Protein kinase C regulation of K+ currents in rat ventricular myocytes andits modification by hormonal status

1. The effects of protein kinase C (PKC) activation on cardiac K+ currents were studied in rat ventricular myocytes, using whole-cell voltage clamp me thods. Control rats were compared to hypothyroid or diabetic rats, in which PKC expression and activity were enhanced. 2. In control myocytes, two calcium-independent outward K+ currents, the tr ansient I-t and the sustained I-ss, were attenuated by 18.9 +/- 2.0 and 16. 8 +/- 3.5%, respectively (mean +/- S.E.M.), following addition of a synthet ic analogue of diacylglycerol, DiC8 (20 mu m). In myocytes from hypothyroid or diabetic rats, I-t and I-ss were not affected by DiC8. 3. The effects of DiC8 were restored in myocytes from thyroidectomized rats by injection of physiological doses of tri-iodothyronine (T-3; 10 mu g kg( -1) for 6-8 days). Incubating cells from diabetic rats with 100 nar insulin for 5-9 h also restored the ability of DiC8 to attenuate I-t and I-ss. 4. The attenuation of K+ currents by DiC8 in control cells was absent in th e presence of a peptide known to inhibit the translocation of the isoform P KC (EAVSKPLT, 24 mu m introduced through the recording pipette). A scramble d peptide (LSETKPAV) was without effect. 5. Under hypothyroid conditions the inhibitory peptide restored the effects of DiC8 on I-t and I-ss. These currents were attenuated by 11.9 +/- 1.5 an d 9.8 +/- 1.5%, respectively, which was significantly (P < 0.001) more than without the peptide or with the scrambled peptide. 6. These results show that the PKC-mediated suppression of cardiac K+ curre nts is normally mediated by PE;Cs translocation. A PKC epsilon translocatio n inhibitor restores the ability of DiC8 to attenuate K+ currents under hyp othyroid conditions. This presumably reflects a (partial) reversal of a chr onic translocation and a shift in the balance between PKC and its anchoring proteins.