The Excitable System in the Cell Surface

Abstract

A membrane model, composed of phospholipid and cholesterol, is described. The electrical resistance and hydration of this model can be controlled by manipulation of ambient ions and by current in ways strongly remiiniscent of the behavior of living cells. The behavior of the model may resemble that of the membrane component of the cell. In addition, an interdependent, lipid-protein molecular structure may exist at the cell surface.T HE ANNOUNCED TITLIE of our paper, "Further Studies on the Nature of the Excitable System in the Cell Surface," tells you the nature of the general area of our interest, but it does not tell you just what we are going to talk about. Therefore, I should like, at the outset, to give you a more precise idea of our subject. What we shall discuss first is the control of the resistance of a phospholipid-cholesterol membrane model by competition between calcium and potassium. We shall describe what we have done, what we know about mechanism, how the system responds reversibly to current flow in a way which may give us insight into the molecular underlay of excitation, and some other parallels between the model and living cells. Following this, we wish to draw attention to another matter which may, at this point, be of interest more to the chemist than to the physiologist, namely, an example of structural interdependence of lipid and protein in periaxoplasmic lamellae.But before we do this, we want to put the work in proper perspective. We have asked a question, and over a period of several years we have made a number of approaches to answering it, of which this is the third. Let ine recapitulate briefly:

Transport of proteins dissolved in organic solvents across biomimetic membranes.

“…Hence realistic biomimetic barriers should do the same. Lipid-impregnated polymeric filters meet these requirements, and indeed they have been used to mimic the barrier properties of the skin and other biological membranes (16,(25)(26)(27). We have selected this approach for the present investigation as well.…”

“…At certain time intervals, samples were withdrawn from the receiver chamber and, after the removal of turbidity (caused by minor stripping of the lipids from the membrane) by centrifugation (30,000 x g, 0.5 hr), were analyzed for the protein or nucleic acid content as described above. All (16,17). The weight gain of the membrane after impregnation and drying was measured to be 220% ± 5%.…”

Excitable behavior can explain the “ping‐pong” mode of communication between cells using the same chemoattractant

“…The ability to directionally relay the activity pulse makes a spatially distributed system consisting of excitable elements a suitable medium for the propagation of information. Not surprisingly, the concept of excitability emerged in biology in the context of nerve pulse propagation [21][22][23][24][25][26]. In 1952 Hodgkin and Huxley [27] developed an elaborate electro-chemical model that for the first time described the initiation and propagation of the action potential along the axon.…”

Electrical Properties of Neurospora crassa

“…9). Without specifying the behavior of E, and R,, the model requires only that R~on behave like the integral resistance across a potassium-specific membrane in order that the internal potential should vary with log [K] o along a slope that is significantly less than the Nernst slope at low for if calcium simply increased R~o~, an effect which is well known in other biological systems (13,43). Potential measurements in this and the preceding paper can be used to calculate the ratio Rio~/R, under a variety of circumstances: raising calcium from 0 to 1 mM appears to increase the ratio by sevenfold, and raising potassium from 10 to 100 mM decreases the ratio threefold.…”