Mechanisms responsible for the in vitro relaxation of ligustrazine on porcine left anterior descending coronary artery

In this study, we have evaluated the underlying mechanisms responsible for the relaxation response of ligustrazine (2,3,5,6-tetra-methylpyrazine; 2,3,5,6-MP) and its structural analogues (2-methyl-pyrazine (2-MP); ethyl-pyrazine (EP); 2,3-di-methyl-pyrazine (2,3-MP); 2,5-dimethyl-pyrazine (2,5-MP); 2,6-di-methyl-pyrazine (2,6-MP) and 2,3,5-tri-methyl-pyrazine (2,3,5-MP)) in porcine left anterior descending coronary artery (tertiary branch, O.D. less than or equal to1 mm). In 5-hydroxytryptamine (3 muM) precontracted preparations, cumulative administration (0.1 - 300 muM) of all pyrazine analogues caused an endothelium-independent, concentration-dependent relaxation. The relative inhibitory potency, as compared at concentration with which 50% relaxation occurred, was 2,3,5,6-MP>2,3,5-MP>EP>2,5-MP greater than or equal to 2,6-MP greater than or equal to 2,3-MP>2-MP. Besides, salbutamol and forskolin caused an endothelium-independent relaxation. The relaxation response of ligustrazine, salbutamol and forskolin was blunted in the presence of cis-N-(2-phenylcyclopentyl) azacyclotridec-1-en-2-amine (MDL 12330A) (10 muM, an adenylate cyclase inhibitor) and N-[2-((bromocinnamyl)amino)ethyl]-5-isoquinoline-sulphonamide (H-89, a protein kinase A inhibitor, 3 muM). Patch-clamp, whole-cell electrophysiological studies using single smooth muscle cells of the left anterior descending coronary artery revealed that ligustrazine (300 muM), salbutamol (30 muM) and forskolin (1 muM) inhibited the nifedipine-sensitive L-type Ca2+ channels, and the inhibitory effect was eradicated by MDL 12330A (10 muM) and H-89 (1 muM). However, neither the Ca2+-dependent K+ channel nor the ATP-dependent K+ channel was modified by ligustrazine (300 muM). In conclusion, our results indicate that ligustrazine-mediated left anterior descending coronary artery relaxation is due to the activation of adenylate cyclase/protein kinase A cascade and the subsequent inhibition of nifedipine-sensitive, voltage-dependent L-type Ca2+ channels. However, opening of K+ channels seems to play no role in mediating the relaxation effect of ligustrazine. (C) 2003 Elsevier Science B.V All rights reserved.