EVALUATION OF PLASMA NOREPINEPHRINE AS AN INDEX OF SYMPATHETIC NEURON FUNCTION IN THE CONSCIOUS, UNRESTRAINED RAT

Measurement of plasma norepinephrine and epinephrine concentrations in the conscious, unrestrained rat yielded values of 138 .+-. 10 and 55 .+-. 8 pg/ml, respectively. Ganglionic blockade reduced basal norepinephrine levels without affecting plasma epinephrine levels. Adrenal demedullation reduced plasma epinephrine to undetectable levels (< 20 pg/ml) and gave rise to an apparent compensatory increase in plasma norepinephrine levels. Adrenal demedullation in combination with ganglionic blockade reduced plasma norepinephrine to the same level as did ganglionic blockade alone. The plasma epinephrine was of adrenal origin. Under these experimental conditions, the major portion of the plasma norepinephrine was apparently of neuronal origin. When specific destruction of the sympathetic nerve terminals without alteration of adrenal medullary function was accomplished with 6-hydroxydopamine, a 5-fold increase in plasma epinephrine concentration was observed at 24 h. Plasma norepinephrine levels of 24 h were not significantly altered from the control levels by the 6-hydroxydopamine suggesting that the rodent adrenal medulla was capable of secreting substantial amounts of norepinephrine under these conditions. Plasma norepinephrine concentrations reflect both sympathetic neuronal and adrenomedullary activity. In the absence of changes in plasma epinephrine, plasma norepinephrine appears to be an index of sympathetic neuron function.