Effect of dopamine on opossum duodenal smooth muscle

Opossum duodenum was cut into 2 mm X 1.5 cm strips. The strips cut along the oral-caudal axis were called longitudinal strips, while those cut 90 degrees to that axis were called circular strips. Dopamine produced contractions in both muscle layers. ED 50 for the longitudinal and circular muscle was 2.8 X 10(-6) M and 1.9 X 10(-5) M, respectively. The longitudinal muscle was 6.5 times more sensitive to dopamine than the circular muscle. The dopamine-produced contractions were completely blocked by phenoxybenzamine, 10(-4) M, and phentolamine, 10(-5) M. Haloperidol, 10(-5) M, or bulbocapnine, 10(-5), completely or partially blocked the contractions produced by dopamine in only one-third of the strips. Tetrodotoxin, propranolol, atropine, and curare did not affect the dopamine-produced contractions. These studies suggest that dopamine mainly stimulates alpha-excitatory adrenergic receptors in opossum duodenal muscle and that the longitudinal muscle is more sensitive to dopamine. However, we cannot exclude the existence of excitatory dopamine receptors in the opossum duodenal smooth muscle.