Effects of chloride gradients on total CO2 flux in the rabbit cortical collecting tubule

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Abstract
Isolated perfused rabbit cortical collecting tubules at 38.degree. C were exposed to Cl- gradients in either lumen-to-bath or bath-to-lumen direction. Total CO2 flux was determined by microcalorimetry of timed collections of luminal perfusion solutions. Transepithelial electrical potential differences were minimized by addition of 0.1 mM amiloride to the perfusate. When 50 mM of bath Cl- was replaced by 50 mM isethioniate or cyclamate and bath perfusate and both contained 25 mM HCO3- total CO2 flux changed from absorption (1.3 .+-. 1.5 pmol .cntdot. mm-1 .cntdot. min-1) to secretion (-2.1 .+-. 1.6 pmol .cntdot. mm-1 .cntdot. min-1) during the gradient period (P < 0.001). Conversely, when a bath-to-lumen Cl- gradient was created (114 mM isethionate replaced 114 mM Cl- in the perfusate), total CO2 absorption increased from +8.1 .+-. 1.9 to +12.9 .+-. 2.7 pmol .cntdot. mm-1 .cntdot. min-1 (P < 0.01). If methylsulfate replaced bath Cl- (50 mM), no change in total CO2 flux was found. If perfusate was initially free and 114 mM Cl- gradient from bath to lumen was created by replacing bath Cl- with 114 mM isethionate, HCO3- secretion decreased from -7.5 .+-. 2.6 to -5.6 .+-. 2.1 pmol .cntdot. mm-1 .cntdot. min-1 (P < 0.05). When the 114 mM bath-to-lumen Cl- gradient was then eliminated by replacing bath Cl- with isethionate in 4 experiments, HCO3- flux returned to control levels, -7.1 .+-. 2.0 pmol .cntdot. mm-1 .cntdot. min-1. Cl- gradients in 1 direction affect HCO3- flux in the opposite direction in cortical collecting tubules. Whether the effect is due to electrical coupling or to an anion exchange mechanism cannot be distinguished.