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Dopamine, when administered in low doses, primarily acts on the renal blood vessels by stimulating dopamine receptors, leading to renal smooth muscle relaxation. This results in vasodilation of the renal vasculature, enhancing renal perfusion and promoting diuresis. The preferential dilation of renal arterioles helps optimize glomerular filtration rate (GFR), making it beneficial for patients with acute renal failure or compromised kidney function.

The other choices do not accurately reflect the primary action of low-dose dopamine. Unlike renal smooth muscle relaxation, renal vasoconstriction is associated with higher doses of dopamine, which can lead to a reduction in renal blood flow and GFR. While dopamine does enhance contractility at moderate doses through beta-1 adrenergic stimulation, this is not the primary effect observed at lower doses. Increased blood pressure is also more likely to occur at higher doses where the drug stimulates alpha receptors leading to vasoconstriction. Understanding these dose-dependent effects of dopamine is crucial for its clinical application in managing various cardiac and renal conditions.