Sodium intake and renin system activity: Effects of metoprolol on the log-linear relationship in conscious rats

Aims: In dogs, plasma renin concentration (PRC) is log-linearly related to sodium intake (NaI)
during steady state (Kjølby & Bie, 2008); i.e., PRC is a linear function of log(NaI). Here,
we hypothesized that this concept also applies to rats, and that the parameters are changed by
chronic blockade of β1 adrenoceptors. Methods: Sprague-Dawley rats were provided with
chronic, femoral, arterial and venous catheters and studied after >5 days of recovery and >3
days of low-salt diet. Sodium intake was varied from 0.07 to16 mmol/kg/d by intravenous infusion. PRC was measured at the end of the dark phase of the diurnal periods. Mean arterial
blood pressure (MABP) was sampled 20 h/d, and glomerular filtration rate (GFR) measured
by inulin clearance using the constant infusion technique. Metoprolol (1 mg/kg/h) was infused
intravenously to block β1 adrenoceptors. Results: Over the ∼200 fold range of NaI, PRC was
log-linearly related to log(NaI). In control experiments, the relation was PRC = -8.2 log(NaI) +
19.6; MABP and heart rate varied significantly with sodium intake, but GFR did not. During
chronic metoprolol infusion, MABP was reduced (lowest salt intake: -23 mmHg, p < 0.01), in
contrast, the PRC- NaI relation followed PRC = -8.7 log(NaI) +19.6, i.e., a relationship indistinguishable from control. Conclusion: The results confirm in the rat that PRC is a log-linear
function of NaI. Surprisingly they indicate either that β1 adrenoceptors are not importantly
involved in renin secretion, or that the β1-mediated effect is quantitatively replaced by the effect of hypotension. Reference: Kjølby M. & Bie P. Am.J.Physiol.Regul.Integr.Comp.Physiol.
294:R17-25,2008.