CALCIUM DIFFUSION-COEFFICIENT IN MYXICOLA AXOPLASM

Calcium diffusion coefficients were measured in Myxicola axoplasm and in agar controls by two independent techniques: one utilizing Ca-45, and one utilizing Ca-specific mini-electrodes. The lowest value, similar to 0.1 x 10(-6) cm(2).s(-1) was measured, using the mini-electrode technique, in axoplasm with intact Ca-sequenstering organelles. With ATP-depleted axoplasm, diffusion coefficients of 0.5-2 x 10(-6) cm(2).s(-1) were obtained by both isotope and mini-electrode techniques. In organelle-free axoplasm with a protein concentration roughly half that in the intact axoplasm, diffusion coefficients of 1.4-3 x 10(-6) cm(2).s(-1) were measured at 0.7 mu M Ca and 7 x 10(-6) cm(2).s(-1) at 3-5 mu M Ca. When compared with measurements of the calcium buffering capacity [Al-Baldawi NF. Abercrombie RF. (1995) Cytoplasmic Ca buffer capacity determined with Nitr-5 and DM-nitrophen. Cell Calcium, 17, 409-422], these diffusion coefficients require that part of the buffer capacity be located on mobile Ca-binding sites.