Local Transmitter Concentrations in Vascular Smooth Muscle during Vasoconstrictor Nerve Activity

Adrenergic excitatory responses of a nerve‐muscle preparation consisting of the rat portal vein and its sympathetic post‐ganglionic nerve supply were studied quantitatively in vitro. The maximal contractile response to nerve stimulation was obtained at 16 imp/sec and it averaged 56 per cent of the maximal response to exogenous noradrenaline, NA, (10– 4 M). Factors such as the myogenic propagation between the individual muscle cells and the sparse distribution of the sympathetic nerve fibres do not allow an estimation of the transmitter concentration during nerve activity from a mere comparison of the magnitude of the responses to nerve stimulation and to exogenous NA, respectively. It was shown, however, that the degree of reduction of the contractions induced by exogenous NA, after partial a‐receptor blockade by phenoxybenzamine, was inversely related to the concentration of NA in the bath. By comparing the reductions of the neural responses with the reductions of the responses to graded concentrations of exogenous NA the effective transmitter concentration could be estimated. The values obtained were (8 ± 4) × 10– 6 M at 4 imp/sec and (2 ± 0.5) × 1‐ ‐5 M at 16 imp/sec. The effectiveness of a‐adrenergic blocking agents in affecting neural and humoral responses to catecholamines in vivo is discussed in the light of the present findings.