Calcium-dependent regulation of potassium permeability in the glial perineurium (blood-brain barrier) of the crayfish

Abstract

The physiological basis of the high selective potassium permeability of the crayfish glial perineurium was studied. The transient spike-like perineurial potential generated in high external [K +] was used as a measure of barrier K + permeability. The medial giant axon membrane potential was used to monitor interstitial [K +]. Perineurial current-voltage relations of the perineurium were used to measure electrical resistance and to determine changes in K + conductance of the perineurial barrier. Of a range of cations studied only Rb +, in addition to K + generated a large transient sheath potential. In some experiments “regenerative” multiple spikes were observed during the continued exposure of the perineurium to high [Rb +] 0. This degree of ion selectivity is typical of glial cell membranes and K channels. Barrier conductance increased only very briefly in Rb +; the potential falling rapidly to a steady 5–10 mV. The P Cl/P Rb, and the P Cl/P k ratios at the peak transient potential were similar suggesting the permeability site for these cations was the same. The permeability of Rb + in the plateau phase was significantly lower than K + suggesting that high [Rb +] 0 may act to block K + channels. The K +-selective permeability was reversibly blocked by extracellular Ba 2+ at both the peak and the plateau phase, in a concentration-dependent manner. Other K-channel blocking agents, tetraethylammonium ions (10 mM), caesium ions (20 mM), and 3,4-diaminopyridine (0.5 mM) were ineffective. The effect of Ba 2+ on the peak potential was similar to the removal of external Ca 2+ or exposure to the Ca 2+-channel blockers, verapamil (10 −4M) or La 3+ (5 mM). The time- and concentration-dependent reversible block of the K + permeability of the perineurium was consistent with the known action of these agents on voltage-gated Ca 2+ channels in nerve and glia. La 3+ caused an irreversible decrease in perineurial conductance and K + influx. Lanthanum titration of the negative charges of glial membranes and mucopolysaccharide matrix of the intercellular space suggest they may be important factors in determining the magnitude of the perineurial leak and paracellular K + permeability. Electron microscopic examination of La 3+ distribution demonstrated a diffusion barrier at the outer layer of perineurial glia. The binding of La 3+ at the basolateral membranes of the glial barrier suggested this was the site at which La 3+ had its physiological actions. The results suggest that the increase in glial membrane K + conductance in high [K +] 0 was most likely due to voltage-gated Ca 2+ and K + channels and Ca 2+-activated K + channels of the membranes of perineurial glia.