CALCITONIN GENE-RELATED PEPTIDE AND CAPSAICIN INHIBIT THE CIRCULAR MUSCLE OF THE GUINEA-PIG ILEUM

Calcitonin gene-related peptide (CGRP) is known to excite, through the release of acetylcholine, the circular muscle (CM) of the guinea-pig ileum in vitro. In the present experiments, the effect of rat CGRP was investigated on the CM of tetrodotoxin-treated, spontaneously active ileum preparations. CGRP (1-10 nM) caused concentration-dependent inhibition of both the amplitude and frequency of spontaneous CM contractions. Capsaicin (a sensory stimulant known to release CGRP from primary afferents) also inhibited CM activity. The effect of 1-mu-M capsaicin underwent rapid desensitization, indicating specific action on afferent structures, whereas a high concentration of the drug (33-mu-M) inhibited CM activity most probably on the smooth muscle itself.