Opossum duodenum was cut into strips measuring 2.0×15.0 mm. Strips cut along the oral-caudal axis were called longitudinal strips, while those cut 90° 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 1.9×10-7 M and 2.8×10-7 M, respectively. Longitudinal muscle was 1.3 times more sensitive to substance P than circular muscle. Phenoxybenzamine, atropine, curare, propanolol, 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.
ASJC Scopus subject areas