Calcium compartmentation in cardiac tissue culture: the effects of extracellular sodium depletion.

The effect of extracellular sodium depletion upon cellular calcium distribution in myocardial tissue culture is studied with the use of sucrose or lithium chloride as substitutes for sodium. With sucrose substitution more than 50% of the increase in calcium at the cellular surface secondary to [Na]0 depletion is probably localized to a screening layer with the remainder bound to the sarcolemma of the cell. Calcium distribution (sucrose substituted) is studied under three different perfusion conditions: (1) In HEPES-buffered perfusate (pH = 7.1-7.35) the 113% gain in Ca at the cellular surface upon 33 mM [Na]0 perfusion is rapidly exchangeable, completely lanthanum (La) displaceable and more than 90% reversible. 5 X 10(-5) verapamil does not affect the response. (2) In 10 mM Pi (phosphate) solution at pH = 7.15, in which a slowly exchangeable mitochondrial Ca compartment is added, the gain in Ca after low [Na]0 is again rapidly exchangeable and 85% reversible that is, similar to the response in HEPES-buffered solution. (3) In 10 mM Pi solution at pH = 7.35 in which slow phase calcium uptake in 133 mM [Na]0 is increased, the initial rapid Ca response to low [Na]0 is similar to that in (1) and (2) but the slow phase uptake rate is further increased by 3.75 times. The major portion of this uptake is not reversible upon return to 133 mM [Na]0. The results distinguish two markedly different effects of [Na]0 depletion in tissue culture dependent upon the pre Na-depletion state of Ca compartmentation: (1) Addition of almost all the calcium to rapidly exchangeable sites at the cellular surface when exchange is limited to these sites and to mitochondria. (2) Further addition of calcium to more slowly exchangeable cellular site(s) when slow phase uptake is increased, by elevation of Pi at pH = 7.35, prior to low [Na]0 perfusion.