Cation transport and membrane potential properties of primary astroglial cultures from neonatal rat brains

Abstract

This paper describes K+ and Na+ content and transport in primary monolayer cultures from dissociated newborn rat brains, considered to consist predominantly of astroglial cells. Net changes in cation content after addition of ouabain, and steady state fluxes using 86Rb+ as a marker for K+ and 22Na+ as a marker for Na+, were measured. The results found indicate that the cells maintained a conventional pattern of cation homeostasis with net efflux of K+ being balanced by its active uptake and net uptake of Na+ balanced by active extrusion mediated by a ouabain sensitive (Na+K) pump. These processes maintained internal measured K+:Na+ ratios of 12--25:1. The cells were normally flat but addition of DBcAMP caused them to round up and form numerous processes, an appearance resembling that of astroglial cells in vivo. DBcAMP treatment also reduced the steady state levels of K+ measured with 86Rb+ by 15--30%, and had no effect on initial rates of 86Rb+ and 22Na+ uptake. The membrane potentials of cells treated with DBcAMP were studied, since only these were easily impaled. The membrane potentials of separate groups of cells gave means ranging from --65 to --75 mV at 35 degrees C, at an external K+ concentration ([K+]o) of 4.5 mM. The dependence of the membrane potentials of individual cells and groups of cells on [K+]o was studied. The slope of the potential per 10-fold change in [K+]o was 55--57 mV, at concentrations of K+ greater than 10--20 mM K+, and diverged from this slope at concentrations below this. This shows that these cells had some permeability to ions other than K+. Assuming that Na+ was the only other ion affecting the membrane potential, it was calculated that the permeability to Na+ was about 30 times less than K+. A similar result was obtained based on estimates of Na+ and K+ permeability from transport experiments on cells also treated with DBcAMP. The results obtained from these cells are compared to those found for other cultured glial cells and glial cells in vivo. We conclude that the membrane potentials of the cultured cells used in the present study show the closest resemblance so far to glia in vivo, since they are large and negative and are determined mainly by K+. However, the cultured cells have different properties from those reported in some studies for glial cells in vivo by showing free permeability to ions other than K+.