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

I have used Ca2+- and pH-sensitive microelectrodes in and on large neurones in isolated ganglia of Helix aspersa to measure changes in intracellular Ca2+ (as VCa) and surface pH caused by caffeine (20 mM) application, and compared these changes with those caused by iontophoretic injection of Ca2+. Mitochondrial uptake was blocked by pressure-injection of Ru360. In 8 experiments the mean increase in VCa in response to caffeine was 27.3 ± 2.8 mV (SEM) equivalent to a peak tenfold increase in ionised Ca2+. Iontophoretic injection of Ca2+ into the same 8 cells showed that the caffeine responses were equivalent to those caused by an average injection charge of 235 ± 41 nC. Surface pH changes produced by the PMCA pumping out Ca2+ released by caffeine averaged 0.025 ± .005 pH units (n = 7), which was equivalent to surface pH changes induced by an average Ca2+ injection charge of 300 ± 86 nC. The average cell volume was 4.2 nl. Assuming that the injection transport index was 0.48 as previously measured, these VCa and pH changes suggest that the total Ca2+ content released by caffeine was about 0.175 mmol/l cell volume.