Hemodynamic responses to aortic depressor nerve stimulation in conscious l-NAME-induced hypertensive rats

Abstract

The present study investigated whether baroreflex control of autonomic function is impaired when there is a deficiency in NO production and the role of adrenergic and cholinergic mechanisms in mediating reflex responses. Electrical stimulation of the aortic depressor nerve in conscious normotensive and nitro-l-arginine methyl ester (l-NAME)-induced hypertensive rats was applied before and after administration of methylatropine, atenolol, and prazosin alone or in combination. The hypotensive response to progressive electrical stimulation (5 to 90 Hz) was greater in hypertensive (−27 ± 2 to −64 ± 3 mmHg) than in normotensive rats (−17 ± 1 to −46 ± 2 mmHg), whereas the bradycardic response was similar in both groups (−34 ± 5 to −92 ± 9 and −21 ± 2 to −79 ± 7 beats/min, respectively). Methylatropine and atenolol showed no effect in the hypotensive response in either group. Methylatropine blunted the bradycardic response in both groups, whereas atenolol attenuated only in hypertensive rats. Prazosin blunted the hypotensive response in both normotensive (43%) and hypertensive rats (53%) but did not affect the bradycardic response in either group. Prazosin plus angiotensin II, used to restore basal arterial pressure, provided hemodynamic responses similar to those of prazosin alone. The triple pharmacological blockade abolished the bradycardic response in both groups but displayed similar residual hypotensive response in hypertensive (−13 ± 2 to −27 ± 2 mmHg) and normotensive rats (−10 ± 1 to −25 ± 3 mmHg). In conclusion, electrical stimulation produced a well-preserved baroreflex-mediated decrease in arterial pressure and heart rate in conscious l-NAME-induced hypertensive rats. Moreover, withdrawal of the sympathetic drive played a role in the reflex bradycardia only in hypertensive rats. The residual fall in pressure after the triple pharmacological blockade suggests the involvement of a vasodilatory mechanism unrelated to NO or deactivation of α1-adrenergic receptor.