The diverse experimental approaches into the actions of dopamine in the
periphery have demonstrated the amine to possess various interesting features.
By activation of distinct receptors it is capable of producing systemic and
renal vasodilatation and of modulating sympatho-adrenal outflow. By a combination of haemodynamic and direct tubular effects it is actively natriuretic, and
influences hormonal release from the kidney and adrenal cortex.
A physiological function in the kidney is strongly suggested by the prod¬
uction and excretion of large amounts of free dopamine, by the close relation¬
ship between dopamine excretion and sodium output in the urine, and by the
characterisation of a widespread distribution of dopamine receptors. Such
observations do not however, prove that endogenous dopamine has physiological
significance and more compelling evidence accrues when an inhibitor of
dopamine can be shown to alter some parameter of normal function. Such
studies have been approached using either dopamine receptor antagonists or
dopa decarboxylase inhibitors to block dopamine synthesis, and a number of
animal models have been utilised, notably in man, dog and rat.
It can be concluded for the present studies and from other reported
work that dopamine exerts independent action in the kidney, without the
requisite mediation of other hormonal, neural or physical factors. More likely
is a system of parallel influences, including dopamine, interacting in an array
of negative and positive feedback loops, forming a highly complex control
mechanism which maintains homeostasis and is able to respond rapidly to
changes in the local and wider environment.