1. Agonists known to increase cyclic AMP levels in gastrointestinal smooth muscles were studied in isolated circular muscles of the canine antrum to investigate the mechanisms of the inhibitory effects of these agents. 2. Muscles were electrically active, generating typical slow wave activity. Cytosolic Ca2+ ([Ca2+]cyt; measured by Indo-1 fluorescence) and tension increased in response to slow waves. 3. Stimulation by isoprenaline (via beta 2-receptors) or forskolin, in the presence or absence of acetylcholine, inhibited the plateau phase and reduced phasic [Ca2+]cyt and contractile responses. 4. Vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP), had similar effects to isoprenaline and forskolin. 5. Increases in the plateau phase of slow waves and the associated increases in [Ca2+]cyt and tension caused by direct activation of voltage-dependent Ca2+ channels by Bay K 8644 (0.1 microM) were also reduced by forskolin. 6. Isoprenaline and forskolin induced negative chronotropic effects, but VIP increased frequency. 7. At a given level of [Ca2+]cyt, contractions were greater under control conditions than in the presence of isoprenaline, VIP and CGRP, suggesting that part of the inhibition produced by these agents may be due to decreased Ca2+ sensitivity of the contractile apparatus. 8. Experiments performed on alpha-toxin-permeabilized muscles confirmed that cyclic AMP-dependent effects involve reduced Ca2+ sensitivity of the contractile apparatus. Addition of cyclic AMP (3-300 microM) caused a reduction in Ca(2+)-induced contraction at a constant level of Ca2+ (pCa 5.5). 9. These results suggest that increased cyclic AMP and probably subsequent activation of protein kinase A: (i) decrease [Ca2+]cyt and contraction by an inhibition of Ca2+ influx during slow waves, and (ii) decrease the sensitivity of the contractile apparatus to [Ca2+]cyt. The membrane effects might occur directly by inhibition of Ca2+ channels or indirectly by increasing the open probability of K+ channels which would tend to cause premature repolarization of slow waves.
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