Effect of substance P on opossum duodenal smooth muscle.

Opossum duodenum was cut into strips measuring 2.0 X 15.0 mm. Strips cut along the oral-caudal axis were called longitudinal strips, while those cut 90 degrees to that axis were called circular strips. Each strip was placed in a heated, oxygenated organ bath and attached to a force-displacement transducer. Substance P produced tonic contraction in longitudinal strips and tonic and phasic contraction in circular strips. The ED50 for longitudinal and circular muscle was 1.9 X 10(-7) M and 2.8 X 10(-7) M, respectively. Longitudinal muscle was 1.3 times more sensitive to substance P than circular muscle. Phenoxybenzamine, atropine, curare, propranolol, haloperidol, and tetrodotoxin had no effect on the substance P-produced contractions in circular and longitudinal muscle. Trifluoperazine (10(-5) and 10(-4) M), D600 (10(-7) M), and nifedipine (10(-8) and 10(-7] inhibited both tonic and phasic contraction in circular and longitudinal strips. These studies suggest that substance P acts on both muscle layers at a site located at the muscle cell and that it produces tonic and phasic contraction through similar calcium-activating pathways.