Sodium cyanide antagonism of the vasodilator action of sodium nitroprusside in the isolated rabbit aortic strip

Resistance to sodium nitroprusside (SNP) is uncommon, but its occurrence has led to massive overdoses of SNP and sometimes death. To examine the mechanism responsible for resistance, aortic smooth muscle strips were prepared and dose-response curves for norepinephrine (NE) obtained. SNP alone caused a shift of the dose-response curve for NE to the right. However, this shift was less when the strips were exposed to both SNP and sodium cyanide (CN^-). When CN^- alone was added to the aortic strips, the response to NE was unchanged. In a further group of aortic muscle strips first contracted with NE and then relaxed with SNP, the addition of CN^- caused the muscles to contract again. It is concluded that CN^- antagonizes the action of SNP in vitro, and that this antagonism is specific for SNP.