DIVALENT CATIONS IN CEREBRAL IMPEDANCE AND CELL MEMBRANE MORPHOLOGY

Electrical impedance of cerebral cortex was measured at 1.0 kHz with low current levels (10-13 A per square micron of electrode surface). Changes in impedance were observed after topical application of calcium and magnesium solutions. These modifications were further evaluated by asphyxiation, and by dehydration with intravenous urea. Asphyxiation of untreated cortex caused an initial fall in resistance and raised capacitance rapidly followed by raised resistance and reduced capacitance. Repeated topical application of small amounts of calcium solutions induced a progressive rise in resistance, typically between 20 and 30% of baseline values 10-20 KΩ. This increased impedance reached a plateau after 30-50 min, and further application of calcium was without effect. By contrast with untreated cortex, asphyxiation was followed by a rapid fall in resistance to values around baseline levels. This impedance curve was thus biphasic, with an inverse relationship between the size of the calcium-induced rise and the ensuing fall during asphyxiation. On the other hand, magnesium ions in equivalent amounts exerted only minor effects on cortical impedance, and subsequent asphyxiation produced a large impedance increase, as in untreated cortex. However, magnesium ions produced effects similar to calcium in normal and asphyxiated cortex, if the cortex was first dehydrated with intravenous urea, and even though urea alone was without major effects on impedance. Prior dehydration with urea had no effect on the course of impedance changes produced by calcium. These findings are interpreted in terms of conductance of impedance measuring currents mainly through an extracellular space containing a hydrated network of macromolecules that blend with similar external coats on cell membrane, and which bear numerous cationic binding sites. Evidence is presented in electron micrographs prepared with aldehyde fixation that excess calcium ions narrow intercellular spaces and that these changes reverse with asphyxiation. No such narrowing was induced by urea. On the other hand, magnesium ions were associated with much augmented intercellular spaces. It is proposed that conductance in the extracellular space is determined by the state of its structural elements, including interdependent factors such as hydration and divalent cation binding, as well as by changes in its cross-sectional area. The preeminent role of calcium ions in altering tissue impedance is discussed in relation to membrane excitation, with competitive binding of calcium and hydrogen ions to anionic macromolecular sites. Evidence relating these results to information storage at the neuronal surface is discussed. © 1969.