Behavior of extracellular K+ and pH in skate (Raja erinacea) cerebellum

Ion-selective microelectrodes were used to measure extracellular K+ concentrations ([K+]o) and extracellular pH (pHo) in skate cerebellum under resting and stimulated conditions. Consistent with earlier ion analysis of elasmobranch cerebrospinal fluid (CSF), [K+]o was 3.6 +/- 0.1 mM. During parallel fiber activation, [K+]o increased to an upper limit of 12-14 mM with an approximately linear dependence on stimulation frequency (1-20 Hz). Post-stimulus undershoots of 0.1-0.6 mM were seen throughout an animal temperature range of 13-18 degrees C. When stimulation produced spreading depression (SD), [K+]o first increased to about 10 mM, then rose more rapidly to about 30 mM. These observations indicate a K+ ceiling of 10-12 mM in elasmobranchs. This ceiling is the same as that seen in mammals, despite marked differences in CSF composition and osmolality between mammalian and elasmobranch species. Extracellular pH (resting pHo was 7.1-7.3) was also altered during parallel fiber stimulation. An initial alkaline shift and subsequent extracellular acidification were characteristic of the response. These pHo transients were similar to those reported in other preparations, although the alkaline shift was enhanced. This may be attributed to the relatively low buffering capacity of elasmobranch CSF and to summation with a generally smaller acid shift.