Effects of Calcium Ions on Prolonged Action Potentials and Hyperpolarizing Responses

Abstract

THE membrane of excitable tissue is capable of maintaining two different states, namely, either the resting state or the acting state. These two states are defined in terms of the membrane permeability to sodium and potassium ions; P_Na/P_K is small in the resting state whereas it is large in the acting state. Our recent experimental results suggest that the membrane potential may be dependent on the state of the membrane calcium which is probably located in a layer of lipoproteins¹. In other words, the resting state of the membrane will be the condition in which calcium ions are associated with the membrane, and the acting state, the condition in which these ions are dissociated from the membrane. This concept is supported by the recent findings that P_Na/P_K is markedly increased when the membrane calcium is removed². It is expected that the dissociation of the membrane calcium will be impeded when the concentration of calcium in the external solution is high, and accelerated when it is low or nullified, The membrane, therefore, will tend to stay in the resting state or acting state depending on the external concentration of calcium. Indeed, the excitable membrane is depolarized and often initiates action potentials spontaneously when the concentration of calcium in the external solution is reduced. Furthermore, it has recently been found that the action potential of frog's pinal ganglion cells is markedly prolonged in Ringer's solution containing no calcium ions³.