Univalent cations and the respiration of brain-cortex slices

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Abstract
Univalent cations fall into three distinct groups as regards their influence on the respiratory rate of rat-brain-cortex slices incubated in media with glucose as substrate. The first group comprises sodium, which exerts the following functions (i) It maintains the unstimulated oxygen uptake of the slices during prolonged incubation; this function is to some extent shared by choline, caesium, rubidium, and potassium, (ii) It serves in low concentration, as cofactor for stimulating agents (electrical pulses; potassium and other univalent ions); this function is unique for sodium. (iii) It antagonizes, in higher concentration, the stimulating cations. In the second group of ions are found ammonium, caesium, rubidium, potassium, and choline; when added to slices in a sodium-containing medium, they produce an initial stimulation and a subsequent, relatively rapid, fall of the respiratory rate. The various ions evoke almost the same maximal stimulation, but the concentrations required for stimulation differ considerably among the ions, with ammonium as the most and choline as the least active ions. Lithium constitutes the third group; even in the absence of sodium, it produces a high initial respiratory rate followed by a rapid fall. The ion presumably acts as its own cofactor. The stimulating ions of the second and third group counteract the effect of high sodium concentrations. There is probably a competition between these ions and sodium for sites on an enzyme or at a cellular structure. The unstimulated and the stimulated respiratory rates are affected by variations in the magnesium and calcium concentrations in the medium, but a competition between these ions and the univalent ions is unlikely.