Ionomycin activates electrogenic Ca2+ influx in rat thymic lymphocytes.
Increases in intracellular Ca2+ concentration ([Ca2+]i) after addition of the carboxylic Ca(2+)-transporting ionophore ionomycin have been reported to be inhibited by depolarization in mouse T- and B-lymphocytes, a result attributed to effects on the plasma-membrane Ca2+ pump. The present experiments were undertaken to test the hypothesis that activation of electrogenic Ca2+ uptake, due to release of Ca2+i from intracellular pools, underlies the membrane-potential (Em)-sensitivity of ionomycin-mediated Ca2+ uptake in T-lymphocytes. To address this proposal we have compared the characteristics of Ca2+ influx induced by ionomycin with those of Ca2+ influx activated by release of Ca2+ from intracellular stores (store-regulated Ca2+ uptake, SRCU). The endosomal Ca(2+)-ATPase inhibitor thapsigargin was used to bring about loss of Ca2+ from intracellular pools. In support of this hypothesis, we report that: (1) ionomycin releases Ca2+ from a thapsigargin-sensitive endosomal pool, a pool previously demonstrated to regulate electrogenic SRCU in rat thymic lymphocytes, (2) Ca2+ and Mn2+ uptake mediated by low doses of ionomycin are inhibited by compounds previously reported to inhibit SRCU, and (3) in the absence of SRCU, ionomycin-mediated Ca2+ uptake displays no sensitivity to Em. We conclude that activation of electrogenic SRCU can adequately account for the Em-sensitivity of Ca2+ flux previously attributed to alterations in Ca(2+)-pump activity. Such a mechanism of action may underlie previous reports of electrogenic Ca2+ transport mediated by ionomycin in other tissues.
Mason
M J
MJ
Department of Physiology, Tulane University School of Medicine, New Orleans, LA 70112.
Grinstein
S
S
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002478:Cells, Cultured; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D007093:Imidazoles / Q000494:pharmacology; D015759:Ionomycin / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012965:Sodium Chloride / Q000494:pharmacology; D013601:T-Lymphocytes / Q000187:drug effects / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D013950:Thymus Gland / Q000378:metabolism
D007093:Imidazoles; D013729:Terpenes; D012965:Sodium Chloride; D015759:Ionomycin; D011189:Potassium Chloride; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; C063159:1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole; D002118:Calcium