FALLIBILITY OF PLASMA NORADRENALINE MEASUREMENTS IN STUDYING POSTPRANDIAL SYMPATHETIC NERVOUS RESPONSES

The use of the plasma noradrenaline (NA) concentration as an index of sympathetic nervous system (SNS) activity in the postprandial state is associated with several problems: (i) it does not take into account t he contribution of alterations in clearance to the plasma NA level, (i i) when antecubital venous blood is sampled, it reflects regional fore arm rather than whole body SNS activity and (iii) no insight is gained into the regional pattern of SNS activation. These potential confound ers were addressed in this study performed in 17 healthy young men. Th e validity of plasma NA measurements in assessing postprandial changes in sympathetic nervous activation was evaluated in relation to that o f whole body and regional plasma NA spillover, derived using isotope d ilution methodology. Plasma clearance of NA is significantly altered f ollowing a meal, with a transient elevation in the early postprandial phase which may lead to an underestimation of SNS activation when asse ssed from arterial plasma NA levels. Forearm plasma NA spillover incre ases postprandially, such that despite significant postprandial elevat ions in arterial plasma NA, the plasma arterial contribution to antecu bital venous plasma NA levels is maintained at less than 40%, the rest being derived locally from the forearm. This makes venous plasma samp les unsuitable for the assessment of SNS activation in organs and vasc ular sites distant from the sampling site. The kidneys and skeletal mu scle are the major regional sites of postprandial sympathetic nervous activation,while cardiac plasma NE spillover is unaltered postprandial ly.;This regional pattern of SNS activation postprandially must be tak en into account when relating increments in plasma NA levels to specif ic physiological events.