Effects of galanin on the opossum internal anal sphincter: structure-activity relationship.

The present study was carried out to investigate the effects of porcine galanin-(1-29), N-terminal fragment galanin-(1-10), C-terminal fragment galanin-(15-29), and the middle fragment galanin-(7-16) on the spontaneous tension of the opossum internal anal sphincter and on the decrease in the resting internal anal sphincter tension in response to neural stimulation by electrical field stimulation. Galanin and galanin-(1-10) caused a concentration-dependent decrease in the resting tension of internal anal sphincter and an augmentation of the percent decrease in the resting tension with electrical field stimulation. Galanin-(15-29), on the other hand, produced an increase in the resting tension of the internal anal sphincter and had no effect on the electrical field stimulation-induced decrease in the resting tension. Galanin-(7-16) produced no significant effect on the internal anal sphincter. The decrease in the internal anal sphincter tension by galanin and galanin-(1-10) was partially antagonized by tetrodotoxin, whereas the increase in the internal anal sphincter tension caused by galanin-(15-29) was not modified by tetrodotoxin. In contrast to its effect in the internal anal sphincter, galanin caused an increase in the resting tension and suppressed a decrease in the lower esophageal sphincter tension in response to electrical field stimulation. From these findings we conclude that (a) galanin exerts an inhibitory effect on the internal anal sphincter by activating galanin receptors both at the intramural inhibitory neurons and at the internal anal sphincter smooth muscle and that the N-terminal portion of galanin may be responsible for these actions; (b) the contractile action of galanin is produced by its action on the smooth muscle; and (c) the actions of galanin on the gastrointestinal tract are tissue specific.