Microvascular effects of atrial natriuretic peptide (ANP) in man: studies during high and low salt diet

OBJECTIVE: Infusion of ANP in anephric dogs causes a decrease in cardiac output and a rise in peripheral vascular resistance. This reduced cardiac output is possibly related to increased resistance to venous return generated in the microcirculation by venular constriction. The aim of the present study was to evaluate in healthy volunteers the effects of low-dose ANP infusion on both conjunctival and skin microcirculation during high or low salt diet. METHODS: ANP (7.5 ng/kg/min) and placebo were infused (i.v.) for 4 h, in random order on two separate days, in two groups of 10 healthy male volunteers each. One group was studied during high salt (ad libitum), and one group during low salt (55 mmol Na+/24 h) diet. Microvascular density and diameters of both conjunctiva and nailfold were studied using intravital videomicroscopy. Nailfold capillary red blood cell velocity (CBV) was studied using intravital videomicroscopy, and skin (thermoregulatory) blood flow (SBF) was studied using laser-Doppler fluximetry. RESULTS: In the high salt group ANP induced a 43% reduction in basal SBF as compared to an 18% reduction by placebo (P < 0.01). Parallel to SBF, ANP significantly reduced CBV (P < 0.02). Conjunctival capillary density decreased by 5% during ANP, while it increased by 28% during placebo (P < 0.05). No such effects of ANP were observed in the low salt group. Blood pressure and heart rate were not influenced by ANP infusion in neither group. CONCLUSION: Infusion of low doses of ANP into humans on an ad libitum salt diet results in vasoconstriction of the microcirculation, probably on the venular side. The lack of effect of ANP on the microcirculation during low salt diet may be related to a higher vascular tone prior to infusion.