ABSTRACT
The possibility of the involvement of the alpha-adrenoceptors in the mechanism of calcium channel blockers inducing vasodilatation was investigated. The effects of verapamil, nifedipine and phentolamine have been compared on the contractions of the rabbit aortic strips produced by different concentrations of noradrenaline [NA] [10-7 - 10-4 M]. Results showed that preincubation of the rabbit aortic strip with a low concentration of verapamil [10-6 M] caused a parallel rightward shift of NA dose response curve, with significant depression of the minimal response induced by NA. A highly marked rightward shift of the NA dose response curve, with significant depression of the minimal response induced by NA. A highly marked rightward shift of the NA dose response curve was noticed with a higher concentration of verapamil [10-5 M]. This was accompanied by a highly significant depression of the minimal [P <0.001] and maximal [P <0.005] responses of NA. Similarly, phentolamine [10-7 M] produced a parallel rightward shift of the NA dose response curve with a highly significant inhibition of the responses induced by lower concentrations of NA [P <0.001]. On the other hand, nifedipine [10-6, 10-5 M] shifted the NA dose response curve to the right in a nonparallel manner and depressed only the maximal responses induced by NA. These observations suggested an alpha-antagonistic property of verapamil on the rabbit aortic strip which was not demonstrated with the other calcium channel blocker, nifedipine