Role of surface dipoles on axon membrane

The cation permeability of erythrocytes in low ionic strength media of various tonicities

The steady state passive efflux of salt from human red blood cells was measured in various low ionic strength media in which the osmotic pressure ranged from 200 to 600 milliosmolar. Sucrose was used as the nonpenetrating nonelectrolyte. If the flux is plotted against the log of the salt concentration, the data for each tonicity can be fitted by three straight-line segments separated by two sharp inflections, one at low external salt concentrations (0.1 to 0.3mM), confirming observations of LaCelle and Rothstein, and a second at higher salt concentrations (20 to 50 mM). As the osmolarity of the medium is increased, the inflection in every case seems to be uniquely determined by the membrane potential calculated from the Nernst equation with use of the chloride ratio. One inflection occurs at about 45 mV and the second at 170 mV in experiments at five different tonicities. Calculations from the Goldman equation suggest that the inflections represent potential-dependent changes to new permeability states. The osmotic pressure of the medium also influences the permeability. The coefficient is systematically reduced as the osmotic pressure is increased.

Modification of optical responses associated with the action potential of lobster giant axons

The sources of optical retardation changes and light scattering changes occurring during the action potential propagation of lobster giant axons have been investigated. A technique has been developed for resolving the total transmitted-light intensity change into a retardation change component, dI-r, and a forward direction light scattering change, dI-s. Trypsin, pronase, neuraminidase and hyaluronidase all reduce the magnitude of dI-r without diminishing the action potential, probably by cleaving charged saccharides. Dithiothreitol has no effect. This suggests that glycoproteins and hyaluronic acid polymers at the surface of the axon are involved in the optical responses, either by being passively realigned or by contributing to compression and expansion forces as the membrane electric field changes. Large dI-s responses are generated by trypsin and pronase treatment. The modifying effects of these proteases may be due to modification of the membrane or to increases in the refractive index of the medium surrounding the axon, since similar large dI-s, responses are produced by increasing the refractive index with sucrose. Since large reductions in dIr can be produced without concurrent reductions in the action potential, a significant portion of the optical retardation responses cannot be attributable to structural changes that are causally related to membrane ionic permeability changes during the action potential.

Electric-field-induced shifts in the infrared spectrum of conducting nerve axons

Difference spectra between resting and excited nerve in the infrared region between 2000 and 1000 cm(-1) have been examined with a resolution of 0.5 cm(-1). Spectra were obtained with a modified Perkin-Elmer model 521 grating infrared spectrophotometer (Perkin-Elmer Corp., Instrument Div., Norwalk, Conn.), and the signal-to-noise (S/N) ratio was improved by time averaging and digital smoothing. Peaks occurring in the regions around 1030 and 1066 cm(-1) are identified as P-O-C stretch, at 1410-1414 cm(-1) as C-H deformation, and at 1750 cm(-1) as carbonyl stretch. The difference peaks appear to be due to a shift of about 1 cm(-1) in the absorption band to lower frequencies for the three lower frequency bands and to a higher frequency for 1750 cm(-1) band. Since the difference peaks appear when the nerve is modulated by a propagated action potential it is concluded that the changing electrical field across the nerve membrane is perturbing the absorption spectrum. From evidence presented it appears likely that these difference peaks are due to phospholipids in the nerve membrane and that they may be related to conformational changes associated with membrane permeability.

Dipole moment of acetylcholine and its relevance to the chemical synaptic transmission

The dipole moment of acetylcholine (AcCh) has been measured in chloroform and a value of 8.49 D was obtained. Such a value actually represents the total dipole moment of the ion pair (AcCh)(+)(Cl)(-). The dipole moment of the (AcCh)(+) cation alone turned out to be 2.65 D whereas its theoretical value obtained after a vectorial calculation was 1.65 D. The discrepancy was related to an interaction between AcCh and the solvent. The meaning of the measured value is discussed on the basis of a recent theory of chemical synaptic transmission based on the assumption of a much higher dipole moment value for the AcCh molecule.

A possible role of phospholipid in nerve excitation

A possible role of phospholipid in regulating the ionic permeability of excitable membranes is proposed. It is assumed that each excitable transport channel in the nerve fiber is lined with two chains of phospholipid dipoles in either parallel or antiparallel directions; a molecular mechanism of nerve excitation is developed that is qualitatively consistent with most of the relevant observations reported in the literature.