Renal vasodilation induced by hypernatraemia: role of alpha-adrenoceptors in the median preoptic nucleus.

1. The renal vasodilation induced by infusion of hypertonic saline (HS) in anaesthetized rats has been shown to depend on the integrity of the median preoptic nucleus (MnPO), as well as noradrenergic afferents to this nucleus. In the present study, we sought to determine the role of alpha(1) and alpha(2)-adrenoceptors in the MnPO in cardiovascular responses induced by intravenous HS infusion (3 mol/L NaCl; 1.8 mL/kg, i.v., over 1 min). 2. Male Wistar rats (320-360 g) were anaesthetized with urethane (1.2 g/kg, i.v.) and instrumented for recording of mean arterial pressure (MAP), renal blood flow (RBF) and vascular conductance (RVC). In one experimental group, rats were injected with yohimbine, prazosin or saline (control) 20 min before HS infusion. In another experimental group, rats were injected with yohimbine or prazosin 20 min after HS infusion. 3. In control rats (n = 7), HS infusion 20 min after saline nanoinjection produced a transient hypertension. Ten minutes after HS infusion, RBF and RVC increased to 159 +/- 14% and 145 +/- 11% of baseline, respectively. Nanoinjection of the alpha(1)-adrenoceptor antagonist prazosin (0.25 mmol/L; n = 6) into the MnPO 20 min before HS infusion increased the HS-induced pressor response. However, HS-induced increases in RBF and RVC were significantly reduced (130 +/- 11% and 105 +/- 6% of baseline, respectively, 10 min after HS). Nanoinjection of the alpha(2)-adrenoceptor antagonist yohimbine (0.23 mmol/L; n = 5) into the MnPO 20 min before HS infusion increased the duration of the pressor response and reduced the increases in RBF and RVC induced by HS (117 +/- 10% and 97 +/- 11% of baseline, respectively, 10 min after HS). 4. We also observed that nanoinjections of the prazosin into the MnPO 60 min after HS infusion resulted in a gradual return of RBF and RVC to baseline values. However nanoinjection of yohimbine 60 min after HS failed to reduce renal vasodilatation induced by hypernatremia. 5. The results of the present study demonstrate that the integrity of adrenergic neurotransmission in the MnPO is essential for the renal vasodilation that follows acute increases in blood sodium concentration.