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An increase in intracellular calcium concentration that is induced by basolateral CO₂ in rabbit renal proximal tubule

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

Submitted 17 March 2003 ; accepted in final form 15 June 2003

Working with isolated perfused S2 proximal tubules, we asked whether the basolateral CO₂ sensor acts, in part, by raising intracellular Ca²⁺ concentration ([Ca²⁺]_i), monitored with the dye fura 2 (or fura-PE3). In paired experiments, adding 5% CO₂/22 mM (constant pH 7.40) to the bath (basolateral) solution caused [Ca²⁺]_i to increase from 57 ± 3 to 97 ± 9nM(n = 8, P < 0.002), whereas the same maneuver in the lumen had no effect. Intracellular pH (pH_i), measured with the dye BCECF, fell by 0.54 ± 0.08 (n = 14) when we added to the lumen. In 14 tubules in which we added to the bath, pH_i fell by 0.55 ± 0.11 in 9 with a high initial pH_i, but rose by 0.28 ± 0.07 in the other 5 with a low initial pH_i. Thus it cannot be a pH_i change that triggers the [Ca²⁺]_i increase. Introducing to the bath an out-of-equilibrium (OOE) solution containing 20% CO₂/no caused [Ca²⁺]_i to rise by 62 ± 17 nM (n = 10), whereas an OOE solution containing 0% CO₂/22 mM caused only a trivial increase. Removing Ca²⁺ from the lumen and bath, or adding 10 µM nifedipine (L- and T-type Ca²⁺-channel blocker) or 2 µM thapsigargin [sarco-(endo) plasmic reticulum Ca²⁺-ATPase inhibitor] or 4 µM rotenone (mitochondrial inhibitor) to the lumen and bath, failed to reduce the CO₂-induced increase in [Ca²⁺]_i. Adding 10 mM caffeine (ryanodine-receptor agonist) had no effect on [Ca²⁺]_i. Thus basolateral CO₂, presumably via a basolateral sensor, triggers the release of Ca²⁺ from a nonconventional intracellular pool.

intracellular pH; carbon dioxide; out-of-equilibrium solutions; fura 2; ions; transport; kidney