Developmental changes of the contractile activity and the drug sensitivity of smooth muscle of the alimentary tract of guinea pig.

The mechanical activity of the smooth muscle of guinea pig alimentary tract changed during fetal and postnatal development. At the F20 (fetus, body weight 20 g) stage of the fetus, the antrum, small intestinal and colonic preparations showed no spontaneous activity. After this stage, spontaneous activity occurred but the patterns of contractile activity of various regions of the alimentary tract differed from those of the adult. However, at later fetal stages (F50 or later), the mechanical patterns of various regions, except the lower region of colon, were similar to adult patterns. An excitatory response to acetylcholine was observed in all regions at the F20 stage but catecholamines were not effective at this stage. Responses to catecholamines appeared later than those to acetylcholine but they were not constant. During development of the colon, the excitatory effect of adrenaline was significant and the effect was blocked by phenoxybenzamine, and isoprenaline inhibited the spontaneous activity. However, sympathetic nerve stimulation at F70 inhibited the mechanical activity and propranolol blocked the inhibitory action of sympathetic nerves. These results indicate the co-existence of alpha-excitatory and beta-inhibitory receptors on the developing colonic smooth muscle cell membrane.