Caffeine‐Sensitive Calcium Stores in Bovine Adrenal Chromaffin Cells

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Abstract
Caffeine was used to study the intracellular Ca2+ pools of bovine chromaffin cells. Its effects on cytosolic Ca2+ concentration ([Ca2+]i) were examined using fura‐2. Caffeine caused a transient increase in [Ca2+]i in the presence or absence of extracellular Ca2+. In the former case, the caffeineinduced [Ca2+]i increase was higher and stayed above the basal value for several minutes. In the latter case, the [Ca2+]i rise was lower and fell to the basal level within 1 min. These results suggest that caffeine increases [Ca2+]i by causing both Ca2+ influx and Ca2+ release from intracellular pools. In the absence of extracellular Ca2+, ionomycin but not caffeine caused a further increase in [Ca2+]i in cells that had been treated with caffeine. Apparently there are at least two intracellular Ca2+ pools, only one of which is sensitive to caffeine. The caffeine‐induced [Ca2+]i rise became smaller when the cells were pretreated with the inositol trisphosphate‐generating agonists, methacholine and bradykinin. In addition, metha‐choline was unable to initiate a [Ca2+]i transient after the cells had been treated with caffeine. The results indicate that the caffeine‐sensitive Ca2+ pools overlap with the inositol trisphosphate‐sensitive pool and that the size, of the latter pool is smaller than that of the former. The caffeine‐sensitive Ca2+ pools were refilled after high K+ treatment, which suggests that the caffeine‐sensitive Ca2+ pools may be important in buffering the cytosolic Ca2+. The effect of caffeine on [Ca2+]i is not due to inhibition of phosphodiesterase. Our results support a Ca2+ entry model in which depletion of intracellular Ca2+ pools controls the rate of Ca2+ entry across the plasma membrane.