THE EFFECT OF ATRIAL-NATRIURETIC-PEPTIDE ON TROUT STOMACH MOTILITY

The effect of atrial natriuretic peptide (ANP) was examined on longitu dinal and circular smooth muscle of esophagus, stomach, anterior and p osterior intestine, and longitudinal muscle of the pyloric ceca from t he rainbow trout (Oncorhynchus mykiss). ANP had the greatest effect in longitudinal stomach (LS) where it reduced spontaneous rhythmical con tractions and baseline tension in otherwise unstimulated strips. ANP r elaxed LS precontracted with 10(-6) M acetylcholine (ACh) in a dose-de pendent manner (pD(2) = 8.39 +/- 0.07; N = 8). Pretreatment of LS with 10(-6) M tetrodotoxin (TTX) did not affect the ability of ANP to rela x ACh contractions (pD(2) before TTX = 8.48 +/- 0.09 versus 8.47 +/- 0 .06 after TTX; N = 7). ANP produced modest inhibitory effects in other longitudinal gastrointestinal (GI) smooth muscle, decreased the rate of, but did not inhibit, rhythmical activity in anterior intestine, an d slightly increased tension of rhythmical contractions in esophageal and anterior intestine. ANP was virtually ineffective on all circular smooth muscle. It is concluded that, in addition to the well known dir ect effects of this peptide on GI absorption, ANP may affect GI functi on through regulation of motility.