Abstract

The medial smooth muscle of the cat's nictitating membrane contains about twice the amount of endogenous norepinephrine found in the inferior muscle. In relation to its weight, the medial muscle takes up and retains about twice the amount of norepinephrine-H³ recovered from the inferior muscle; however, in relation to the content of endogenous norepinephrine, uptake is equal for both muscles. These results are consistent with the view that the medial muscle has a denser adrenergic innervation than the inferior muscle. It is proposed that this difference is causally related to the fact that the innervated medial muscle is significantly less sensitive to l-norepinephrine and l-epinephrine than the innervated inferior muscle. The differences in sensitivity disappear after denervation. Apparently the density of adrenergic innervation is one of the factors which influence the concentration of certain amines at the receptors of effector organs.