Studies of axon-glial cell interactions and periaxonal K + homeostasis—III. The effect of anisosmotic media and potassium on the relationship between the resistance in series with the axon membrane and glial cell volume

Abstract

The effect of anisosmotic physiological solutions and [K +] 0 on the resistance in series with the axon membrane were studied in medial giant axons of the crayfish, Procambarus clarkii, to determine if changes in series resistance are correlated with changes in glial cell volume and volume regulatory responses. Series resistance was estimated from computer analysed voltage waveforms generated by constant current and space clamp techniques using piggy-back axial wire current passing and glass pipette recording electrodes. Axons subjected to anisosmotic physiological solution in the range of 23 to 175% of isosmolar solution demonstrated that the series resistance of axons changes in a manner similar to that expected for a volume change in isolated cells. In hyperosmotic solution the series resistance changes biphasically, initially decreasing followed by a recovery of the series resistance, similar to the regulatory volume increase described for glial cells in culture. The increase in series resistance following the initial decrease is inhibited by bumetanide (0.1 mM). Ouabain (1 mM), an inhibitor of the volume decreasing Na-K pump, causes the series resistance to increase significantly above that seen for the no-drug control. Bumetanide, an inhibitor of the volume increasing Na-K-Cl cotransporter, inhibits the volume regulatory response to anisosmotic media. Treating the axon with three times normal external [K +] causes the series resistance to decrease approximately 15% while five times normal [K +] leads to a 15% increase in series resistance. Both ouabain and d-tubocurare (10 −8M) prevent the three-fold [K +]-induced decrease in series resistance while carbachol (10 −8M) and bumetanide have little effect. On the other hand, ouabain enhances the five-fold [K +]-induced increase in series resistance while carbachol and bumetanide cause the five-fold [K +] response to be in a decreasing direction. d-Tubocurare has little effect on the five-fold [K +]-induced increase in series resistance. The study demonstrates that under the conditions of these experiments changes in series resistance are a reflection of changes in cell volume modulated by ouabain- and bumetanide-sensitive K + uptake mechanisms. The effects of carbachol and d-tubocurare on the series resistance suggest that their effects are modulated through their actions on the glial cell membrane potential and the electrochemical gradient for K +, which in turn controls the amount of K + that appears in the periaxonal space. A comprehensive model for K + homeostasis of the periaxonal space is presented and integrates the influences of glial transport processes and cholinergically controlled membrane potential of the glial cell at rest and during action potential generation.