Involvement of the Endoplasmic Reticulum of Chromaffin Cells of the Rat Adrenal Gland in Calcium Signaling

Abstract

We studied the involvement of the endoplasmic reticulum (ER) in calcium signaling in rat chromaffin cells. For this purpose, the following agents influencing the activity of the ER were used: (i) Caffeine that activates the release of Ca2+ from the endoplasmic store and (ii) thapsigargin that suppresses accumulation of calcium in the ER. The intracellular Ca2+ concentration was measured with the help of a calcium-sensitive dye, Fura-2AM, using the microfluorescent technique. Applications of caffeine led to a rise in the level of free Ca2+ in the cell cytosol and also to a decrease in the amplitude of calcium transients induced by depolarization of the plasma membrane under the action of a hyperpotassium solution. Under conditions of repeated caffeine applications, the amplitude of transients decreased to 9% of its initial value, which is explained by exhaustion of the calcium stores. The action of caffeine was restored when the calcium stores were re-filled under the action of depolarization of the plasma membrane. Thapsigargin completely removed the effect of caffeine and did not influence KCl-induced transients. Therefore, our experiments are indicative of a significant importance of the ER calcium stores for calcium signaling in chromaffin cells, which allows us to hypothesize that these stores play an important role in the control of secretion of catecholamines.