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

Abstract

The present study investigated active tone development in isolated ring segments of rabbit epicardial coronary artery. Endothelium-denuded (E−) or endothelium-intact (E+) vessels treated with the NO synthase inhibitor Nω-nitro-l-arginine (100 μM) 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 μM) and chelerythrine (IC50= 4–5 μM) and the classical PKC inhibitor Gö-6976 (IC50= 0.3–0.4 μM) blocked both active tone and 10 nM 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 (30 nM). 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 Gö-6976 (200 nM). SNP (500 nM) reduced Cavcurrents only in the presence of the PKA blocker 8-bromo-2′- O-monobutyryl-cAMPS, Rp isomer (10 μM). In conclusion, active tone development in coronary artery is suppressed by basal NO release and is dependent on both enhanced Cavactivity 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 Cavcurrent by PKC.