The contribution of aldehyde dehydrogenase type 2 (ALDH2) to bioactivation of glyceryl trinitrate (GTN) and isosorbide dinitrate (ISDN) was systematically examined in excised rabbit aorta and anesthetized whole animal with cyanamide, an ALDH2 inhibitor. In excised aortic preparation, the degree of inhibition by cyanamide in GTN-induced vasorelaxation (concentration ratio, calculated as EC 50 in the presence of cyanamide/EC 50 in the absence of cyanamide; 5.61) was twice that in ISDN-induced relaxation (2.78). However, the degree of inhibition by cyanamide, as assessed by the dose ratio (as described above, but calculated with doses) in anesthetized rabbits was 2.29 in GTN-induced hypotension (assessed by area under the curve (AUC) of 50 mmHg·min) and 7.68 in ISDN-induced hypotension. Thus, the inhibitor was 3 times more potent in ISDN-induced hypotension, a finding in conflict with to that obtained in excised aortic preparation. The rate of increase in plasma nitrite (NO 2 − ) concentration at certain hypotensive effect (50 mmHg·min of AUC) in the presence and absence of cyanamide (ΔNO 2 − ratio) was larger in ISDN-induced hypotension (15.01) than in GTN-induced hypotension (3.28). These results indicate that the bioactivation pathway(s) of GTN is ALDH2-dependent in aortic smooth muscle, while ADLH2-independent mechanism(s) largely take place in the whole body. In contrast, the activation mechanism(s) of ISDN is largely ALDH2-dependent in both aortic smooth muscle and whole body. Plasma NO 2 − may be derived from pathways other than the cyanamide-sensitive metabolic route.Key words glyceryl trinitrate; isosorbide dinitrate; cyanamide; rabbit; nitrite Glyceryl trinitrate (GTN) has been widely used in the treatment of ischemic heart disease for more than a century 1) and the main mechanism of its vascular action is the activation of the intracellular nitric oxide (NO) receptor enzyme, soluble guanylate cyclase, leading to increased bioavailability of guanosine 5′-cyclic monophosphate (cGMP) and activation of cGMP-dependent protein kinases and/or cyclic nucleotidegated ion channels.2) However, the mechanism of bioactivation of the agent, upstream of intracellular signal transduction, was not elucidated until recently.Another new concept argues that the high potency nitrate, GTN, is mainly bioactivated by mitochondrial aldehyde dehydrogenase type 2 (ALDH2) when used at low, clinically relevant concentrations (<1 µm, high affinity pathway), and this notion is supported by evidence from various laboratories. [3][4][5] On the other hand, the low potency nitrate, isosorbide dinitrate (ISDN), is suggested to be mainly metabolized by P450 in the endoplasmic reticulum, directly yielding nitric oxide. 5-7)Although the above scenarios are based on many molecular, biochemical, mechanical (excised vascular preparation) and whole animal studies, there are no systematic studies that include both excised vascular and whole animal preparations to elucidate the difference between GTN and ISDN in one species.The present study was de...