INVOLVEMENT OF DIHYDROPYRIDINE-SENSITIVE CA-2+ CHANNELS IN ADENOSINE-EVOKED INHIBITION OF ACETYLCHOLINE-RELEASE FROM GUINEA-PIG ILEAL PREPARATION

The effects of adenosine and nifedipine on endogenous acetylcholine (ACh) release evoked by electrical stimulation from guinea pig ileal longitudinal muscle preparations exposed to physostigmine were evaluated using an HPLC with electrochemical detection (ECD) system. Resting ACh release, which was sensitive to tetrodotoxin (0.3 μM), was enhanced by Bay K. 8644 (0.5 μM; a Ca2+ antagonist) or 4‐aminopyridine (30 μM; a K+ channel blocker) but not by theophylline (100 μM; a P1 purinoceptor antagonist) or atropine (0.3 μM). The enhancement of the resting ACh release by Bay K 8644 was virtually unaffected by atropine. Electrically evoked ACh release was enhanced by around two‐ to fourfold in the presence of theophylline, atropine, Bay K 8644, 4‐aminopyridine, or atropine. On the other hand, the evoked ACh release was reduced by adenosine (10–30 μM), nifedipine (0.1–0.3 μM; a dihydropyridine Ca2+ channel antagonist), or bethanechol (1–3 μM) in a concentration‐related fashion. The reduction induced by adenosine or nifedipine was almost abolished by either theophylline or Bay K. 8644, whereas that induced by bethanechol was virtually unaffected by these drugs. The inhibition by adenosine of ACh release was not influenced in the presence of 4‐aminopyridine or atropine. However, this inhibition by adenosine was considerably enhanced by halving the Ca2+ concentration in the Krebs solution and was diminished by doubling the Ca2+ concentration. These findings suggest that adenosine produces a cholinergic neuromodulation presumably via modifying dihydropyridine‐sensitive Ca2+ channel activities in the cholinergic neurons, and thus L‐type Ca2+ channels may exist on the nerve terminals. It was also demonstrated that the HPLC‐ECD method used for detecting endogenous ACh was suitable for evaluating mechanisms of the neuromodulation. Copyright © 1990, Wiley Blackwell. All rights reserved