Effective Transmitter Concentrations in the Rat Portal Vein as Reflected by Neurogenic Potentiation of Responses to Exogenous Noradrenaline

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Abstract
In the rat portal vein exogenous noradrenaline (NA) and the transmitter released during nerve activity seem to react with the same set of α‐receptors, located on muscle cells close to the nerve terminals. Very shortlasting, high transmitter concentration peaks have been calculated to occur at the receptors for each nerve impulse. In the present study an indirect approach was utilized to evaluate to what extent these peak concentrations, in view of their short duration, are effective in activating the smooth muscle. Electrical field stimulation of the adrenergic nerve supply of the rat portal vein was applied during fully developed responses to graded concentrations of exogenous NA. Transmural stimulation at 2–32 imp/s was found to increase the established responses to exogenous NA in concentrations up to 10‐5M. These effects were abolished by tetrodotoxin. The findings imply that the high, shortlasting NA concentrations, which occur at the α‐receptors for each nerve impulse, lead to high transient degrees of receptor occupancy during nerve activity. The effective transmitter concentrations, over a wide range of impulse frequencies, thus seem to approach the levels of NA which produce maximum responses when applied exogenously.