ATTENUATION OF CONTRACTION OF ISOLATED CANINE CORONARY-ARTERIES BY ENFLURANE AND HALOTHANE

Contraction of vascular smooth muscle such as that existing in coronary arteries is regulated in part by Ca++ entry into cells via Ca++ channels. Volatile anaesthetics are known to attenuate agonist-induced coronary artery constriction. The purpose of this experiment was to determine if 1.5 MAC concentrations of halothane or enflurane attenuated contractions evoked by activation of one type of Ca++ channel - the potential operator channel. In the current experiment, potential operator channels were activated by depolarizing isolated canine coronary artery rings with high concentration of K+, causing Ca++ entry and vessel contraction. Rings without endothelium were suspended for isometric force measurement in organ chambers containing aerated Krebs-Ringer solution. Maximum response to Ca++ in rings depolarized with K+ was 120 +/- 5 per cent in untreated versus 101 +/- 3 percent in rings treated with enflurane (P < 0.01). The maximum response was 123 +/- 6 per cent in untreated versus 111 +/- 5 per cent during halothane administration (P < 0.05). In contrast, nifedipine 10(-9) M depressed maximum contractions from 114 +/- 5 percent to 37 +/- 4 per cent (P < 0.01) and nifedipine 10(-8) M depressed contractions to 30 +/- 4 per cent (P < 0.01). In a further series of experiments, sustained contractions were depressed by continued administration of the anaesthetics, indicating no loss of anaesthetic effect with time. The results indicate that 1.5 MAC halothane and enflurane attenuate contractions of canine coronary arteries evoked by depolarization and Ca++ entry through potential operated channels. However, neither halothane nor enflurane exhibited the marked depressant effect exerted by nifedipine.