Transmural differences in rat ventricular protein kinase C epsilon correlate with its functional regulation of a transient cardiac K⁺ current

The effects of PKC activation on a transient (It) and a sustained (Iss) cardiac K+ current and the subcellular distribution of the epsilon isoform of PKC (PKC(epsilon)) were compared in epicardial and endocardial regions of the rat ventricle. Activation of PKC(epsilon) with a diacylglycerol analogue (di-octanoyl-glycerol (DiC8), 20 (mu)M) leads to differential effects in epicardial and endocardial cells. In epicardial cells (n = 20) It and Iss are attenuated by 17.7 +/- 2.1 % and 11.9 +/- 3.1 %, respectively (means +/- S.E.M.). In endocardial cells It attenuation was significantly smaller (4.6 +/- 1.6 %, n = 14, P < 0.0005). Iss attenuation was similar to that in epicardial cells (10.5 +/- 3.8 %). PKC[epsilon] expression was measured by Western blotting. Calculated endocardial/epicardial ratios showed no regional differences in total protein extracts (1.04 +/- 0.11, mean +/- S.E.M, n = 4), but PKC[epsilon] distribution in the cytosolic fraction showed a marked difference, with significantly (P < 0.05) higher levels in endocardial extracts. The cytosolic endocardial/epicardial PKC[epsilon] ratio was 2.64 +/- 0.24 (n = 4), indicating a reduced amount of PKC[epsilon] in the membrane fraction of the endocardium. This could account for the reduced effect of DiC8 on It in endocardial myocytes. Under both hypothyroid and streptozotocin-induced diabetic conditions the difference in endocardial and epicardial cytosolic PKC[epsilon] levels was absent (ratios of 0.86 +/- 0.21 (n = 4) and 1.09 +/- 0.16 (n = 3), respectively; means +/- S.E.M.). Ratios in the total protein extracts were not significantly different from those in control conditions. The results show transmural differences in the functional effects of PKC(epsilon) activation on a cardiac K+ current, and in the subcellular distribution of PKC(epsilon). These differences are absent in diabetic and hypothyroid conditions.