EFFECTS OF NATRIURETIC PEPTIDES AND NITROPRUSSIDE ON VENOUS FUNCTION IN TROUT

Active venous regulation of cardiovascular function is well known in m ammals but has not been demonstrated in fish. In the present studies, the natriuretic peptides (NP) rat atrial natriuretic peptide (ANP) and trout ventricular natriuretic peptide (VNP), clearance receptor inhib itor SC-46542, and sodium nitroprusside (SNP) were infused into unanes thetized trout fitted with pressure cannulas in the ventral aorta, dor sal aorta, and ductus Cuvier, and a ventral aorta (VA) flow probe was used to measure cardiac output (GO). In another group, in vivo vascula r (venous) capacitance curves were obtained during ANP or SNP infusion . The in vitro effects of NP on vessels and the heart were also examin ed. ANP, VNP, and SC-46542 decreased central venous pressure (P-Ven), CO, stroke volume (SV), and gill resistance (R-G), whereas systemic re sistance (R-S) and heart rate (HR) increased. Dorsal aortic pressure ( P-DA) transiently increased and then fell even though R-S remained ele vated. ANP decreased mean circulatory filling pressure (MCFP), increas ed vascular compliance at all blood volumes, and increased unstressed volume in hypovolemic fish. ANP had no direct effect on the heart. ANP responses in vivo were not altered in trout made hypotensive by prior treatment with the angiotensin-converting enzyme inhibitor lisinopril . SNP reduced ventral aortic pressure (P-VA), P-DA, and R-S, increased CO and HR, but did not affect P-Ven, SV, or R-G. SNP slightly decreas ed MCFP but did not affect compliance or unstressed volume. In vitro, large systemic arteries were more responsive than veins to NP, whereas SNP relaxed both. These results show that, in vivo, NP decrease venou s compliance, thereby decreasing venous return, CO, and arterial press ure. Conversely, SNP hypotension is due to decreased R-S. This is the first evidence for active regulation of venous capacitance in fish, wh ich probably occurs in small veins or venules. The presence of venous baroreceptors is also suggested.