Effects of some gastrointestinal hormones on two muscle layers of duodenum

strips 2 mm wide and 2-2.5 cm long. Strips cut in the oral-caudal axis were called longitudinal strips; those cut at 90 ” to that axis were called circular strips. Cholecystokinin (CCK) and cerulein stimulated phasic contractions of circular muscle of opossum duodenum, but had no effect on the longitudinal muscle. The effect of CCK was not blocked by tetrodotoxin (lo- ’ M), indicating a direct muscle stimulation. CCK had no effect on both muscle layers of the cat duodenum. Vasoactive intestinal peptide raised tension in longitudinal muscle, but reduced tension in circular muscle of opossum duodenum. Glucagon slightly reduced tension in both longitudinal and circular muscle of opossum duodenum. It also inhibited con-tractions of circular muscle caused by acetylcholine. Pentagas-trin and secretin had no effect on either muscle layer in either species. These findings suggest that the circular an