Dopamine

Dopamine is an endogenous catecholamine neurotransmitter. Exogenously administered dopamine cannot cross the blood-brain barrier, so it acts only in the periphery and not in the central nervous system. At low doses, dopamine primarily stimulates D₁ receptors on vascular smooth muscle, particularly in the renal, mesenteric, and coronary circulation, causing vascular smooth muscle relaxation and increasing renal blood flow, urine output, and sodium excretion. At moderate doses, it mainly stimulates β₁ receptors in the myocardium, increasing cardiac contractility and stroke volume, thereby improving cardiac output. At high doses, it significantly stimulates α₁ receptors, causing peripheral vasoconstriction and increasing systemic vascular resistance and blood pressure.