Renal extraction of endogenous and radiolabelled catecholamines in the dog

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Abstract
Efferent renal nerve activity can be studied by measurements of the renal venous outflow of noradrenaline and dopamine. Accurate estimates of the intrarenal release to plasma of these catecholamines, however, require determinations of the net contribution of the catecholamines in arterial plasma to their renal venous outflow. We therefore studied the extractions of endogenous noradrenaline, dopamine and adrenaline, as well as 3H-labelled tracer amounts of noradrenaline, and dopamine in innervated and denervated canine kidneys. Approximately two-thirds of noradrenaline and dopamine in arterial plasma were extracted in the kidney, while 80-90% of adrenaline in arterial plasma was extracted. The fractional extractions of the three catecholamines were not substantially altered when the sympathetic nervous system was moderately activated by bilateral carotid occlusion or when the renal nerve activity was abolished by acute denervation. It is concluded that biochemical assessment of renal sympathetic nerve activity by studies of the renal venous noradrenaline and dopamine outflow requires some estimate of the net arterial contribution to the renal venous outflow. Ideally, catecholamine extraction by the kidney should be evaluated by studies of the renal extraction of 3H-labelled noradrenaline and dopamine, but the extraction of endogenous adrenaline may also be useful for this purpose.