The present study investigated active tone development in isolated ring segments of rabbit epicardial coronary artery. Endothelium denuded vessels (E-) or E+ vessels treated with the NOS inhibitor L-NA (100µM) developed active tone which was enhanced by stretch and reversed by the NO-donor sodium nitroprusside (SNP; IC₅₀=9nM). Nifedipine abolished active tone and the contractile response to phorbol dibutyrate (PDBu 10nM) with the same potency (IC₅₀=8nM) whereas 300nM PDBu responses were only partially blocked by nifedipine. The classical and novel PKC inhibitors GF109203x (IC₅₀=1-2µM) and chelerythrine (IC₅₀=4-5 µM) and the classical PKC inhibitor Go6976 (IC₅₀=0.3-0.4 µM) blocked both active tone and 10nM PDBu responses with similar potency. Active tone development was associated with depolarization of membrane potential (Em) and a shift to the left of the Em vs contraction relationship determined by varying extracellular potassium. The depolarization and leftward shift were reversed by either chelerythrine (10 µM) or SNP (30nM). PDBu (100-300 nM) increased peak L-type calcium channel (Cav) currents in isolated coronary myocytes and this effect was reversed by chelerythrine (1 µM) or Go6976 (200 nM). SNP (500 nM) reduced Cav currents only in the presence of the PKA blocker Rp-8-Br-MB-cAMPS (10 µM). In conclusion, active tone development in coronary artery is suppressed by basal NO release and is dependent upon both enhanced Cav activity and classical PKC activity. Both Em-dependent and -independent processes contribute to contraction. Our results suggest that one Em-independent process is direct enhancement of Cav current by PKC.