The potential of intact human platelets for sulfoconjugation of norepinephrine in vitro

The aim of the present study was to investigate the potential of human platelets for sulfoconjugation of plasma norepinephrine. For this purpose the uptake and the efflux of norepinephrine and norepinephrine sulfate were studied in vitro in intact human platelets. Incubation of platelets with 0.5 mumol/l norepinephrine led to a rapid uptake of norepinephrine within 1 min (22.76 +/- 4.14 pmol/2.5 x 10(9) platelets/min), whereby 63.7% was transformed to norepinephrine sulfate. The substrate kinetics showed a different slope of norepinephrine uptake with increasing norepinephrine concentrations in the incubation medium (0.1-0.5 mumol/l). At 0.1 mumol/l norepinephrine the uptake was more efficient than at higher norepinephrine concentrations. Incubation with norepinephrine sulfate slightly decreased intraplatelet norepinephrine sulfate (p < or = 0.01). Incubation of human platelets in a catecholamine free medium led to an efflux of norepinephrine and norepinephrine sulfate (21.8%, 11.2% of total intraplatelet content, respectively). The present findings indicate a net uptake of norepinephrine into the platelets. At physiological norepinephrine concentrations the uptake seems to be achieved predominantly by an active transport mechanism. Moreover, data indicate a net efflux of norepinephrine sulfate. Therefore it is concluded that human platelets might contribute to the formation of plasma norepinephrine sulfate by norepinephrine uptake, intraplatelet sulfoconjugation and release of norepinephrine sulfate.