Effect of endogenous prostaglandins on acetylcholine release from dog trachealis muscle

We used a radioenzymatic technique to measure effects of the prostaglandin synthesis inhibitor indomethacin and of exogenous prostaglandin E2 (PGE2) and prostaglandin I2 (PGI2) on acetylcholine (ACh) efflux from canine tracheal smooth muscle (TSM) during sustained electrical field stimulation (EFS; 2 Hz, 2 ms pulse duration, 50 V for 15 min). ACh efflux from indomethacin (INDO, 10(-6) M)-pretreated and control TSM increased with consecutive stimulations. However, efflux of ACh was greater in INDO-treated than control muscles. INDO increased the tension produced by TSM in response to EFS. Neither PGE2 (10(-8) M) nor PGI2 (10(-6) M) had any effect on ACh efflux from INDO-pretreated TSM during the first of three periods of EFS. However, PGI2 and PGE2 prevented the progressive increase in ACh efflux observed on subsequent stimulations. PGE2 but not PGI2 decreased contractions of TSM caused by EFS. Our results demonstrate that endogenous prostaglandins, probably PGE2, do inhibit EFS-evoked ACh release from canine TSM in vitro, but suggest that these prostaglandins modulate EFS-evoked contractions predominantly by postsynaptic mechanisms.