Estimation of intrasynaptic norepinephrine concentrations at vascular neuroeffector junctions in vivo.

We estimated vascular neuroeffector junctional norepinephrine concentrations and their relation to pressor responses by measuring plasma norepinephrine levels and blood pressure during sympathetic stimulation or norepinephrine infusion in pithed, vagotomized, alpha 2-adrenoceptor blocked, adrenal-demedullated rats with and without uptake1 blockade by desipramine. For an increment in mean arterial pressure of 50 mm Hg, the estimated mean junctional norepinephrine concentration ( ES50m ) was about 7 nmol/l. Norepinephrine concentration gradients between the site of norepinephrine release and plasma appeared to be equal and reciprocal for sympathetic stimulation and for norepinephrine infusion. These gradients were reduced equally (by about two-thirds) after desipramine treatment, indicating that removal of both released and infused norepinephrine is mainly by neuronal uptake.