Fluoride mobilizes intracellular calcium and promotes Ca²⁺ influx in rat proximal tubules

In the renal proximal tubule, external Ca²⁺ ([Ca²⁺](o)) is required for parathyroid hormone to elevate cytosolic Ca²⁺ ([Ca²⁺](i)). However, other hormones increase [Ca²⁺](i) in the absence of [Ca²⁺](o). These differences may arise from a diversity of signal transduction pathways acting on external and internal Ca²⁺ pools. However, Ca²⁺ influx may be necessary to expedite and maintain the rise of [Ca²⁺](i) for a period after the initial surge. In this study, F^- was used to probe the roles of intracellular Ca²⁺ mobilization, Ca²⁺ influx, and phosphoinositide (PI) hydrolysis on the surge of [Ca²⁺](i) in rat proximal tubules. In the presence of external Ca²⁺; 1-20 mM F^- evoked incremental rises of [Ca²⁺](i) in tubules loaded with aequorin. Whereas 10 mM F^- increased [Ca²⁺](i) in the absence of [Ca²⁺](o), the time constant for the [Ca²⁺](i) surge was increased. These findings are consistent with a role of Ca²⁺ influx on the effect of F^- on [Ca²⁺](i). Indeed, 10 mM F^- also enhanced the uptake of ⁴⁵Ca²⁺, and promoted Ca²⁺ influx in aequorin- and fura-2-loaded, Ca²⁺-deprived tubules. In tubules, F^- also activated PI hydrolysis with a time course that paralleled Ca²⁺ mobilization. The effect of F^- on [Ca²⁺](i) was not altered when the 39-kDa pertussis toxin substrate was inactivated with the toxin. This G protein was most likely G(i), because prostaglandin E₂, an activator of G(i) in tubules, dissociated the pertussis toxin-sensitive protein. The results support the notion that activation of a signal-transduction complex, the F^- substrate, causes Ca²⁺ influx, mobilizes internal Ca²⁺, and activates PI hydrolysis in rat proximal tubules. The effect on Ca²⁺ mobilization does not appear to be regulated by the pertussis toxin-sensitive (G(i)) protein. Although [Ca²⁺](o) is not necessary for the expression of the effect of F^- on [Ca²⁺](i), Ca²⁺ influx may be a critical determinant for the [Ca²⁺](i) surge within a finite period of time.