Developmental changes in agonist-mediated colonic smooth muscle contraction in the rabbit.

ABSTRACT: We studied smooth muscle strips from rabbit distal colon to determine age-related changes in length-tension properties and agonist-mediated contraction. Strips from newborn (1-d-old) and weanling (11-wk-old) rabbits were oriented to measure isometric tension in longitudinal muscle. Active tension comprised 47 ± 4 and 75 ± 5% of the total tension in the newborn and weanling, respectively. Total and active tensions in the weanling were greater than in the newborn (p < 0.001). Although the potencies for bethanechol were similar, the maximal response was nearly 9-fold greater in weanlings (6900 ± 292 mN/cm2) versus newborns (753 ± 112 mN/cm2) p < 0.001. Maximal stress increased with age for bethanechol, high extracellular potassium, substance P, neurokinin A, cholecystokinin octa-peptide, bombesin, and serotonin. ED50 for bethanechol, substance P, neurokinin A, and bombesin did not change with age. Serotonin was 12 times more potent in newborns versus weanlings (p < 0.05). In contrast, cholecystokinin octapeptide was five times less potent in newborns (18.6 nM versus 3.4 nM, respectively, p < 0.05). Substance P-induced contractions were inhibited partially by atropine. We conclude that length-tension properties of longitudinal colonic smooth muscle differ, and responses to agonists increase with age.