title: Mechanisms of low Na+-induced increase in intracellular calcium in KCl-depolarized rat cardiomyocytes.
abstract: Although low Na+ is known to increase the intracellular Ca2+ concentration ([Ca2+]i) in cardiac muscle, the exact mechanisms of low Na+ -induced increases in [Ca2+]i are not completely defined. To gain information in this regard, we examined the effects of low Na+ (35 mM) on freshly isolated cardiomyocytes from rat heart in the absence and presence of different interventions. The [Ca2+]i in cardiomyocytes was measured fluorometrically with Fura-2 AM. Following a 10 min incubation, the low Na+ -induced increase in [Ca2+], was only observed in cardiomyocytes depolarized with 30 mM KCl, but not in quiescent cardiomyocytes. In contrast, low Na+ did not alter the ATP-induced increase in [Ca2+]i in the cardiomyocytes. This increase in [Ca2+]i due to low Na+ and elevated KCl was dependent on the extracellular concentration of Ca2+ (0.25-2.0 mM). The L-type Ca2+ -channel blockers, verapamil and diltiazem, at low concentrations (1 microM) depressed the low Na+, KCl-induced increase in [Ca2+]i without significantly affecting the response to low Na+ alone. The low Na+, high KCl-induced increase in [Ca2+]i was attenuated by treatments of cardiomyocytes with high concentrations of both verapamil (5 and 10 microM), and diltiazem (5 and 10 microM) as well as with amiloride (5-20 microM), nickel (1.25-5.0 mM), cyclopiazonic acid (25 and 50 microM) and thapsigargin (10 and 20 microM). On the other hand, this response was augmented by ouabain (1 and 2 mM) and unaltered by 5-(N-methyl-N-isobutyl) amiloride (5 and 10 microM). These data suggest that in addition to the sarcolemmal Na+ - Ca2+ exchanger, both sarcolemmal Na+ - K+ ATPase, as well as the sarcoplasmic reticulum Ca2+ -pump play prominent roles in the low Na+ -induced increase in [Ca2+]i.
authors.affiliation: Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Winnipeg, Manitoba, Canada.
authors.forename: Satyajeet S
authors.initials: SS
authors.lastname: Rathi
authors.forename: Harjot K
authors.initials: HK
authors.lastname: Saini
authors.forename: Yan-Jun
authors.initials: YJ
authors.lastname: Xu
authors.forename: Naranjan S
authors.initials: NS
authors.lastname: Dhalla
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D004110:Diltiazem / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives*; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D009532:Nickel / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020746:Calcium Channels, L-Type; D005456:Fluorescent Dyes; D019831:Sodium-Calcium Exchanger; D017923:Sodium-Hydrogen Exchangers; C049925:fura-2-am; D011189:Potassium Chloride; D000584:Amiloride; D009532:Nickel; D012964:Sodium; D014700:Verapamil; D000254:Sodium-Potassium-Exchanging ATPase; D004110:Diltiazem; D002118:Calcium; D016257:Fura-2