Effects of chronic treatment with losartan and enalaprilat on [H-3]-norepinephrine release from isolated atria of Wistar-Kyoto and spontaneously hypertensive rats

The present study was designed to evaluate the effect of chronic treatment with losartan, an AT(1) angiotensin II receptor antagonist, and enalaprilat, an angiotensin converting enzyme inhibitor, on the presynaptic modulation of [H-3]-norepinephrine release from isolated atria of spontaneously hypertensive rats (SHR) and their respective control, the Wistar-Kyoto rats (WKY). The rats received either losartan (5 mg/kg/day) or enalaprilat (1 mg/kg/day) for 12 days by means of osmotic minipumps. The atria were isolated and incubated with [H-3]-norepinephrine and the release of radioactivity was used as an index of norepinephrine release. The experimental protocol consisted of two electrical stimulations and the drugs were administered 20 min before the second stimulation. The modulatory action of angiotensin II (0.01 and 1 mu mol/L), the alpha(2)-adrenoceptor agonist, oxymetazoline (1 mu mol/L), the alpha(2)-adrenoceptor antagonist, idazoxan (1 mu mol/L) and the beta(2)-adrenoceptor agonist fenoterol (1 mu mol/L) were tested. The results show that losartan or enalaprilat both similarly reduced the blood pressure in SHR. However, only the chronic losartan treatment, and not enalaprilat, abolished the facilitatory effect of exogenously administered angiotensin II on the release of radioactivity. The prejunctional alpha(2)- and beta(2)-adrenoceptor modulatory mechanisms were not altered by either chronic treatments, Similarly, the facilitatory effect of angiotensin II was blocked by acute administration of losartan but not by enalaprilat. Finally, the facilitatory action of bradykinin on the release of radioactivity was unchanged by chronic enalaprilat treatment. These results confirm the presence of facilitatory AT(1) angiotensin II receptors on sympathetic nerve terminals of rat atria. These results also confirm that sympathetic nerve terminal blockade by losartan or the blockade of endogenous angiotensin II formation by enalaprilat are likely to participate in the antihypertensive action of AT(1) angiotensin II receptor antagonists and angiotensin converting enzyme.