Inositol 1, 4, 5-Trisphosphate- and Caffeine-Sensitive Ca²⁺-Storing Organelle in Bovine Adrenal Chromaffin Cells

Abstract

The uptake and release properties of Ca²⁺ by several subcellular fractions of the bovine adrenal medulla were investigated. Investigation by the ⁴⁵Ca²⁺ tracer method showed that permeabilized cells and the fractions of mitochondria (MT) and microsomes (MC) caused ATP-dependent Ca²⁺ uptake in a Ca²⁺ concentration-dependent manner (pCa 8-4), whereas permeabilized cells and the fractions of secretory granules (SG) were able to accumulate a significant amount of Ca²⁺ even in the absence of ATP, which was completed by the addition of hexokinase and glucose. In these organelle fractions, Ca²⁺ uptake in the presence of ATP at pCa 7 and pCa 5.8 was well-correlated with the activity of the NADPH cyiochrome c reductase (marker enzyme for the endoplasmic reticulum) and cytochrome c oxidase (marker enzyme for mitochondria), respectively. As detected by Fura-2 ratiometry, both inositol 1, 4, 5-trisphosphate (IP₃) and caffeine caused concentration-dependent Ca²⁺ releases from permeabilized cells and MC, but not from MT and SG. In an ATP-depleted condition, homogenates still took up a significant amount of Ca²⁺ but was not able to respond to IP³ and caffeine. These results suggest that the endoplasmic reticulum is a major Ca²⁺-storing organelle, which releases Ca²⁺ in response to IP₃ and caffeine in bovine adrenal chromaffin cells.