Effects of anions and dinitrophenol on resting membrane potentials of insect muscle fibres

Exchange of part of the chloride ions in the saline for an equivalent amount of sulphate or for divalent or trivalent carboxylate ions, results in transient depolarizations of insect muscle fibres. Exchange of chloride for monocarboxylates (with a pK_a≥4·0) results in reversible hyperpolarizations, preceded by short lasting depolarizations. A positive correlation between the pK_a values and the hyperpolarizing activity of the different monocarboxylates has been demonstrated.Exchange of chloride for bicarbonate causes a reversible increase in membrane conductance.The hyperpolarizations are absent in aged preparations, in fresh preparations at low temperature, and after depolarization by dinitrophenol (DNP). The initial depolarizations remain unaltered under these conditions.Omission of potassium enhances the hyperpolarizing effect of monocarboxylates and the presence of monocarboxylates enhances the effect of omission of potassium. The ‘potassium-electrode’ properties of the muscle fibres are not markedly changed either by dinitrophenol or by monocarboxylates.Bicarbonate and propionate cause swelling of the cytoplasm. The swelling by these ions does not occur in a potassium-free saline.It is proposed that the hyperpolarizing effects of monocarboxylates are caused by an efflux of protons formed intracellularly from monocarboxylates penetrated in the protonated form through the membrane.