NEW APPROACHES TO EVALUATE SYMPATHOADRENAL SYSTEM ACTIVITY IN EXPERIMENTS ON EARTH AND IN-SPACE

In previous studies the activity of the sympathoadrenal system (SAS) i n cosmonauts during space flights was evaluated by measuring plasma ca techolamines (CA) levels and urinary CA and their metabolites concentr ations. Plasma CA levels are accepted indicators of SAS activity, howe ver, they are determined by the plasma clearances as well as the rates of CA release (spillover-SO) into the bloodstream. Nowadays methods a re available which evaluate not only plasma levels of CA but also thei r release, spillover, uptake, reuptake, degradation and also CA synthe sis in vivo measured by plasma levels of dihydroxyphenylalanine (DOPA) . Plasma concentrations of DOPA, the CA noradrenaline (NE), adrenaline (ADR), and dopamine (DA), the deaminated catechol metabolites dihydro xyphenylglycol (DHPG) and dihydroxyphenylacetic acid (DOPAC), and the O-methylated metabolites methoxyhydroxyphenylglycol (MHGP) and homovan illic acid (HVA) were measured during immobilization stress (IMO) in c onscious rats. Radiotracer methods were used to measure NE SO. IMO mar kedly increased arterial NE levels but NE SO was less elevated because the NE clearance was slightly reduced in IMO rats. Simultaneous measu rements of plasma CA and their metabolites provide another means to ob tain information about SAS function. For instance, dissociation betwee n changes of plasma DHPG and NE levels can indicate changes in neurona l reuptake of NE. We found marked parallel increases in plasma NE and DHPG levels during acute IMO; however after repeated IMO, plasma NE le vels were increased but DHPG responses were less pronounced suggesting a reduced NE reuptake. DOPA, the CA precursor, circulates in plasma a t a concentration higher than NE. During stress, increased sympathoneu ral outflow stimulates DOPA synthesis and release into the circulation supporting the view that changes in plasma DOPA levels during stress reflect in vivo changes in the rate of CA synthesis. We propose to mea sure the new plasma indicators of SAS activity in cosmonauts and/or in animals before, during and after space flights.