Tonic suppression of spontaneous baroreceptor reflex by endogenous angiotensins via AT₂ subtype receptors at nucleus reticularis ventrolateralis in the rat

We evaluated the role of endogenous angiotensins at the rostral nucleus reticularis ventrolateralis (NRVL) in the modulation of spontaneous baroreceptor reflex (BRR) response and the subtype of angiotensin receptors involved using rats anesthetized and maintained with pentobarbital sodium. Bilateral microinjection of angiotensin II (ANG II) or its active metabolite angiotensin III (ANG III) (5, 10, or 20 pmol) into the NRVL significantly suppressed the spontaneous BRR response, as represented by the magnitude of transfer function between systemic arterial pressure and heart rate signals. The inhibitory effect of ANG III (20 pmol) was discernibly reversed by coadministration with its peptide antagonist, [Ile⁷]ANG III (1.6 nmol), or the nonpeptide AT₂ receptor antagonist, PD-123319 (1.6 nmol), but not by the nonpeptide AT₁ receptor antagonist, losartan (1.6 nmol). On the other hand, the peptide antagonist, [Sar¹, Ile⁸]ANG II (1.6 nmol) or both non-peptide antagonists appreciably reversed the suppressive action of ANG II (20 pmol). Whereas losartan produced minimal effect, blocking the endogenous activity of the angiotensins by microinjection into the bilateral NRVL of PD-123319, [Sar¹, Ile⁸]ANG II or [Ile⁷]ANG III elicited significant enhancement of the spontaneous BRR response. We conclude that under physiologic conditions both endogenous ANG II and ANG III may exert a tonic inhibitory modulation on the spontaneous BRR response by acting selectively on the AT₂ subtype receptors at the NRVL.