Effects of Dopamine on the Contractility of the Circular Muscles in the Guinea-pig Gastric Antrum and Fundus'

=Abstract=The effects of dopamine on gastric motility were investigated using circular muscle strips prepared from the antral and' fundic regions of the guinea-pig stomachs. The spontaneous and phasic contractions induced by electrical stimulation were recorded with a force transducer. Dopamine potentiated the spontaneous contraction of the antral circular muscle, which was suppressed completely by a-blocker phentolamine and partially by an-blocker yohimbine but was not blocked by TTX. Dopamine increased the basal tone of both the antral and the fundic circular muscles, which was suppressed by phentolamine but not affected by TTX. Dopamine increased the contraction evoked by direct electrical stimulation to the fundic circular muscle, which was suppressed by phentolamine but unaffected by TTX. Dopamine increased the basal tone and relaxation component of the phasic contraction evoked by transmural nerve stimulation in the fundic region, while it decreased the contraction component. Atropine decreased the contraction component in the fundic region, while it increased the relaxation component. In the presence of atropine dopamine increased the basal tone and relaxation component, which were suppressed by phentolamine. In conclusion, dopamine increases the basal tone of both the antral and fundic circular muscles and the amplitude of spontaneous contraction of the antral circular muscle via a-adren-ergic receptors, especially via the al-receptor in the plasma membrane of circular muscle cells. Dopamine increases the relaxation component through the enhanced release of transmitters from non-adrenergic non-cholinergic nerve terminals, and suppresses contraction by inhibiting transmitter release from cholinergic nerves in the fundic region.