The effects of N omega-nitro-L-arginine methyl ester, sodium nitroprusside and noradrenaline on venous return in the anaesthetized cat.

1. The vascular actions of N omega-nitro-L-arginine methyl ester (L-NAME), sodium nitroprusside and noradrenaline were investigated in cats under chloralose anaesthesia with controlled vascular tone and ventilation. Cardiac output, heart rate, vascular pressures and mean circulatory filling pressure (MCFP) were measured. Total peripheral resistance (TPR) and resistance to venous return (Rvr) were calculated from steady-state readings. 2. L-NAME (37 mumol kg-1, i.v.) administered to ten cats receiving noradrenaline (6 nmol kg-1 min-1, i.v.) increased aortic pressure by 47.5 +/- 7.1 mmHg from 106 mmHg, and MCFP by 1.56 +/- 0.36 mmHg from 10.0 mmHg (means +/- s.e. means). Mean changes in portal venous pressure, RAP and heart rate were not significant. Cardiac output fell by 29.7 +/- 3.3% from 130 ml min-1 kg-1. TPR rose by 108 +/- 7.2% from 796 mmHg l-1 min kg and Rvr by 58.4 +/- 4.5% from 64 mmHg l-1 min kg. 3. Infusion of sodium nitroprusside into cats receiving noradrenaline evoked dose-related falls in aortic pressure, MCFP, TPR and Rvr. Changes in portal venous pressure, RAP and heart rate were not significant and cardiac output fell slightly. After L-NAME, sensitivity to nitroprusside was increased by 139 +/- 34% for MCFP, 176 +/- 19% for TPR and 351 +/- 39% for Rvr, and cardiac output rose slightly. The nitroprusside infusion required to restore TPR after L-NAME was estimated to be 5.8 x 10(+/- 0.41) nmol kg-1 min-1, which was approximately three times more than that required to restore MCFP. 4. Infusion of noradrenaline evoked dose-related increases in aortic and portal venous pressures, heart rate, cardiac output, MCFP, TPR and Rvr. After L-NAME and nitroprusside (4.4 nmol kg-1 min-1, i.v.),TPR and Rvr were not significantly different, but MCFP was reduced by 1.76 +/- 0.24 mmHg, and cardiac output by 22 +/- 1.9%. After subsequent expansion of the circulating blood volume (5-7.5 ml kg-1 dextran-saline), mean values for all parameters were restored to their previous levels. Sensitivity to noradrenaline was not significantly altered for heart rate, TPR and Rvr but was reduced by 31.8 +/- 12%for MCFP and by 66.5 +/- 18% for cardiac output.5. The depression of cardiac output by L-NAME is attributed to the increase in Rvr, partly compensated by the rise in MCFP. For a given rise in MCFP, the increase in R, was seven times greater after L-NAME than after noradrenaline, and the difference in the relative actions of the two drugs on resistance and capacitance vessels largely accounts for their contrasting effects on venous return. A procedure is suggested for replacement of vascular nitric oxide by nitroprusside infusion and blood volume expansion.