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 andNarose to 70 m . After addition of 6 m Mg2+and lowering extracellular Na2+to 29 m , 30m Rb+was added, and over 10–15 minaiNarecovered to 3–7 m . Two load–recovery cycles were conducted in 10 fibers. During one of the cycles Ca2+(0.1–1.0 m ) was added before Rb+, causing a contracture. During recoveryaiNafell faster during Ca2+contracture than in control cycles. Between 30 and 20 m the rates were −10.0±1.6 and −5.4±0.6 m /min, respectively (P<0.05). In Ca2+-exposed fibers tension fell almost parallel withaiNa. Na,K-pump reactivation caused membrane potential (Vm) to hyperpolarize transiently to −70 mV. Ca2+did not affect membrane conductance. For a givenaiNaduring reactivation, Vmwas 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 ( ) 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% andiNathat causes 50% pump activation (k0.5) was lowered from 21.2±1.6 to 15.5±1.4 m