In vivo and in vitro studies to elucidate the hemodynamic differences of sodium nitroprusside, nitroglycerin, and two isosorbide nitrates.

In isolated renal veins of the rabbit, isosorbide dinitrate and isosorbide-5-mononitrate were seven to twenty times more potent as relaxants than in renal arteries which explains their predilection for the capacitance vascular bed in vivo. For sodium nitroprusside (SNP) and nitroglycerin (NTG), the sensitivity was slightly greater in veins at threshold concentrations (EC10), but similar in veins and arteries at higher concentrations (EC50). After 30 min of exposure, the relaxant effect to NTG faded partially in arteries, but not in veins, which may underlie its preference for the capacitance vessels in vivo. In anaesthetized rats, SNP and NTG were infused i.v. or into the femoral artery. The hypotensive response to NTG was the same by either route of infusion, whereas that to SNP was considerably lower on infusion into the femoral artery; a 34% inactivation of SNP on passage through the hind leg was calculated. The result decrease in venous over arterial blood levels of SNP at a somewhat greater sensitivity of veins than of arteries may account for the balanced effect of SNP on resistance and capacitance vessels in vivo.