Effect of intracellular sodium on calcium uptake in isolated guinea-pig diaphragm and atria

Abstract

(1) Effects of cellular sodium on the 45Ca uptake of isolated guinea-pig diaphragm and atria were studied. (2) Cellular sodium and calcium contents were higher in diaphragm compared to atria after incubating the tissues in normal Krebs-Henseleit solution. (3) Cellular sodium content in atria and diaphragm were reduced significantly by incubating the tissues in high potassium Krebs-Henseleit solution (K+ = 34.7 mM), while it was increased by incubating the tissues in the ice-cold low potassium and low calcium Krebs-Henseleit solution (K+ = 0.65 mM, Ca2+ = 0.2 mM). Cellular potassium content was changed inversely to the sodium content. (4) In atria, cellular content of calcium was not altered significantly by the above conditions. But in diaphragm, the cellular content of calcium was decreased slightly but significantly after incubation in the ice-cold low potassium and low calcium Krebs-Henseleit solution. (5) At normal cellular sodium levels, the 45Ca uptake of both tissues was similar. (6) The reduction of the cellular sodium content caused a significant decrease in the 45Ca uptake in both tissues. (7) When the cellular sodium content was increased in atrial preparations, a marked increase in the 45Ca uptake was observed. On the other hand, in diaphragm preparations, only a slight increase was observed, even when cellular sodium content was much higher than the normal level. (8) These results indicate that even when the intracellular sodium is increased by some physiological of pharmacological events, calcium influx through Na+/Ca2+ exchange mechanism is very slight and slow in diaphragm.