density was similar in the three cell types, whereas IKs (E-4031-insensitive component) tail current density was significantly smaller in the M cells. Our results suggest that the distinctive phase-3 repolarization features of M cells are due in part to a lesser contribution of IKs and that this distinction may also explain why M cells are the main targets for agents that prolong APD in ventricular myocardium.(ABSTRACT TRUNCATED AT 400 WORDS)