Changes in the responsiveness of the cat ileocecal sphincter to adrenergic agents during the postnatal period

The changes in the responsiveness of the cat ileocecal sphincter (ICS) to adrenergic agents during the postnatal period were studied by the effect of noradrenaline (applied cumulatively: 0.01-100 mumol) on the mechanical activity:smooth muscle tone and phasic contractions. Noradrenaline contracted the ICS smooth muscle through alpha 1-adrenergic receptors. These receptors were differentiated in the first postnatal days but continue to develop functionally even after 60 days postnatal. An age-determined decrease in the EC50 values of noradrenaline, inducing tonic contractions, was established. Noradrenaline inhibited the phasic contractions of ICS. This effect increased after beta-adrenergic receptor blockers, and disappeared after blocking of alpha 1-adrenergic receptors, which suggests the existence of a postjunctional inhibitory adrenergic receptor of alpha 1 type. The beta-adrenergic receptors in the ICS smooth muscle are not differentiated until 15 days postnatal. They developed functionally towards 30 days postnatal.