Calcium induced contracture stimulates Na,K-pump rate in isolated sheep cardiac Purkinje fibers.
By keeping intracellular Na+ (aiNa) low, the Na,K-pump can prevent Ca2+ overload of cardiomyocytes. We therefore examined whether Ca2+ stimulates Na,K-pump activity in sheep cardiac Purkinje fibers. By removing Ca2+, Mg2+ and K+, the fibers depolarized and aiNa rose to 70 mM. After addition of 6 mM Mg2+ and lowering extracellular Na2+ to 29 mM, 30mM Rb+ was added, and over 10-15 min aiNa recovered to 3-7 mM. Two load-recovery cycles were conducted in 10 fibers. During one of the cycles Ca2+ (0.1-1.0 mM) was added before Rb+, causing a contracture. During recovery aiNa fell faster during Ca2+ contracture than in control cycles. Between 30 and 20 mM the rates were -10.0+/-1.6 and -5.4+/-0.6 mM/min, respectively (P<0.05). In Ca2+-exposed fibers tension fell almost parallel with aiNa. Na, K-pump reactivation caused membrane potential (Vm) to hyperpolarize transiently to -70 mV. Ca2+ did not affect membrane conductance. For a given aiNa during reactivation, Vm was more negative during Ca2+ contracture and depolarized faster (P<0.05). Intracellular pH (pHi) fell from 7.11+/-0.05 to 6.92+/-0.08 (n.s.) during control load-recovery cycles and was 6.83+/-0.14 at the end of the Ca2+ cycles. ATP content of the fibers did not change significantly through two complete load-recovery cycles, but creatine phosphate (CrP) fell by about 40%. By fitting the data to a model incorporating the Hill equation we show that during Ca2+-induced contracture maximum Na,K-pump rate (Vmax) was increased by about 40% and aiNa that causes 50% pump activation (k0.5) was lowered from 21. 2+/-1.6 to 15.5+/-1.4 mM.
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Institute for Experimental Medical Research, University of Oslo, Ullevaal Hospital, Oslo, N-0407, Norway.
Sejersted
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D000255:Adenosine Triphosphate / Q000032:analysis; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D004558:Electric Stimulation; D004789:Enzyme Activation / Q000187:drug effects; D006003:Glycogen / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D010725:Phosphocreatine / Q000032:analysis; D011188:Potassium / Q000378:metabolism; D011690:Purkinje Fibers / Q000187:drug effects* / Q000201:enzymology; D012756:Sheep; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D010725:Phosphocreatine; D000255:Adenosine Triphosphate; D006003:Glycogen; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D002118:Calcium