Deleterious effect of exogenous angiotensin-I on the extent of regional ischaemia and its inhibition by captopril

The endogenous activity of the local renin-angiotensin system(RAS) and the anti-ischaemic properties of captopril were investigatedin electrically driven rabbit Langendorff hearts (constant pressure:70 cmH₂O, Tyrode solution, Ca²⁺ 1.8 mmol.l^–1). Cumulativeconcentration-response curves showed no significant difference(P>0.05) between the reduction of the global coronary flow(CF) by exogenous angiotensin-I or angiotensin-II (EC₅₀ = 10^–10mol.l^–1). It is concluded that the local RAS in isolatedrabbit hearts is highly sensitive, whereas its endogenous activityis very low due to very low endogenous angiotensin-I content.Myocardial ischaemia (MI) was induced by the occlusion of aleft coronary artery branch and MI was quantified from NADHsurface fluorescence photography. MI was significantly enlarged(+35%) (P <0.05) by exogenous angiotensin-I (6x10^–9mol.l^–1). The reduction in CF and the increment in MIby angiotensin-I could be completely prevented by adding captoprilat a low concentration (10^–6 mol.l^–1) to the perfusionbuffer. In the absence of exogenous angiotensin-I, captoprilalone (10^–6 mol.l^–1) neither significantly enhancedCF (P >0.05), nor diminished MI (P >0.05), supportingthe finding of very low endogenous activity of tile local RASin this model. We, moreover, conclude that at a low concentration(10^–6 mol.l^–1) captopril does not possess directcardioprotective properties independent of its ACE inhibitingaction.