Submitted on March 17, 2003
Accepted on June 15, 2003
An increase of intracellular calcium concentration that is induced by basolateral CO2 in the rabbit renal proximal tubule
Patrice Bouyer1*, Yuehan Zhou1, and Walter F. Boron1
1 Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, CT, USA
* To whom correspondence should be addressed. E-mail: Patrice.Bouyer{at}yale.edu.
Working with isolated perfused S2 proximal tubules, we asked whether the basolateral CO2 sensor acts, in part, by raising intracellular [Ca++], monitored with the dye fura2 (or fura-PE3). In paired experiments, adding 5% CO2/22 mM HCO3- (constant pH 7.40) to the bath (basolateral) solution caused [Ca++]i to increase from 57 ± 3 to 97 ± 9 nM (n = 8, P < 0.002), whereas the same maneuver in the lumen had no effect. Intracellular pH (pHi), measured with the dye BCECF, fell by 0.54 ± 0.08 (n = 14) when we added CO2/HCO3- to the lumen. In 14 tubules in which we added CO2/HCO3- to the bath, pHi fell by 0.55 ± 0.11 in 9 with a high initial pHi, but rose by 0.28 ± 0.07 in the other 5 with a low initial pHi. Thus, it cannot be a pHi change that triggers the [Ca++]i increase. Introducing to the bath an out-of-equilibrium (OOE) solution containing 20% CO2/no HCO3-/pH 7.40 caused [Ca++]i to rise by 62 ± 17 nM (n = 10), whereas an OOE solution containing 0% CO2/22 mM HCO3-/pH 7.40 caused only a trivial increase. Removing Ca++ from lumen and bath-or adding 10 µM nifedipine (L- and T-type Ca++-channel blocker) or 2 µM thapsigargin (SERCA inhibitor) or 4 µM rotenone (mitochondrial inhibitor) to lumen and bath-failed to reduce the CO2-induced increase in [Ca++]i. Adding 10 mM caffeine (ryanodine-receptor agonist) had no effect on [Ca++]i. Thus, basolateral CO2, presumably via a basolateral sensor, triggers the release of Ca++ from a non-conventional intracellular pool.
