[Patterns in the inhibition of adrenergic response by catecholamines]

Alterations of adrenergic reaction under the effect of norepinephrine (NE), epinephrine (E), phenylephrine (Ph) and isopropylnorepinephrine (ISO) were studied in isolated segments of the rat small intestine. The dissociation constants of adrenoreceptors with NE, E, Ph and ISO were found to be 5.1 +/- 0.70 X 10(-7) M, 1.20 +/- 0.30 X 10(-7) M, 7.60 +/- 1.40 X 10(-7) M, 1.40 +/- 0.02 X 10(-7) M, resp. Ph, NE, E, were shown to begin to function in 10-20 sec as competitive inhibitors of the adrenergic reaction with Ki 2.20 +/- 0.80 X 10(-7) M, 6.80 +/- 1.20 X 10(-7) M, 0.30 +/- 0.09 X 10(-7) M, resp., and ISO as an inhibitor of a mixed type with Ki of competitive inhibition 1.80 +/- 0.05 X 10(-7) M and Ki of the noncompetitive one 0.37 +/- 0.04 X 10(-7) M. The autoinhibition seems to take part in adrenergic process control chiefly through a considerable catecholamine release from nerve endings and adrenals in extreme conditions.