Changes in plasma catecholamines in response to reflex modulation of sympathetic vasoconstrictor tone by cardiopulmonary receptors.

The effects of selective deactivation and stimulation of cardiopulmonary receptors on plasma noradrenaline (radioenzymatic method) were studied in nine normotensive subjects by reducing and increasing central venous pressure for 20 min via lower body suction and leg-raising manoeuvres that did not alter arterial blood pressure and heart rate. Deactivation of cardiopulmonary receptors was accompanied by a rise in plasma noradrenaline that achieved a peak within 5 min (91.8 +/- 22%, mean +/- SE) and was then sustained. Stimulation of cardiopulmonary receptors was accompanied by a fall in plasma noradrenaline (-16.6 +/- 3.4%) that levelled off at the second minute and was then sustained. On average the increase and the reduction in plasma noradrenaline had a time course and a magnitude similar to the increase (80.5 +/- 10.5%) and the reduction (-28.4 +/- 5%) in forearm vascular resistance (derived from plethysmographic flow measurement) concomitantly caused by cardiopulmonary receptors. Furthermore, analysis of individual data showed that changes in plasma noradrenaline and forearm vascular resistance were linked by a positive relationship (r = 0.64). Thus the cardiopulmonary receptor reflex can produce rapid, marked and sustained changes in both plasma noradrenaline and forearm vasomotor tone. This is in sharp contrast with the previously observed inability of the carotid baroreflex to alter both these humoral and haemodynamic variables. Taken together these findings support the hypothesis that sympathetic tone to skeletal muscle is an important determinant of the concentration of plasma noradrenaline in blood.