Renin secretion and total body sodium: pathways of integrative control.

1. Herein, we review mechanisms of sodium balance operating at constant mean arterial blood pressure (MABP); that is, under conditions where MABP does not provide the primary signal to the kidney. 2. Relative constancy of body fluids requires accurate regulation of total body sodium (TBS). Normally, plenty of sodium is ingested and balance is achieved by control of renal excretion driven by multiple central nervous, cardiovascular, endocrine and renal tubular mechanisms. Subtle changes in sodium balance are associated with parallel changes in extracellular volume (due to fast and precise osmoregulation), but not necessarily in MABP. Therefore, signals other than MABP seem to be the primary link between TBS and kidney function. 3. Renal functions involved in sodium homeostasis include: (i) the rate of glomerular filtration (GFR) determined by renal haemodynamics, including tubuloglomerular feedback (TGF); (ii) proximal tubular reabsorption involving glomerulotubular balance (GTB) and neurohumoral control; (iii) macula densa mechanisms influencing TGF and renin secretion; and (iv) distal tubular reabsorption dominated by the renin-angiotensin-aldosterone system (RAAS). 4. The present review focuses on the interactive, homeostatic operation of TBS, MABP, GTB, TGF and the RAAS. Regulation of sodium balance involves neurohumoral control of tubular sodium reabsorption, including proximal reabsorption. Central nervous system-mediated regulation of the latter modulates renin secretion. Homeostatically, the RAAS-TGF interaction seems analogous to a spring-shock absorber set-up: non-adaptive RAAS functions determine the new steady state position, whereas TGF controls the rate of change. Recruitment of renin-secreting cells during sustained stimulation may be essential for chronic adaptation, although details of this afferent arteriolar cell plasticity are unclear at present.