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(Circulation Research. 1995;77:354-360.)
© 1995 American Heart Association, Inc.

Articles

Effects of Rest Interval on the Release of Calcium From the Sarcoplasmic Reticulum in Isolated Guinea Pig Ventricular Myocytes

Cesare M. N. Terracciano, Ruby U. Naqvi, Kenneth T. MacLeod

From the Department of Cardiac Medicine, National Heart and Lung Institute, University of London, London, UK.

Correspondence to Kenneth T. MacLeod, Department of Cardiac Medicine, National Heart and Lung Institute, University of London, Dovehouse St, London, SW3 6LY, UK. E-mail 100044.761@compuserve.com k.t.macleod@ucl.ac.uk.

Abstract Guinea pig cardiac myocytes were loaded with the fluorescent dye indo 1, and cell contraction was measured by a video edge-detection system. Ca²⁺ was released from the sarcoplasmic reticulum (SR) by rapidly cooling the myocytes or by rapid application of 10 mmol/L caffeine. Estimates of the amount of Ca²⁺ released from the SR after different rest intervals (ie, under different loading conditions) were obtained by measuring the current evoked by rapid application of 10 mmol/L caffeine, which we call Na⁺/Ca²⁺ exchange current. This current is completely inhibited by removal of extracellular Na⁺ and Ca²⁺ or by application of 5 mmol/L Ni²⁺. SR Ca²⁺ release after rest intervals of 5 to 120 seconds (assuming cell volume to be 30x10^-12 L) was estimated to be 57.8±5.7 to 25.7±4.5 µmol/L accessible cell volume, respectively, equivalent to 23 to 10 µmol/kg wet wt, respectively. There was an exponential decline in Ca²⁺ released from the SR after rest intervals of 2 to 120 seconds (rate constant, 0.029 s^-1; t_1/2, 24 seconds); thereafter, there remained a portion (56%) of Ca²⁺ releasable to caffeine application. We found a similar exponential decay (rate constant, 0.020 s^-1; t_1/2, 35 seconds) of the size of rapid cooling contractures with increasing rest intervals. The time to peak of the Na⁺/Ca²⁺ exchange current in the presence of caffeine slowed at long rest intervals, ie, at smaller SR loads. A decrease in SR load of 50% increased the time to peak of the exchange current by 213±37% (n=6). The rate of generation of a rapid cooling contracture was faster after short rest intervals and was associated with a faster rise in the corresponding increase in indo 1 fluorescence. A decrease in SR load of 50% decreased the rate of cell shortening to 34±5% and decreased the rate of change in fluorescence to 54±2% (n=5).

Key Words: sarcoplasmic reticulum • cardiac myocyte • Ca²⁺ release • caffeine • Na⁺/Ca²⁺ exchange current

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