ELECTRICAL STIMULATION OF THE SYMPATHETIC NERVES OF THE ISOLATED RABBIT EAR AND THE FATE OF THE NEUROHORMONE RELEASED

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Abstract
The rate of in vivo destruction of exogenous circulating (nor)adrenaline in the isolated perfused rabbit ear is so slow as to be negligible. Electrical stimulation of the superior cervical ganglion produces a marked constriction of the blood vessels in the rabbit ear which rapidly disappears when the stimulation ceases, presumably because the neurohormone released has either been locally inactivated or washed out by the circulation.Direct stimulation of the sympathetic plexus of the artery at the base of the isolated ear was found to be quite practical and frequency–response determinations were made. The response occurred in two parts. An initial 'spike' response had an optimum frequency of approximately 25 c.p.s. while that of the 'steady state' response was at 15 c.p.s.No evidence of a pressor substance was found in the perfusate after prolonged stimulation during recirculation of 4 ml of perfusate even when inhibitors of amine oxidase and catechol-O-methyl transferase were used separately. The most likely explanations are that either (a) the neurohormone is destroyed locally, or (b) it never accumulated in the perfusate above the threshold concentration of the preparation. In view of the negligible destruction of circulating adrenaline in the ear it is suggested that suggestion (b) may be more appropriate.