Factors influencing free intracellular calcium concentration in quiescent ferret ventricular muscle

1. The photoprotein aequorin was injected into cells of ferret papillary muscles to monitor the resting intracellular free Ca concentration ([Ca2+](i)). 2. Increasing the external Ca concentration ([Ca2+](o)) increased both resting [Ca2+](i) and resting tension. The tension and [Ca2+](i) both rose to a peak and then declined to a steady-state level which was higher than the control. Qualitatively similar, but larger, effects were observed if [Ca2+](i) was first elevated with strophanthidin. The increase of [Ca2+](i) was accompanied by the development of spontaneous oscillations of [Ca2+](i). 3. When a steady level of [Ca2+](i) had been reached in high [Ca2+](o), [Ca2+](o) was reduced back to the control level for a brief period. A subsequent increase of [Ca2+](o) produced a rise of [Ca2+](i) to the same steady level as that previously found in the high [Ca2+](o) but the initial peak and subsequent decline were absent. It is suggested that the decline of [Ca2+](i) from the initial peak is mediated by a fall of intracellular Na concentration (Na+](i)) limiting Ca entry on a Na-Ca exchange. 4. Increasing external K concentration (K[K+](o)) from 5 to 30 ml/l had no detectable effect on [Ca2+](i) under control conditions. However, if [Ca2+](i) was first increased either by applying strophanthidin or by increasing [Ca2+](o), increasing [K+](o) produced a transient rise of [Ca2+](i) and tension. This rise was unaffected by D600. It is suggested that the secondary decline of [Ca2+](i) after the initial rise may, again, be produced by a fall of [Na+](i) acting on an Na-Ca exchange. 5. Acification produced by increasing [CO2] had no detectable effect on [Ca2+](i) under control conditions. However, if [Ca2+](i) was increased by strophanthidin, acidification produced a rise of [Ca2+](i). This rise of [Ca2+](i) was partly transient even when the intracellular acidification was presumably maintained (raising CO2 at constant [HCO3-]). Acidification in Na-free solutions had qualitatively similar effects to those in Na-containing solutions. 6. In Na-free solutions (Na replaced by K) the [Ca2+](i) could be maintained at a low level for at least several hours. Increases of [Ca2+](o) in Na-free solutions led to a decrease of [Ca2+](i), and similarly decreasing [Ca2+](o) led to an increase in [Ca2+](i). These anomalous effects of [Ca2+](o) on [Ca2+](i) could be abolished by Mn ions or D600.It is suggested that changes in [Ca2+](o) may have reciprocal effects on Ca permeability and hence on [Ca2+](i). 7. The application of the mitochondrial uncoupler FCCP in Na-free solutions led to an increase of resting tension followed, after a substantial delay, by an increase of [Ca2+](i). It is therefore suggested that in these conditions the maintenance of a low [Ca2+](i) involves a metabolism-dependent process which fails at an [ATP] lower than that at which rigor develops. 8. It is concluded that: (i) in Na-containing solutions changes of [Na+](i) may affect the response of [Ca2+](o) or membrane potential and (ii) there is a metabolism-dependent mechanism which is capable of maintaining a low [Ca2+](i), even in the absence of Na-Ca exchange.