Dynamic and static calcium gradients inside large snail (Helix aspersa) neurones detected with calcium-sensitive microelectrodes

We have used quartz Ca(2+)-sensitive microelectrodes (CASMs) in large voltage-clamped snail neurones to investigate the inward spread of Ca(2+) after a brief depolarisation. Both steady state and [Ca(2+)](i) transients changed with depth of penetration. When the CASM tip was within 20 μm of the far side of the cell the [Ca(2+)](i) transient time to peak was 4.4 ± 0.5 s, rising to 14.7 ± 0.7 s at a distance of 80 μm. We estimate that the Ca(2+) transients travelled centripetally at an average speed of 6 μm(2) s(−1) and decreased in size by half over a distance of about 45 μm. Cyclopiazonic acid had little effect on the size and time to peak of Ca(2+) transients but slowed their recovery significantly. This suggests that the endoplasmic reticulum curtails rather than reinforces the transients. Injecting the calcium buffer BAPTA made the Ca(2+) transients more uniform in size and increased their times to peak and rates of recovery near the membrane. We have developed a computational model for the transients, which includes diffusion, uptake and Ca(2+) extrusion. Good fits were obtained with a rather large apparent diffusion coefficient of about 90 ± 20 μm(2) s(−1).This may assist fast recovery by extrusion.