CA2+ ENTRY THROUGH NA+-CA2+ EXCHANGE CAN TRIGGER CA2+ RELEASE FROM CA2+ STORES IN NA+-LOADED GUINEA-PIG CORONARY MYOCYTES

1. The ionized cytosolic calcium concentration ([Ca2+]i) was monitored in voltage-clamped coronary myocytes at 36-degrees-C and 2.5 mm [Ca2+]o using the Ca'' indicator indo-1. [Ca2+]i was transiently increased by fast application of 10 mm caffeine, and the mechanisms involved in decay of [Ca2+]i were analysed. 2. Resting [Ca2+]i Was 166 +/- 62 nm (mean +/- S.D.). Caffeine increased [Ca2+]i within 1-2 s to 1618 +/- 490 nm. In the continuous presence of caffeine [Ca2+]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 [Ca2+]i to 105 +/- 30 nm. When caffeine was applied repetitively the [Ca2+]i transients were of reduced amplitude indicating that the store had lost a part of releasable Ca2+. 3. After a 1 s caffeine application [Ca2+]i decayed with a half-time of 2.3 +/- 0.8 s to the undershoot of 112 +/- 57 nm. The decay of [Ca2+]i was largely prevented by 3 mm [La3+]o; after wash-out of La3+ [Ca2+]i fell to the resting value without an undershoot. The results demonstrate that La3+-sensitive Ca2+ extrusion contributes to the decay of the [Ca2+]i transient and to the undershoot. 4. With 10 mm [Na+]i, sodium removal from the bath incremented [Ca2+]i in three out of ten cells by 71 +/- 11 nm; in the other cells [Ca2+]i did not change. In the absence of extracellular sodium the decay of [Ca2+]i after wash-out of caffeine was not retarded. 5. To stimulate Na+-Ca2+ exchange, cells were dialysed with pipette solution containing 150 mm NaCl. Elevation of [Na+]i had no significant effect on the resting [Ca2+]i (180 +/- 47 nM) or on the caffeine-induced [Ca2+]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 [Ca2+]i, although responses varied in amplitude (from 130 to 2300 nm) and rate of rise. In the absence of sodium [Ca2+]i remained elevated. After a 1 s caffeine application the undershoot of [Ca2+]i was abolished in sodium-free solution. When caffeine was applied in sodium-free solution, the [Ca2+]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 Ca2+ stores with caffeine. Ryanodine pretreatment abolished the caffeine-induced [Ca2+]i transients and reduced [Ca2+]i response due to sodium removal. The ryanodine-insensitive component of the [Ca2+]i transient may originate directly from Ca2+ influx through Na+-Ca2+ exchange, the ryanodine-sensitive one from sarcoplasmic reticulum (SR) Ca2+ release triggered by Ca2+ influx through Na+-Ca2+ exchange. 8. The results suggest that the Na+-Ca2+ exchanger is of minor importance for the [Ca2+]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, Ca2+ influx through Na+-Ca2+ exchange can trigger Ca2+ release from the SR which, thus, amplifies the Ca2+ signals according to the SR Ca2+ load.