title: Evidence that store-operated Ca2+ channels are more effective than intracellular messenger-activated non-selective cation channels in refilling rat hepatocyte intracellular Ca2+ stores.
abstract: Liver cells possess store-operated Ca2+ channels (SOCs) with a high selectivity for Ca2+ compared with Na+, and several types of intracellular messenger-activated non-selective cation channels with a lower selectivity for Ca2+ (NSCCs). The main role of SOCs is thought to be in refilling depleted endoplasmic reticulum Ca2+ stores [Cell Calcium 7 (1986) 1]. NSCCs may be involved in refilling intracellular stores but are also thought to have other roles in regulating the cytoplasmic-free Ca2+ and Na+ concentrations. The ability of SOCs to refill the endoplasmic reticulum Ca2+ stores in hepatocytes has not previously been compared with that of NSCCs. The aim of the present studies was to compare the ability of SOCs and maitotoxin-activated NSCCs to refill the endoplasmic reticulum in rat hepatocytes. The experiments were performed using fura-2FF and fura-2 to monitor the free Ca2+ concentrations in the endoplasmic reticulum and cytoplasmic space, respectively, a Ca2+ add-back protocol, and 2-aminoethyl diphenylborate (2-APB) to inhibit Ca2+ inflow through SOCs. In cells treated with 2,5-di-t-butylhydroquinone (DBHQ) or vasopressin to deplete the endoplasmic reticulum Ca2+ stores, then washed to remove DBHQ or vasopressin, the addition of Ca2+ caused a substantial increase in the concentration of Ca2+ in the endoplasmic reticulum and cytoplasmic space due to the activation of SOCs. These increases were inhibited 80% by 2-APB, indicating that Ca2+ inflow is predominantly through SOCs. In the presence of 2-APB (to block SOCs), maitotoxin induced a substantial increase in [Ca2+](cyt), but only a modest and slower increase in [Ca2+](er). Under these conditions, Ca2+ inflow is predominantly through maitotoxin-activated NSCCs. It is concluded that SOCs are more effective than maitotoxin-activated NSCCs in refilling the endoplasmic reticulum Ca2+ stores. The previously developed concept of a specific role for SOCs in refilling the endoplasmic reticulum is consistent with the results reported here.
authors.affiliation: Department of Medical Biochemistry, School of Medicine, Faculty of Health Sciences, Flinders University, G.P.O. Box 2100, South Australia 5001, Adelaide, Australia.
authors.forename: R B
authors.initials: RB
authors.lastname: Gregory
authors.forename: D
authors.initials: D
authors.lastname: Sykiotis
authors.forename: G J
authors.initials: GJ
authors.lastname: Barritt
mesh_terms: D000818:Animals; D001896:Boron Compounds / Q000494:pharmacology; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D004837:Epinephrine / Q000494:pharmacology; D016257:Fura-2 / Q000031:analogs & derivatives / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism*; D006873:Hydroquinones / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology; D007473:Ion Channels / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology*; D007700:Kinetics; D008297:Male; D008387:Marine Toxins / Q000494:pharmacology; D008856:Microscopy, Fluorescence; D035964:Oxocins / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D019284:Thapsigargin / Q000494:pharmacology; D014667:Vasopressins / Q000494:pharmacology; D000095702:Polyether Toxins
chemical_list: D001896:Boron Compounds; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium; D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004837:Epinephrine; D016257:Fura-2; D006873:Hydroquinones; D015544:Inositol 1,4,5-Trisphosphate; D007473:Ion Channels; D008387:Marine Toxins; D035964:Oxocins; D019284:Thapsigargin; D014667:Vasopressins; C019359:2,5-di-tert-butylhydroquinone; C020394:maitotoxin; D000095702:Polyether Toxins