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

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 HCO3- (constant pH 7.40) to the bath (basolateral) solution caused [Ca2+]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 [Ca2+]i increase. Introducing to the bath an out-of-equilibrium (OOE) solution containing 20% CO2/no HCO3-/pH 7.40 caused [Ca2+]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 Ca2+ from the lumen and bath, or adding 10 microM nifedipine (L- and T-type Ca2+-channel blocker) or 2 microM thapsigargin [sarco-(endo) plasmic reticulum Ca2+-ATPase inhibitor] or 4 microM 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.