Characterization of inositol 1,4,5-trisphosphate-sensitive (IsCaP) and-insensitive (IisCaP) nonmitochondrial Ca²⁺ pools in rat pancreatic acinar cells

Summary

We have measured Ca²⁺ uptake and Ca²⁺ release in isolated permeabilized pancreatic acinar cells and in isolated membrane vesicles of endoplasmic reticulum prepared from these cells. Ca²⁺ uptake into cells was monitored with a Ca²⁺ electrode, whereas Ca²⁺ uptake into membrane vesicles was measured with⁴⁵Ca²⁺. Using inhibitors of known action, such as the H⁺ ATPase inhibitors NBD-Cl and NEM, the Ca²⁺ ATPase inhibitor vanadate as well as the second messenger inositol 1,4,5-trisphosphate (IP₃) and its analog inositol 1,4,5-trisphosphorothioate (IPS₃), we could functionally differentiate two non-mitochondrial Ca²⁺ pools. Ca²⁺ uptake into the IP₃-sensitive Ca²⁺ pool (IsCaP) occurs by a MgATP-dependent Ca²⁺ uptake mechanism that exchanges Ca²⁺ for H⁺ ions. In the absence of ATP Ca²⁺ uptake can occur to some extent at the expense of an H⁺ gradient that is established by a vacuolar-type MgATP-dependent H⁺ pump present in the same organelle. The other Ca²⁺ pool takes up Ca²⁺ by a vanadate-sensitive Ca²⁺ ATPase and is insensitive to IP₃ (IisCaP). The IsCaP is filled at “higher” Ca²⁺ concentrations (∼10⁻⁶ mol/liter) which may occur during stimulation. The low steady-state [Ca²⁺] of ∼10⁻⁷ mol/liter is adjusted by the IisCaP.

It is speculated that both Ca²⁺ pools can communicate with each other, the possible mechanism of which, however, is at present unknown.