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Effect of isolated removal of either basolateral or basolateral CO₂ on reabsorption by rabbit S2 proximal tubule

Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520

Submitted 13 January 2003 ; accepted in final form 8 April 2003

The equilibrium had made it impossible to determine how isolated changes in basolateral CO₂ ([CO₂]) or concentration ([]), at a fixed basolateral pH, modulate renal or reabsorption. In the present study, we have begun to address this issue by measuring reabsorption (J_HCO3) and intracellular pH (pH_i) in isolated perfused rabbit S2 proximal tubules exposed to three different basolateral (bath) solutions: 1) equilibrated 5% CO₂/22 mM 7.40, 2) an out-of-equilibrium (OOE) solution containing 5% CO₂/pH 7.40 but minimal ("pure CO₂"), and 3) an OOE solution containing 22 mM 7.40 but minimal CO₂ ("pure "). Tubule lumens were constantly perfused with equilibrated 5% CO₂/22 mM . Compared with the equilibrated bath solution (J_HCO3 = 76.5 ± 7.7 pmol·min^–1·mm^–1, pH_i = 7.09 ± 0.04), the pure CO₂ bath solution increased J_HCO3 by 25% but decreased pH_i by 0.19. In contrast, the pure bath solution decreased J_HCO3 by 37% but increased pH_i by 0.24. Our data are consistent with two competing hypotheses: 1) the isolated removal of basolateral (or CO₂) causes a pH_i decrease (increase) that in turn raises (lowers) J_HCO3; and 2) removal raises J_HCO3 by reducing inhibition of basolateral Na/HCO₃ cotransport and/or reducing backleak, whereas CO₂ removal lowers J_HCO3 by reducing stimulation of a CO₂ sensor.

bicarbonate; carbon dioxide; intracellular pH; acid-base; volume reabsorption; out-of-equilibrium solutions

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