Sex Differences in Solute and Water Handling in the Human Kidney: Modeling and Functional Implications
Besides the excretion of metabolic wastes, the kidneys regulate homeostasis of electrolytes, pH, metabolites, volume and blood pressure. Sex differences in kidney function and blood pressure have been widely described across many species. Immunoblot analysis has revealed that the kidney of a female rat is not simply a smaller version of a male kidney. Rather, male and female rat kidneys exhibit dimorphic patterns of transporter expression and salt handling, the functional implications of which have been analyzed in a series of previously published modeling studies of rat kidney function. In the present study, we extend the analysis to the human kidney: we developed sex-specific models of solute and water transport in the human kidney, and identified epithelial transport parameters, consistent with patterns found in male and female rats, that yield urine output and excretion rates consistent with known human values. The model predicts that the lower sodium hydrogen exchanger 3 (NHE3) activity in women reduces the fractional reabsorption of Na+, K+, Cl-, and water along the proximal tubule, compared to men, and that the larger load on the distal nephron can be handled by enhanced activities in key Na+ transporter such as epithelial sodium channel (ENaC) and sodium chloride cotransporter (NCC) in women. Model simulations further indicate that the larger distal transport capacity and proximal transport reserve may better prepare women for elevated demands of pregnancy and lactation. The larger distal transport capacity may also contribute to reduced efficacy of angiotensin converting enzyme inhibitors to lower blood pressure in women. Author summaryThe kidneys maintain homeostasis by controlling the amount of water, ions, and other substances in the blood. That function is accomplished by the nephrons, which transform glomerular filtrate into urine by an exquisite transport process mediated by a number of membrane transporters. Recently, the distribution of renal transporters along the nephron has been shown to be markedly different between male and female rodents. We postulate that similar sexual dimorphism exists between men and women, and we seek to reveal its physiological implications. We hypothesize that the larger abundance of a renal Na+ transport in the proximal tubules in females may also better prepare them for the fluid retention adaptations required during pregnancy and lactation, durint which renal and systemic hemodynamics are both drastically altered by the marked volume expansion and vasodilation. Also, kidneys play a key role in blood pressure regulation, and a popular class of anti-hypertensive medications, angiotensin converting enzymes (ACE) inhibitors, have been reported to be less effective in women. Model simulations suggest that the blunted natriuretic and diuretic effects of ACE inhibition in women can be attributed, in part, to their higher distal baseline transport capacity.