Perinatal changes in bombesin-stimulated muscle contraction in rabbit stomach and colon

Abstract Bombesin and its mammalian homologue, gastrin-releasing peptide, stimulate smooth muscle contraction and may promote the growth of gastrointestinal tissues. Isometric contraction of strips from circular muscle of the gastric fundus and longitudinal muscle of the distal colon were used to compare changes in the response to bombesin in newborn and weanling rabbits. There was an age-related qualitative change in gastric muscle from biphasic contractions including phasic and tonic components in the newborn to phasic contractions alone in the weanling. The colon contractions were tonic at both ages. In both tissues there was an age-related fivefold increase in stress in response to maximally effective concentrations of bethanechol ( P 2 (98% of the maximal response to bethanechol) in the newborn to 565 ± 81 mN/cm 2 (4% of the maximal response to bethanechol) in the weanling ( P 2 (82% of the response to bethanechol) in the newborn to 862 ± 11 mN/cm 2 (29% of the response to bethanechol) in the weanling ( P