Predicted Effect of Circadian Regulation of Renal Transporters in a Hypertensive Rat

Abstract

The kidney regulates extracellular fluid and electrolyte homeostasis. Its function is modulated by the renin-angiotensin-aldosterone system (RAAS). Overactivation of the RAAS, such as in chronic Ang II infusion, promotes vasoconstriction, anti-natriuresis, and hypertension. Kidney function is also modulated by circadian clocks. Indeed, glomerular filtration rate, filtered electrolyte loads, urine volume, and urinary excretion all exhibit notable diurnal rhythms, which reflect, in part, the regulation of renal transporter proteins by circadian clock genes. Key renal transporters that are regulated by clock gene proteins include sodium-hydrogen exchanger 3 (NHE3), sodium-glucose cotransporter 1 (SGLT1), Na+-K+-2Cl- cotransporter (NKCC2), Na+-Cl- cotransporter (NCC), and epithelial sodium channel (ENaC), which are targeted by diuretics used to treat hypertension. The objective of the present study was to assess the effect of administration time on the natriuretic and diuretic effects of loop, thiazide, and K+-sparing diuretics, which are common treatments for hypertension, and how those effects differ between a normotensive and hypertensive kidney. Loop diuretics inhibit NKCC2 on the apical membrane of the thick ascending limb; notably, In Ang II-induced hypertension, NKCC2 is differentially regulated along the medullary versus cortical thick ascending limb. Thiazide diuretics inhibit NCC on the distal convoluted tubule, and K+-sparing diuretics inhibit ENaC on the connecting tubule and collecting duct. We simulated Na+ transporter inhibition using computational models of kidney function at different times of day. The simulation results predicted qualitatively similar diurnal oscillations in segmental transport in the normotensive and hypertensive kidneys, and highlighted significant time-of-day differences in the natriuresis, diuresis, and kaliuresis responses. NEW & NOTEWORTHYChronic infusion of angiotensin II promotes vasoconstriction, salt retention, and hypertension. Blood pressure and kidney function exhibit circadian rhythms. Given the diurnal variations in the expression levels of key renal electrolyte transporters, how do the natriuretic and diuretic effects of diuretics, a common treatment for hypertension that targets renal transporters, vary during the day? To answer this question, we simulated Na+ transporter inhibition using computational models of kidney function at different times of day.