Sympathetic nerve function--assessment by radioisotope dilution analysis.

Radioisotope dilution measurements of norepinephrine spillover (rate of entry of the transmitter into plasma) provide more accurate assessments of sympathoneural transmitter release than allowed by measurements of plasma catecholamine concentrations alone. Measurements of total body norepinephrine spillover, as an index of global sympathetic outflow, allow effects on plasma clearance to be distinguished from effects on release of catecholamines into plasma, while spillovers from specific tissues enable examination of regionalized sympathetic responses. However, spillovers of norepinephrine represent only a fraction of the transmitter that escapes neuronal and extraneuronal uptake after release by nerves. Numerous factors may influence this fraction and measures spillovers independently of transmitter release by nerves. Modified radioisotope dilution methods for assessment of rate processes operating within and between intracellular and extracellular compartments have further improved our understanding of the relationships of norepinephrine release, uptake, spillover, turnover, and metabolism. This article reviews the breadth of information about sympathetic nerve function attainable using catecholamine radioisotope dilution analyses against a backdrop of the relative advantages and methodological limitations associated with the methodology.