Functional Asymmetry in Excitable Surface Membrane of Protoplasmic Droplet Isolated from <I>Nitella</I> by Photosensitized Oxidation

Abstract

ABSTRACT.Photooxidations of protoplasmic droplets of Nitella were performed by crystal violet (C.V.), rose bengal (R.B.) and methylene blue (M.B.) from the inside or outside of the membrane.Various physical properties of the droplet were examined. External treatment of the membrane by C.V. induced a temporary increase of resistance and a gradual depolarization of the membrane potential. Excitability disappeared after the depolarization, but the surface tension remained constant. Both membrane potential and resistance remained constant when the droplet was modified by C.V. from the inside, and the excitability was retained. Tension at the surface, however, decreased sharply by a factor of 100. Internal application of R.B. decreased resistance and depolarized the membrane potential by a maximum 20 mV. External treatment with R.B. caused a temporary decrease of resistance with depolarization by about 100 mV. Adsorption of M.B. on the surface membrane led to a decrease in resistance without changing the membrane potential under light illumination, and this was followed by abrupt depolarization. These results indicate that membrane potential and excitability of the protoplasmic droplet are governed largely by the structure at the outermost surface between the protoplasm and external solution and that the tension at the surface is maintained by the internal structure of the protoplasm. 155 Plasma membranes of cells are generally located in an extremely asymmetric environment, i.e., the concentration differences of ionic species on the two sides of the membrane vary by a factor of over 100, the electrical potential differs by 50 to 100 mV and the intracellular potential is negative with respect to the external solution. It is not unreasonable to assume that membranes may have a high anisotropic structure.The asymmetric organization of proteins in erythrocyte membrane was examined by Bender, Garan and Berg (2), Bretcher (3), Phyllips and Morison (12), Sergest et al. (13) and Zwaal, Roelofsen and Colley (20) by treating the membrane with non-penetrating reagents. Asymmetric distributions of phospholipids were reported by Bretcher (4), Stanley, Gordesky and Marinetti (14) and Zwaal, Roelofsen and Colley (20) who studied erythrocyte membranes by digesting phospholipids with specific enzymes. Among these findings, it is interesting to note that the molecular composition of both the proteins and phospholipids differs between the intact and ghost cell membranes (15).Functional asymmetry in the excitable membrane has been studied only in internally perfused squid giant axons. Tasaki (16) demonstrated that internal and external

Intracellular chloride and potassium ions in relation to excitability ofChara membrane

“…In squid axon, effects of monovalent or divalent cations on membrane excitability from the inside were different from those from the outside (cf Tasaki, 1968). Also, the functional asymmetry of the surface membrane of protoplasmic droplets of Nitella was revealed by effects of photooxidizing treatment from the inside and the outside of the membrane on Em and the tension at the surface (Ueda, Muratsugu & Kobatake, 1976). Na + could shorten the duration of the action potential (90 sec), although the effect was weaker than that of K + (6 sec).…”