Responsiveness to noradrenaline of longitudinal and circular smooth muscles of cat caecum during the postnatal period.

1. The changes in the responsiveness of the isolated longitudinal and circular smooth muscles from cat caecum to adrenergic agents during the postnatal period were studied by the effect of noradrenaline (0.01-100 mumol) on the smooth muscle tone and phasic contractions. 2. Noradrenaline (10 mumol) contracted the preparations of longitudinal and circular muscles isolated from cats between 15 and 60 days postnatal via stimulatory alpha 1-adrenoceptors. 3. Noradrenaline applied cumulatively (0.01-100 mumol) produced a concentration-determined but age-independent inhibitory effect on the phasic contractions of longitudinal muscle and exercised a concentration- and age-determined effect on the phasic activity of circular muscle. 4. The adrenoceptors of cat caecum are not differentiated until 60 days postnatal.