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WATER AND ELECTROLYTE HOMEOSTASIS

Normotensive sodium loading in conscious dogs: regulation of renin secretion during β-receptor blockade

Department of Physiology and Pharmacology, University of Southern Denmark, Odense, Denmark

Submitted 5 September 2008 ; accepted in final form 1 December 2008

Renin secretion is regulated in part by renal nerves operating through β₁-receptors of the renal juxtaglomerular cells. Slow sodium loading may decrease plasma renin concentration (PRC) and cause natriuresis at constant mean arterial blood pressure (MAP) and glomerular filtration rate (GFR). We hypothesized that in this setting, renin secretion and renin-dependent sodium excretion are controlled by via the renal nerves and therefore are eliminated or reduced by blocking the action of norepinephrine on the juxtaglomerular cells with the β₁-receptor antagonist metoprolol. This was tested in conscious dogs by infusion of NaCl (20 µmol·kg^–1·min^–1 for 180 min, NaLoad) during regular or low-sodium diet (0.03 mmol·kg^–1·day^–1, LowNa) with and without metoprolol (2 mg/kg plus 0.9 mg·kg^–1·h^–1). Vasopressin V₂ receptors were blocked by Otsuka compound OPC31260 to facilitate clearance measurements. Body fluid volume was maintained by servocontrolled fluid infusion. Metoprolol per se did not affect MAP, heart rate, or sodium excretion significantly, but reduced PRC and ANG II by 30–40%, increased plasma atrial natriuretic peptide (ANP), and tripled potassium excretion. LowNa per se increased PRC (+53%), ANG II (+93%), and aldosterone (+660%), and shifted the vasopressin function curve to the left. NaLoad elevated plasma [Na⁺] by 4.5% and vasopressin by threefold, but MAP and plasma ANP remained unchanged. NaLoad decreased PRC by 30%, ANG II by 40%, and aldosterone by 60%, regardless of diet and metoprolol. The natriuretic response to NaLoad was augmented during metoprolol regardless of diet. In conclusion, PRC depended on dietary sodium and β₁-adrenergic control as expected; however, the acute sodium-driven decrease in PRC at constant MAP and GFR was unaffected by β₁-receptor blockade demonstrating that renin may be regulated without changes in MAP, GFR, or β₁-mediated effects of norepinephrine. Low-sodium diet augments vasopressin secretion, whereas ANP secretion is reduced.

blood pressure; sodium balance; angiotensin; aldosterone; vasopressin; atrial natriuretic peptide

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