Effects of adenosine and calcium entry blockers on 3,4-diaminopyridine-induced rhythmic contractions in dog coronary artery.

3,4-Diaminopyridine, a potassium (K+) channel blocker, was used to induce phasic contractions in an isolated K+-contracted dog left anterior descending coronary artery ring preparation. The effects of adenosine, N6-L-phenyl-isopropyl-adenosine (L-PIA) and 5'-N-ethylcarboxamide-adenosine (NECA) were compared with those of calcium (Ca2+) entry blockers (nifedipine, verapamil, diltiazem) on the maximum developed force, minimum developed force and contraction frequency in this model. Adenosine, L-PIA and NECA significantly relaxed the minimum force and decreased the contraction frequency without any effect on the maximum force. The order of potency was: NECA greater than L-PIA greater than adenosine. Nifedipine, verapamil and diltiazem significantly relaxed the maximum force and increased the contraction frequency without a significant relaxing effect on minimum force. It is, therefore, likely that adenosine (and its analogs) and Ca2+ entry blockers have different mechanisms for the relaxation of coronary smooth muscle and that adenosine probably relaxes the vessels through A2 receptor.