Coronary vasodilation induced by captopril and zofenoprilat: evidence for a prostaglandin-independent mechanism.

In this study, the vasodilating properties of captopril and zofenoprilat, two angiotensin-converting enzyme (ACE) inhibitors containing the sulfhydryl group, are investigated in the isolated rat heart. It is demonstrated that both compounds increase coronary flow in a dose-dependent manner. However, the mean pD2 of zofenoprilat appears to be significantly higher than the mean pD2 of captopril (4.55 +/- 0.06 and 3.35 +/- 0.02 respectively), indicating that zofenoprilat is about ten times more potent in increasing coronary flow than captopril. Possibly this difference in potency between captopril and zofenoprilat can be explained by their physicochemical properties. Since zofenoprilat is more lipophilic than captopril, its concentration in cardiac and vascular tissues at distribution equilibrium is thought to be higher than the tissue concentration of captopril, which may result in a more pronounced vasodilatory action. The precise mechanism of coronary vasodilation induced by ACE inhibitors containing the sulfhydryl group is not yet understood. Several factors have been proposed, such as stimulation of prostacyclin production. However, in this study, concomitant administration of 10(-6) mol/l acetylsalicylic acid shows no antagonism, indicating that under normoxic conditions the vasodilatory effects of captopril and zofenoprilat are independent of the production of vasodilating prostaglandins. Therefore, other factors than stimulation of prostacyclin synthesis seem to be involved, such as prevention of bradykinin breakdown and/or potentiation of endothelium derived relaxing factor (EDRF). Furthermore, despite a marked inhibition of prostacyclin production, 10(-6) mol/l acetylsalicylic acid itself has no effect on coronary flow. These results suggest that prostacyclin does not play an important role in the regulation of coronary flow, at least in the normoxic isolated rat heart.