Dopamine: An Important Neurohormone of the Sympathoadrenal System. Significance of Increased Peripheral Dopamine Release for the Human Stress Response and Hypertension*
I. Introduction THE RECOGNITION of dopamine (DA) as an important peripheral neurotransmitter is largely product of the increased sophistication and interest in receptor research of the last decade. Starting with Goldberg's (1) work on vascular receptors in the late 1960s, DA receptors have been identified and characterized in many peripheral organs. Among these are the inhibitory presynaptic DA receptors on noradrenergic nerve terminals and postsynaptic DA receptors in kidney, carotid bodies, peripheral arteries, including the coronary and cerebral vasculature, and parts of the gastrointestinal, genitourinary and endocrine systems (1–4). The physiologic functions of these DA receptors include vasodilatation, increased sodium excretion, and increased myocardial contractility (1–4). The sources of endogenous DA that may activate these receptors have been less clearly defined. This is especially true for DA in plasma (3), which in humans at rest accounts for approximately ⅕ of the free catecholamines and more than ½ of the total catecholamines (Fig. 2) (3, 5, 6).