Effects of yohimbine, rauwolscine and corynanthine on contractions and calcium fluxes induced by depolarization and prostaglandin F2α in rat aorta

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Abstract
1 The effects of the selective α2-adrenoceptor antagonists yohimbine and its stereo-isomer rauwolscine and the selective α1 -adrenoceptor antagonist corynanthine (a third yohimbine stereoisomer) on contractions induced in rat aorta by depolarization and prostaglandin F2α (PGF2α) have been compared. 2 In calcium-free solution, depolarization with 100 mM K+ failed to produce a contraction of rat aorta but PGF2α (3 μM) stimulated a contraction equal to about 23% of maximal elicited in normal physiological solution. 3 Yohimbine had no significant effect on depolarization-induced contractions except at concentrations greater than 30 μM. Rauwolscine and corynanthine (1 to 100 μM) depressed depolarization-induced contractions in a concentration-dependent manner, but the characteristics of inhibition were not identical. 4 Contractions induced by PGF2α (3 μM) were depressed in a concentration-dependent manner by rauwolscine (3 to 100 μM) but were unaffected by yohimbine or corynanthine. 5 Depolarization-stimulated 45Ca influx was depressed by rauwolscine and corynanthine to about the same extent as were the contractions; while rauwolscine (100 μM) completely inhibited PGF2α-stimulated 45Ca influx, it also depressed part of the PGF2α-stimulated contraction dependent on intracellular calcium. 6 Rauwolscine (100 μM) partly inhibited PGF2α-stimulated release of 45Ca from aortic smooth muscle in calcium-free solution. 7 It is concluded that the yohimbine structure possesses a calcium entry blocking action as well as a depressant action on contractions not dependent on calcium entry. The predominant effect depends on the structural configuration and the nature of the stimulating agent.