Enalapril is an ACE inhibitor. Actually it is a prodrug that is converted into active moiety in vivo.
It has an ester moiety which on hydrolysis produces free acid moiety which is active.
The main action of enalapril is to inhibit the action of ACE inhibiting the conversion of AT I to AT II.
In this way ACE inhibitors block three important actions of AT II.
As it inhibits the actions of AT II, they produce vasodilatation leading to fall in blood pressure.
Further they decrease AT II induced aldosterone secretion resulting in decreased sodium and water retention.
They also protect vascular smooth muscle against hypertrophy produced by AT II.
All these effects of ACE inhibitors decrease both after load and preload decreasing cardiac work.
This may be one of the reasons why ACE inhibitors improve the outcomes in patients with heart failure.