Jpn. J. Pharmacol. 72, 307-315 (1996)

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

Hiroki Teraoka (1), Ryo Takai (2), Tetsuro Taneike (2), Takeo Hiraga (1) and Akira Ohga (1)

(1) Department of Toxicology and (2) Department of Veterinary Pharmacology, School of Veterinary Medicine, Rakuno Gakuen University, Ebetsu 069, Japan

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 cytochrome 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.

Keywords: Caffeine, Ca2+ store, Chromaffin cell, Inositol 1,4,5-trisphosphate, Organelle