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

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Abstract
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 (PKCε) were compared in epicardial and endocardial regions of the rat ventricle. Activation of PKCε with a diacylglycerol analogue (di‐octanoyl‐glycerol (DiC8), 20 μm) 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 %). PKCε 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 PKCε distribution in the cytosolic fraction showed a marked difference, with significantly (P < 0.05) higher levels in endocardial extracts. The cytosolic endocardial/epicardial PKCε ratio was 2.64 ± 0.24 (n = 4), indicating a reduced amount of PKCε 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 PKCε 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 PKCε activation on a cardiac K+ current, and in the subcellular distribution of PKCε. These differences are absent in diabetic and hypothyroid conditions.