The sarcolemmal mechanisms involved in the control of diastolic intracellular calcium in isolated rat cardiac trabeculae.
We performed experiments using the calcium indicator Indo-1 to determine the relative roles of the sarcolemmal mechanisms involved in the regulation of diastolic intracellular calcium concentration ([Ca2+]i) in trabeculae from the rat heart. Ryanodine was used to eliminate sarcoplasmic reticulum (SR) function. In the functional absence of the SR, 76.8 +/- 3.9% of the calcium was extruded by the Na-Ca exchange carrier in the [Ca2+]i range of diastolic concentration +/- 200-400 nM. This was assessed by measuring the recovery of [Ca2+]i from small perturbations in the presence and absence of extracellular sodium. The steady-state relationship between [Ca2+]o and [Ca2+]i was linear over the range of 1-40 mM, a 20-fold increase of [Ca2+]o produced a 1.97-fold +/- 0.13-fold increase in [Ca2+]i (n = 5). In the absence of extracellular sodium raising [Ca2+]o had a variable effect. In some preparations there was little change of [Ca2+]i while in others the response was almost as large as in control conditions. We conclude that the Na-Ca exchanger contributes approximately 77% of sarcolemmal calcium extrusion following small perturbations in [Ca2+]i and that this fraction does not diminish as the [Ca2+]i declines. In addition we have shown a sodium-independent entry of calcium into quiescent cardiac muscle under resting conditions.
Lamont
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Department of Veterinary Preclinical Sciences, University of Liverpool, PO Box 147, Liverpool, L69 3BX, UK.
Eisner
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D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism; D003971:Diastole; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D007211:Indoles; D008297:Male; D009206:Myocardium / Q000648:ultrastructure*; D051381:Rats; D017208:Rats, Wistar; D012433:Ryanodine / Q000494:pharmacology; D012508:Sarcolemma / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000502:physiology; D012964:Sodium / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007211:Indoles; D019831:Sodium-Calcium Exchanger; D012433:Ryanodine; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; C048960:indo-1; D002118:Calcium