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

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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 < 0.05). In contrast, in the stomach age-related decreases in the response to maximally effective concentrations of bombesin were observed, from 2930 ± 179 mN/cm2 (98% of the maximal response to bethanechol) in the newborn to 565 ± 81 mN/cm2 (4% of the maximal response to bethanechol) in the weanling (P < 0.005). In the colon, a twofold increase in response to bombesin was observed, from 446 ± 59 mN/cm2 (82% of the response to bethanechol) in the newborn to 862 ± 11 mN/cm2 (29% of the response to bethanechol) in the weanling (P < 0.05). No age-related changes were observed in the potency of bombesin in either tissue. Neither atropine nor tetrodotoxin altered the contractions in either tissue, suggesting that bombesin interacted directly with myocytes. There was three times as much bombesinlike immunoreactivity in the stomach compared with the colon, but no age-related changes in either tissue. In summary, by the age of weaning the stomach lost the tonic component of contraction and 80% of the efficacy of bombesin-stimulated phasic contraction that had been present in the newborn. The loss of efficacy, absolute in the stomach and relative to bethanechol in the colon, suggest that bombesin may be most important in stimulating motility in the neonatal period.