Calcium content of the endoplasmic reticulum of snail neurones releasable by caffeine.

I have used Ca(2+)- and pH-sensitive microelectrodes in and on large neurones in isolated ganglia of Helix aspersa to measure changes in intracellular Ca(2+) (as V(Ca)) and surface pH caused by caffeine (20mM) application, and compared these changes with those caused by iontophoretic injection of Ca(2+). Mitochondrial uptake was blocked by pressure-injection of Ru360. In 8 experiments the mean increase in V(Ca) in response to caffeine was 27.3±2.8mV (SEM) equivalent to a peak tenfold increase in ionised Ca(2+). Iontophoretic injection of Ca(2+) into the same 8 cells showed that the caffeine responses were equivalent to those caused by an average injection charge of 235±41nC. Surface pH changes produced by the PMCA pumping out Ca(2+) released by caffeine averaged 0.025±.005 pH units (n=7), which was equivalent to surface pH changes induced by an average Ca(2+) injection charge of 300±86nC. The average cell volume was 4.2nl. Assuming that the injection transport index was 0.48 as previously measured, these V(Ca) and pH changes suggest that the total Ca(2+) content released by caffeine was about 0.175mmol/l cell volume.