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Calcium entry through sodium-calcium exchange can trigger calcium release from calcium stores in sodium-loaded guinea-pig coronary myocytes



Calcium entry through sodium-calcium exchange can trigger calcium release from calcium stores in sodium-loaded guinea-pig coronary myocytes



Journal of Physiology (Cambridge) 468(0): 225-243



1. The ionized cytosolic calcium concentration ((Ca-2+)-i) was monitored in voltage-clamped coronary myocytes at 36 degree C and 2.5 mM (Ca-2+)-o using the Ca-2+ indicator indo-1. (Ca-2+)-i was transiently increased by fast application of 10 mM caffeine, and the mechanisms involved in decay of (Ca-2+)-i were analysed. 2. Resting (Ca-2+)-i was 166 +- 62 nM (mean +- S.D.). Caffeine increased (Ca-2+)-i within 1-2 s to 1618 +- 490 nM. In the continuous presence of caffeine (Ca-2+)-i fell close to resting values with a half-decay time of 5.0 +- 1.6 s. Wash-out of caffeine induced an undershoot of (Ca-2+)-i to 105 +- 30 nM. When caffeine was applied repetitively the (Ca-2+)-i transients were of reduced amplitude indicating that the store had lost a part of releasable Ca-2+. 3. After a 1 s caffeine application (Ca-2+)-i decayed with a half-time of 2.3 +- 0.8 s to the undershoot of 112 +- 57 nM. The decay of (Ca-2+)-i was largely prevented by 3 mM (La-3+)-o; after wash-out of La-3+ (Ca-2+)-i fell to the resting value without an undershoot. The results demonstrate that La-3+-sensitive Ca-2+ extrusion contributes to the decay of the (Ca-2+)-i transient and to the undershoot. 4. With 10 mM (Na)-i, sodium removal from the bath incremented (Ca-2+)-i in three out of ten cells by 71 +- 11 nm; in the other cells (Ca-2+)-i did not change. In the absence of extracellular sodium the decay of (Ca-2+)-i after wash-out of caffeine was not retarded. 5. To stimulate Na+-Ca-2+ exchange, cells were dialyzed with pipette solution containing 150 mM NaCl. Elevation of (Na+)-i had no significant effect on the resting (Ca-2+)-i (180 +- 47 nM) or on the caffeine-induced (Ca-2+)-i transients (peak 1614 +- 530 nm, half-time of decay 3 s, undershoot 107 +- 40 nm). 6. With 150 mM (Na+)-i, sodium removal resulted in an increase of (Ca-2+)i, although responses varied in amplitude (from 130 to 2300 nm) and rate of rise. In the absence of sodium (Ca-2+)-i remained elevated. After a 1 s caffeine application the undershoot of (Ca-2+)-i was abolished in sodium-free solution. When caffeine was applied in sodium-free. solution, the (Ca-2+)-i transient decayed to a sustained level and the following caffeine response was attenuated. 7. With 150 mM (Na+)-i, the effects of sodium removal were strongly suppressed by a preceding depletion of the Ca-2+ stores with caffeine. Ryanodine pretreatment abolished the caffeine-induced (Ca-2+)-i transients and reduced (Ca-2+)-i response due to sodium removal. The ryanodine-insensitive component of the (Ca-2+)-i transient may originate directly from Ca-2+ influx through Na+-Ca-2+ exchange, the ryanodine-sensitive one from sarcoplasmic reticulum (SR) Ca-2+ release triggered by Ca-2+ influx through Na+-Ca-2 exchange. 8. The results suggest that the Na+-Ca-2+ exchanger is of minor importance for the (Ca-2+)-i transients of guinea-pig coronary myocytes as long as (Na+)-i is at the physiological level of 10 mM. When stimulated by 150 mM (Na+)-i, Ca-2+ influx through Na+-Ca-2+ exchange can trigger Ca" release from the SR which, thus, amplifies the Ca-2+ signals according to the SR Ca-2+ load.

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