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

1. The effects of PKC activation on a transient (I-t) and a sustained (I-ss ) cardiac K+ current and the subcellular distribution of the epsilon isofor m of PKC (PKC epsilon) were compared in epicardial and endocardial regions of the rat ventricle. 2. Activation of PKC epsilon with a diacylglycerol analogue (di-octanoyl-gl ycerol (DiC8), 20 muM) leads to differential effects in epicardial and endo cardial cells. In epicardial cells (n = 20) I-t and I-ss are attenuated by 17.7 +/- 2.1% and 11.9 +/- 3.1 %, respectively (means +/- S.E.M..). In endo cardial cells I-t attenuation was significantly smaller (4.6 +/- 1.6%, n = 14, P < 0.0005). I-ss, attenuation was similar to that: in epicardial cells (10.5 <plus/minus> 3.8%). 3. PKC epsilon expression was measured by Western blotting. Calculated endo cardial/epicardial ratios showed no regional differences in total protein e xtracts: (1.04 +/- 0.11, mean +/- S.E.M, n = 4), but PKC epsilon distributi on in the cytosolic fraction showed a marked difference, with significantly (P< 0,05) higher levels in endocardial extracts acts. The cytosolic endoca rdial/epicardial PKC<epsilon> ratio was 2.64 +/- 0.24 (n = 4), indicating a reduced amount of PKC epsilon in the membrane fraction action of the endoc ardium. This could account for the reduced effect of DiC8 on I-s in endocar dial myocytes. 4. Under both hypothyroid and streptozotocin-induced dial,diabetic conditio ns the difference in endocardial and epicardial cytosolic PKC epsilon level s was absent (ratios of 0.86 +/- 0.21 (n = 4) and 1.09 +/- 0.16 (fl = 3), r espectively; means +/- S.E.M.). Ratios in the total protein extracts were n ot significantly different from those in control conditions. 5. The results show transmural differences in the functional effects of PKC epsilon activation on a cardiac K+ current, and in the subcellular distrib ution of PKC epsilon. These differences are absent in diabetic and hypothyr oid conditions.