Microvascular effects of atrial natriuretic peptide in rat cremaster

Experiments utilized the open cremaster preparation to test the hypothesis that atrial natriuretic peptide (ANP)-induced volume changes result from microvascular resistance alterations. Atrial natriuretic peptide (25, 100, and 500 ng/kg/min, IV) or vehicle was infused into anesthetized rats. At the two highest ANP infusion rates, mean arterial pressure was significantly reduced from 104 +/- 3 (control) to 87 +/- 2 and 77 +/- 2 mmHg, respectively. Hematocrit was 41.0 +/- 0.8 and 45.6 +/- 0.9% (p < 0.05) at the end of vehicle and ANP infusions, respectively. Despite these effects of ANP, there were no significant arteriolar or venular diameter alterations. Thirty microM nitroprusside significantly dilated all vessel segments except large venules. These observations suggest that resistance alterations in the skeletal muscle microvasculature are not the cause of ANP-induced fluid movement.