Role of L-type calcium channels and PKC in active tone development in rabbit coronary artery

The present study investigated active tone developinent in isolated ring segments of rabbit epicardial coronary artery. Endothelium-denuded (E-) or endothelium-intact (E+) vessels treated with the NO synthase inhibitor N-omega-nitro-L-arginine (100 LM) developed active tone , which was enhanced by stretch and reversed by the NO donor sodium nitroprusside (SNP: IC50 = 9 nM). Nifedipine abolished active tone and the contractile response to phorbol dibutyrate (PDBu; 10 nM) with the same potency (IC50 = 8 nM), whereas 300 nM PDBu responses were only partially blocked by nifedipine. The classical and novel PKC inhibitors GF-109203X (IC50 = 1-2 mu M) and chelerythrine (IC50 = 4-5 mu M) and the classical PKC inhibitor Go-6976 (IC50 = 0.3-0.4 mu M) blocked both active tone and 10 nM PDBu responses with similar potency. Active tone developinent was associated with depolarization of membrane potential (E) and a shift to the left of the E-m-vs.-contraction relationship determined by varying extracellular potassium. The depolarization and leftward shift were reversed by either chelerythrine (10 I-LM) or SNP (30 nM). PDBu (100-300 nM) increased peak L-type calcium channel (Ca-v) Currents in isolated coronary myocytes, and this effect was reversed by chelerythrine (1 mu M) or Go-6976 (200 nM). SNP (500 nM) reduced Ca-v currents only in the presence of the PKA blocker 8-bromo-2'-O- monbutyryl-cAMPS. Rp isomer (10 mu M). III Conclusion, active tone developinent in coronary artery IS Suppressed by basal NO release and is dependent on both enhanced Ca-v activity and classical PKC activity. Both E-m-dependent and -independent processes contribute to contraction. Our results suggest that one E-m-independent process is direct enhancement of Ca-v current by PKC.