Salt sensitivity of renin secretion, glomerular filtration rate and blood pressure in conscious Sprague-Dawley rats

Aim: We hypothesized that in normal rats in metabolic steady state, (i) the plasma renin concentration (PRC) is log-linearly related to Na ⁺ intake (NaI), (ii) the concurrent changes in mean arterial pressure (MABP) and glomerular filtration rate (GFR) are negligible and (iii) the function PRC = f(NaI) is altered by β ₁-adrenoceptor blockade (metoprolol) and surgical renal denervation (DNX). Methods: In catheterized, conscious rats on low-Na ⁺ diet (0.004% Na ⁺), NaI was increased by up to 120-fold, in four 3-day steps, by intravenous saline infusion. MABP was recorded continuously, PRC measured in arterial blood, and GFR estimated by inulin clearance. Results: Steady states were achieved within 3 days. PRC [mIU L ^-1] was log-linearly related to NaI [mmol kg ^-1 day ^-1]: PRC = -9.9 log (NaI) + 22. Set point (22 mIU L ^-1 at NaI = 1) and slope (9.9 mIU per decade NaI) were independent of metoprolol administration and DNX. MABP and GFR were markedly salt-sensitive: MABP [mmHg] = 4.9 log (NaI) + 99 (P < 0.01), and GFR [mL min ^-1] = 1.4 log (NaI) + 8.3 (P < 0.01). MABP increased similarly (approx. 10%, P < 0.001) irrespective of pre-treatment. Metoprolol, but not DNX, reduced MABP, HR, and GFR (all P < 0.01). Salt sensitivity of GFR was not observed in DNX rats. Conclusion: Log-linear relations to sodium intake exist not only for PRC, but also for MABP and GFR, which per 10-fold increase in sodium intake rose by 5 mmHg and 1.4 mL min ^-1 respectively. Steady-state levels of PRC appear independent of renal nerves. MABP and GFR seem markedly salt sensitive in normal rats.