Nitroprusside increases cyclic guanylate monophosphate concentrations during relaxation of rabbit aortic strips and both effects are antagonized by cyanide.

The authors have confirmed previous observations that sodium cyanide (CN-) partially reverses the vasodilator effects of sodium nitroprusside (SNP) on vascular smooth muscle. As tested on rabbit aortic strips contracted by norepinephrine (NE), the final tension is independent of the order of addition of reagents. In the same concentration, CN- alone had no effect on tension also as reported by others. The ED50 values for relaxation of aortic strips for a series of directly acting agonists ("nitric oxide vasodilators") were: sodium azide (N-3) 2.1 X 10(-7) M; SNP 2.7 X 10(-7) M; hydroxylamine (H2NOH) hydrochloride 2.5 X 10(-6) M; human nitric oxide hemoglobin (HbNO) 3.5 X 10(-6) M; and sodium nitrite (NO-2) 1.2 X 10(-4) M. In addition to SNP, CN- antagonized the vasodilator effects of N-3 and H2NOH, but it failed to reverse relaxation by HbNO, NO gas, NO-2 (as observed by us), glyceryl trinitrate, adenosine, or papaverine (as observed by others). The only change noted in cyclic-adenosine monophosphate (c-AMP) concentrations in aortic strips exposed to 1) NE, 2) NE + NO-2 or SNP, or 3) NE + NO-2 or SNP + CN- was an increase due to NE. The only statistically significant change noted in cyclic-guanosine monophosphate (c-GMP) concentrations exposed to 1) NE, 2) NE + NO-2 or 3) NE + NO-2 + CN- was also an increase due to NE. In contrast, SNP resulted in further increases in c-GMP after NE, and when cyanide was added, a significant decrease in c-GMP followed. These results are only partially consistent with a role for c-GMP in relaxation of vascular smooth muscle, but cyanide may become a useful tool for the study of mechanisms of action of the nitric oxide vasodilators.