Intracellular calcium and sodium activity in sheep heart Purkinje fibres - Effect of changes of external sodium and intracellular pH

Intracellular Ca, Na and H selective microelectrodes were used to study the effects of reduction of the extracellular Na concentration, [Na]₀, on the free intracellular Ca concentration, [Ca]_i, Na activity, (a_Na ⁱ), and intracellular pH (pH_i) in sheep heart Purkinje fibres. 1. Reduction of [Na]₀ from 140 mM to 14 mM produced a decrease of a_Na ⁱ, and increase of free [Ca]_i, and normally an increase of resting tension. 2. Inhibition of the Na-K pump by 10^-5 M acetylstrophanthidin produced a slow rise of [Ca]_i and resting tension. 3. The magnitude of the increase of free [Ca]_i (and tension) produced by [Na]₀ reduction was greatly enhanced when the Na-K pump is inhibited by either acetylstrophanthidin or K-free solutions. 4. We suggest that this enhanced rise of free [Ca]_i in the presence of Na-K pump inhibition is due to Ca loading of intracellular Ca buffering systems during the pump inhibition. 5. Addition of NH₄Cl produced a transient decrease of free [Ca]_i that accompanied an alkaline change in pH_i. Removal of NH₄Cl (which produced a transient intracellular acidification) produced a transient increase of free [Ca]_i. We conclude that a close relationship exists between the control of free [Ca]_i and pH_i which may be due to competition at, or common use of, intracellular buffering systems.