Acidosis inhibits calcium release from intracellular stores in cardiac muscle (e.g. Hulme & Orchard, 1998) but little is known about its effect on calcium release in neurones (Willoughby et al. 2001). To explore the effects of low pHi on the response to caffeine in snail (Helix aspersa) neurones in isolated ganglia I have recorded [Ca2+]i and pHi using ion-sensitive microelectrodes, and lowered pHi by injecting HCl. This method minimises the risk of changing pH inside subcellular organelles inherent in the more widely used method of adding or removing weak acids or bases.
In the experiment shown in Fig. 1, caffeine always produced a clear calcium release, followed by an undershoot, as long as pHi was higher than 7. At more acid pHi values caffeine had no effect. In this and another similar experiment, in both of which pHi was held steady while caffeine was applied, 10 mM caffeine caused a clear release of calcium when applied over a range from 7.1 to 7.9 (n = 19), but no release when pHi was less than 7.1 (n = 6). In other experiments in which the caffeine concentration was raised to 50 mM, it was possible to induce a calcium release at pHi values as low as 6.7. Such low values are, however, hard to sustain. Thus calcium signalling may be impaired in snail neurones rendered acidotic by exercise or anoxia.
This work was funded by the MRC.
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| Figure 1. Experiment to show the effect of acidosis on the response of [Ca2+]i to superfusion with caffeine; methods as described previously (Willoughby et al. 2001). The snail Ringer solution was bicarbonate free to inhibit pHi regulation except where indicated. The membrane potential was recorded with a microelectrode filled with 2 M KCl, and the voltage-clamp electrode was filled with 1 M CsCl. The ion-sensitive microelectrodes were made from boro- or aluminosilicate glass micropipettes silanized as previously described (Collins & Thomas, 2001). To decrease pHi I injected HCl by passing current through a fifth microelectrode filled with 1 M HCl, inserted (arrow) some time after the other electrodes. Thus the clamp current is the sum of the currents used to clamp the membrane potential and inject HCl. |
![\"Figure 1. Experiment to show the effect of acidosis on the response of [Ca2+]i to superfusion with caffeine; methods as described previously (Willoughby et al. 2001). The snail Ringer solution was bicarbonate free to inhibit pHi regulation except where indicated. The membrane potential was recorded with a microelectrode filled with 2 M KCl, and the voltage-clamp electrode was filled with 1 M CsCl. The ion-sensitive microelectrodes were made from boro- or aluminosilicate glass micropipettes silanized as previously described (Collins & Thomas, 2001). To decrease pHi I injected HCl by passing current through a fifth microelectrode filled with 1 M HCl, inserted (arrow) some time after the other electrodes. Thus the clamp current is the sum of the currents used to clamp the membrane potential and inject HCl.\"](https://static.physoc.org/app/uploads/2019/01/22203808/S256.gif)