Acid-Induced Chloride Current in Distal Convoluted Tubule

Abstract

The distal convoluted tubule (DCT) of the kidney is responsible for the fine-tuning of urinary filtrate before excretion. The DCT is known to facilitate the movement of Na, K, Ca, Mg, and Cl via transporters and ion channels. Recent research has focussed on the functional activity of TRP channels in the DCT, with heavy emphasis on TRPM6/7 (Mg at neutral pH 7.4; Na at acidic pH) and TRPV5/6 (Ca). Several studies have suggested that these channels should give rise to large macroscopic currents in DCT cells. Therefore we undertook this study to determine the ionic species representing the dominant current in mouse DCT (MDCT) cells via whole-cell voltage-clamp electrophysiology. Using ionic permeation studies and analysis of reversal potential changes, our results unequivocally demonstrate that MDCT cells exhibit a significant Cl current under both neutral and acidic extracellular solutions. We further identified that this current is rapidly activating, differentiating it from the endogenous acid-induced anion current discovered in HEK293 cells. We noted that currents elicited under acidic pH were substantially larger than those at neutral pH, and thus hypothesized that extracellular acid induced a different, larger Cl current in MDCT cells. This was supported pharmacologically, where only the acid-induced Cl current was blocked by furosemide, a clinically prescribed loop-diuretic. To further understand the significance of a large acid-induced Cl current in the late nephron, we investigated inner medullary collecting duct (IMCD) cells as they represent the most acidic portion of the nephron. IMCD current was not only acid-induced, but shared an identical I-V relationship, pH sensitivity, and time-course of activation with the acid-induced induced Cl current discovered in MDCT cells. We thus conclude that the late nephron contains a large acid-induced Cl current that may play a significant role in anion reabsorption.