Caerulein receptors on the cholinergic neurons in guinea-pig ileum

The release of [3H]ACh and the contractions of guinea-pig ileal longitudinal muscle preparations, with myenteric plexus attached were measured and recorded simultaneously. Caerulein in concentrations of 10(-11) M to 10(-8) M caused dose-dependent increase of the contractions and the [3H]ACh release. This excitatory effect of caerulein was sensitive to TTX (10(-6) M). Proglumide selectively antagonized both the contractions and the [3H]ACh-releasing effect of caerulein. Electrical field stimulation (O.1 Hz)-evoked contractions were sensitive to atropine. Caerulein (10(-9) M) did not influence the electrically-evoked release of [3H]ACh and the electrically-evoked contractions. Nifedipine (10(-6)M) decreased them about 50%. In the presence of nifedipine caerulein produced an inhibitory effect on the electrically-induced contractions. This nifedipine-unmasking inhibitory effect of caerulein was accompanied by a decrease of the [3H]ACh release and was prevented by proglumide.