Caffeine-sensitive calcium stores in bovine adrenal chromaffin cells.
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 caffeine-induced [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, methacholine 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.
Liu
P S
PS
Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan, R.O.C.
Lin
Y J
YJ
Kao
L S
LS
D000311:Adrenal Glands / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D001920:Bradykinin / Q000494:pharmacology; D002110:Caffeine / Q000008:administration & dosage / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D002838:Chromaffin System / Q000187:drug effects / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2; D015759:Ionomycin / Q000494:pharmacology; D016210:Methacholine Chloride / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology
D005456:Fluorescent Dyes; D016210:Methacholine Chloride; D012433:Ryanodine; D002110:Caffeine; D004533:Egtazic Acid; D015759:Ionomycin; D011188:Potassium; D001920:Bradykinin; D002118:Calcium; D016257:Fura-2