Selectivity of Ca2+ channel blockers in inhibiting muscular and nerve activities in isolated colon

Abstract

1. Potency and efficacy of nifedipine, verapamil and diltiazem and of Bay K 8644 in modifying propulsion and nerve or smooth muscle activities have been compared in the guinea-pig isolated distal colon. Both the neuronal and muscular effects of Ca2+ channel blockers seem to develop at concentrations that are devoid of any significant effect apart from that on Ca2+ channels. 2. Nifedipine, verapamil and diltiazem were all able to impair propulsion, resting and stimulated acetylcholine (ACh) release and smooth muscle contractility in a concentration-dependent way. However, some degree of selectivity for neuronal and muscular effects could be observed. Nifedipine was more than 500 fold more potent than verapamil in relaxing musculature but less than twice as potent in reducing ACh release. On the other hand, verapamil was the most efficacious Ca2+ channel blocker tested in inhibiting ACh release, its effects being inversely correlated to the external Ca2+ concentration, and completely abolished by Bay K 8644. 3. By comparing the potencies exhibited by each drug against peristaltic reflex, smooth muscle contractility and ACh release, verapamil proved to be almost as potent in slowing the peristaltic reflex as in reducing ACh release, while nifedipine was about 100 fold more potent against the peristaltic reflex than against ACh release, but nearly equal against the peristaltic reflex and smooth muscle tone. Therefore, interference with cholinergic neurotransmission is likely to play a major role in the antipropulsive effect of verapamil, while peristaltic reflex impairment by nifedipine is likely to be dependent on inhibition of smooth muscle. 4. A facilitatory effect of Bay K 8644 on both the efficiency of the peristaltic reflex and the nonadrenergic, non-cholinergic (NANC) nerve-mediated relaxation could be observed at concentrations at least 10 fold lower than those required to affect ACh release or smooth muscle. 5. It is concluded that the effects of Ca2+ channel blockers on neurotransmitter release may be relevant to their effects on the gastrointestinal motor function.