On the haemodynamic differences between sodium nitroprusside, nitroglycerin, and isosorbide nitrates. Comparison of their vasorelaxant effects in vitro and of their inactivation in vivo.

In organ bath studies, the sensitivity of isolated renal arteries and veins of the rabbit to sodium nitroprusside (SNP), nitroglycerin (GTN), and two isosorbide nitrates was compared. For isosorbide-dinitrate and isosorbide-5-mononitrate, a 7 to 20 times greater sensitivity of veins than of arteries was found explaining their known in vivo predilection for the capacitance vessels. To SNP and GTN, the immediate response was practically identical; a moderately greater sensitivity of the veins at threshold concentrations ( EC10 ), and a similar sensitivity of veins and arteries at the EC50 was observed with both drugs. However, on prolonged exposure (30 min), the relaxant response to GTN faded to a greater extent in arteries than in veins which may be a factor in the in vivo preference of GTN for the capacitance vessels. In anaesthetized rats, the hypotensive response to SNP and to GTN either infused i.v. or into the femoral artery, was compared. With GTN, the hypotensive response was the same by either route of administration. In contrast, the hypotensive response to SNP was considerably reduced on infusion into the femoral artery; an inactivation of SNP by 34% on passage through the hind-leg was calculated. The resulting decrease in venous over arterial blood levels of SNP could outweigh the somewhat greater sensitivity of veins than of arteries found in the organ bath, and may account for the balanced effect of SNP on resistance and capacitance vessels known to exist in vivo.