Objectives: 

The key source of Ca2+ for intracellular signalling is the endoplasmic reticulum. Its total releasable content of calcium in excitable cells has been directly measured in cardiac myocytes. (Varro A et al 1993 Pflugers arch 423,158-160) but not in neurones.

Materials: 

I have used Ca2+-sensitive in, and pH-sensitive microelectrodes on, large neurones in isolated ganglia of Helix aspersa to measure intracellular Ca2+ (as VCa) and surface pH changes caused by caffeine (20 mM) application, and compared them with changes caused by iontophoretic injection of Ca2+ (for methods see Thomas RC 2009 J Physiol 587; 315-327). Mitochondria appeared to take up about half the Ca2+ released by caffeine, so all experimental cells were pressure-injected with Ru360 to block this uptake.

Results: 

In 8 experiments the mean increase in VCa in response to caffeine was 27.3 ± 2.8 mV (SEM). This was equivalent to almost a ten-fold increase in ionised Ca2+. By injecting Ca2+ iontophoretically into the same 8 cells I was able to estimate that the caffeine responses were equivalent to those caused by an average injection charge of 235 ± 41 nC.

For 7 experiments in which the surface pH changes (recorded in snail Ringer with reduced buffering power) produced by the PMCA pumping in 2H+ in exchange for each Ca2+ were compared, the average response to caffeine of 0.025 ± .005 pH units was equivalent to that induced by an average injection charge of 300 ± 86 nC.

Conclusions: 

Assuming that about half the estimated average cell volume of 4.2 nl was cytoplasm, and that the injection transport index was 0.48, these VCa and pH changes suggest that the total Ca2+ content released by caffeine was about 0.32 mmol/litre, almost 3 times more than for cardiac myocytes.

I am grateful to the department of PDN for support.