Impaired arterial baroreflex regulation of heart rate after blockade of P2-purinoceptors in the nucleus tractus solitarius.

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of ATP mimics baroreflex responses (bradycardia, hypotension); however, the physiological role of these receptors in cardiovascular control remains unclear. We tested whether blockade of these receptors attenuates arterial baroreflex control of heart rate (HR). Baroreflex-induced changes in HR (via graded i.v. infusion of phenylephrine and nitroprusside) were observed in seven alpha-chloralose/urethane anesthetized male Sprague-Dawley rats before and after microinjection of the purinergic P2 receptor antagonist suramin (0.5 nmol in 50 nL) into the subpostremal NTS. Before suramin, typical baroreflex changes in HR were observed (maximum gain, Gmax = 2.94 +/- 0.54 bpm/mmHg). Suramin markedly impaired baroreflex-induced changes in HR (gain = 0.02 +/- 0.08 and 0.18 +/- 0.09 bpm/mmHg for increases and decreases in mean arterial blood pressure, respectively); however, after 90-130 min, HR and baroreflex reactivity returned to control levels. Microinjections of vehicle into the same area did not alter baroreflex function. In addition, suramin did not alter the depressor responses to microinjections of glutamate into the same site of the NTS. We conclude that normal P2x-purinoceptor function in subpostremal NTS may be necessary for baroreflex regulation of HR.