Ca2+ uptake and release properties of a thapsigargin-insensitive nonmitochondrial Ca2+ store in A7r5 and 16HBE14o- cells

Abstract

In a previous study we overexpressed the thapsigargin (tg)-insensitive Pmr1 Ca2+ pump of the Golgi apparatus of Caenorhabditis elegans in COS-1 cells and studied the properties of the Ca2+ store into which it was integrated. Here we assessed the properties of an endogenous tg-insensitive nonmitochondrial Ca2+ store in A7r5 and 16HBE14o- cells, which express a mammalian homologue of Pmr1. The tg-insensitive Ca2+ store was considerably less leaky for Ca2+ than the sarco(endo)-plasmic-reticulum Ca2+-ATPase (SERCA)-containing Ca2+ store. Moreover like for the worm Pmrl Ca2+ pump expressed in COS-1 cells, Ca2+ accumulation into the endogenous tg-insensitive store showed a 2 orders of magnitude lower sensitivity to cyclopiazonic acid than the SERCA-mediated transport. 2,5-Di-(tert-butyl)-1,4-benzohydroquinone was only a very weak inhibitor of the tg-insensitive Ca2+ uptake in A7r5 and 16HBE14o-cells and in COS-1 cells overexpressing the worm Pmr1. Inositol 1,4,5-trisphosphate released 11% of the Ca2+ accumulated in permeabilized A7r5 cells pretreated with tg with an EC50 that was 5 times higher than for the SERCA-containing Ca2+ store but failed to release Ca2+ in 16HBE14o- cells. In the presence of tg, 15% of intact A7r5 cells responded to 10 μm arginine-vasopressin with a small rise in cytosolic Ca2+ concentration after a long latency. In conclusion, A7r5 and 16HBE14o- cells express a Pmr1-containing Ca2+ store with properties that differ substantially from the SERCA-containing Ca2+ store.