Background: 

Sodium intake (NaI), renin system activity, and blood pressure regulation are intimately related. However, precise quantitative insight into these relationships is virtually absent. We hypothesized that in rats (i) the steady-state plasma renin concentration (PRC) is log-linearly related to NaI, (ii) the function PRC = f(NaI) is altered by b1 adrenoceptor blockade or by renal denervation, and (iii) a step-up in NaI generates a new steady-state PRC within 3 d at constant renin mRNA expression.

Methods: 

In chronically catheterized, conscious rats on low-Na⁺ diet (0.004 % Na⁺), Na⁺ input was increased by up to 120-fold, in 3-day steps by intravenous infusions of saline with different [NaCl]. MABP was recorded continuously, PRC measured in arterial blood and GFR measured by inulin clearance. Metroprolol (1 mg/kg/h) was used to block b1 adrenoceptors; renal denervation was performed by standard procedures.

Results: 

PRC (mIU/l) was log-linearly related to NaI (mmol/kg/d): PRC = -9.9log(NaI) + 21.6; metoprolol and renal denervation had no effect. MABP and GFR were markedly salt sensitive: MABP [mmHg] = 4.9log(NaI) + 99.3, and GFR [ml/min] = 1.4log(NaI) + 8.3. Metoprolol, but not denervation, reduced blood pressure and heart rate (p<0.001) as well as glomerular filtration rate (p<0.01). Salt-sensitivity of GFR was not observed in denervated rats. With increase in NaI, PRC decreased with of T½ of 4.6 h.

Conclusion: 

A log-linear relation to salt intake was found not only for PRC, but also for MABP and GFR, which per decade increase in NaI changed by 5 mmHg and 1.4 ml/min respectively. PRC = f(NaI) is unaffected by metoprolol or renal denervation. New steady states are reached within 1 d at constant renin mRNA.