title: Involvement of sarcoplasmic reticulum in changing intracellular calcium due to Na+/K+-ATPase inhibition in cardiomyocytes.
abstract: Earlier studies have demonstrated that ouabain-induced increase in [Ca2+]i, as a consequence of sarcolemma (SL) Na+/K+-ATPase inhibition, is associated with activation of both the SL Na+/Ca2+ exchanger and SL Ca2+ channels. In view of the importance of sarcoplasmic reticulum (SR) in the regulation of [Ca2+]i, this study examined the role of SR in ouabain-induced increase in [Ca2+]i in both quiescent and KCl-depolarized cardiomyocytes. For this purpose, adult rat cardiomyocytes were loaded with fura-2 and ouabain-induced changes in [Ca2+]i were monitored upon treatment with or without different agents that are known to influence Ca2+ handling by the intracellular organelles. Ouabain not only increased the basal [Ca2+]i and augmented KCl-induced increase in [Ca2+]i but also produced similar effects on the ATP-induced increase in [Ca2+]i. Treatments of cardiomyocytes with caffeine, ryanodine, or cyclopiazonic acid, which affect SR Ca2+ stores, attenuated the ouabain-induced increase in basal Ca2+ as well as augmentation of the KCl response. Both ryanodine and cyclopiazonic acid produced additional effects, when used in combination with a SL Ca2+ channel inhibitor (verapamil), but not with a Na+/Ca2+ exchange inhibitor (KB-R7943). Inhibitors of Ca2+/calmodulin kinase, protein kinase A, and inositol-3-phosphate receptors were also observed to depress the ouabain-induced increase in [Ca2+]i in cardiomyocytes. On the other hand, mitochondrial Ca2+ transport inhibitors did not exert any effect on the ouabain-induced alterations in [Ca2+]i in cardiomyocytes. Furthermore, ouabain did not show any direct effect on the Ca2+ uptake and Ca2+ release activities of SR or mitochondria. These results suggest an indirect involvement of SR Ca2+ stores in the ouabain-induced increase in [Ca2+]i in cardiomyocytes and indicate the participation of both Ca2+-induced Ca2+ release and regulatory mechanisms in this action.
authors.affiliation: Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, University of Manitoba, Winnipeg, MB R2H 2A6, Canada.
authors.forename: Harjot K
authors.initials: HK
authors.lastname: Saini-Chohan
authors.forename: Ramesh K
authors.initials: RK
authors.lastname: Goyal
authors.forename: Naranjan S
authors.initials: NS
authors.lastname: Dhalla
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D066298:In Vitro Techniques; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000037:antagonists & inhibitors; D008297:Male; D008929:Mitochondria, Heart / Q000187:drug effects / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012433:Ryanodine / Q000494:pharmacology; D012508:Sarcolemma / Q000187:drug effects / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors*
chemical_list: D053496:Inositol 1,4,5-Trisphosphate Receptors; D012433:Ryanodine; D002110:Caffeine; D010042:Ouabain; D011189:Potassium Chloride; D000255:Adenosine Triphosphate; D017868:Cyclic AMP-Dependent Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium