Comparison of the bradycardic effects of alinidine (St 567), AQ-A 39 and verapamil on guinea-pig sinoatrial node superfused with different Ca2+ and NaCl solutions.

Two new substances, alinidine (St 567) and AQ-A 39, previously described as 'specific bradycardic agents' were investigated with respect to their effects on the rate of electrical discharges from sinus nodes of isolated right guinea-pig atria. Both substances decreased the discharge rate concentration dependently within a wide range (3-100 micrograms/ml). Low external Ca2+ (0.18 mM) increased and low external NaCl (62 mM) decreased the rate lowering effect of alinidine as well as of AQ-A 39. In contrast, the reference compound verapamil (an inhibitor of slow inward current) was less effective in low Ca2+ and more effective in low NaCl. The similar reactions of alinidine and AQ-A 39 are discussed with respect to their different chemical structure and different electrophysiological action patterns as described so far.