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.
|Number of pages||15|
|Journal||Journal of Physiology|
|Publication status||Published - 1984|