We have considered whether lecithin surface films below the gel-liquid crystal transition temperature, Tc, are in unique physical states. In general, below Tc, equilibrium films do not exist when surface pressures, pi, exceed about 0.1 dyn/cm. Since surface pressure-surface area isotherms of lecithin films below Tc always encompass pi's much greater than 0.1 dyn/cm, the film states are metastable. We show that the film properties under these conditions depend strongly on the history of the film, particularly the method of film formation. Lecithin surface films below Tc are thus in arbitrary metastable states, so that pi-area isotherms are difficult to interpret. The physical significance of such isotherms remains to be determined. The utility of pure lecithin surface layers below Tc as models for biological systems is also challenged by our results.

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A new method is proposed for measuring the dynamic properties of a membrane transporter by means of steady-state fluxes. Any voltage-sensitive transporter will give a flow of substrate in the presence of a steady-state periodic membrane potential. The periodic steady-state flow, averaged over one period, is a flux that can be measured by traditional steady-state techniques, such as the radioactive tracer method. The average flux, solely due to the periodic field, is described by a set of Lorentzian functions that depend on the applied periodic field amplitude and frequency. The normal mode amplitudes and frequencies of these Lorentzians are model-independent parameters of the transport mechanism. Measurement of the average flux as a function of the applied periodic frequency permits determination of system relaxation times as the reciprocals of the midpoints of the Lorentzian curves, which in turn can be used to estimate individual rate constants of specific models. It was found by simulation of a six-state model of the electrogenic Na+/glucose cotransporter, using published estimates of the model rate constants, that the periodic field effects can be large and rich with measurable details that can be used to study the mechanism thoroughly. The new method serves in this case to complement and expand on the information obtainable by means of the voltage clamp method. It was also found by means of simulations of a nonelectrogenic six-state cotransporter model that experimentally measurable effects are expected and that results can be used to distinguished among alternative kinetic models as well as to estimate individual rate constants. The range of dynamic information available with this method is not accessible by voltage clamp or other pre-steady-state methods presently in use.

The transport of L-glutamate (Glu) by single muscle fibers of Balanus nubilus was studied by means of internal dialysis in an effort to obtain transport data under well-defined intracellular conditions. It is shown that the method effectively controls the sarcoplasmic amino acid composition and Glu metabolism. It was found that the classic neutral amino acid transport systems are either absent or inactive, while Glu is strongly transported by a single system. Nonsaturable Glu "leak" is small relative to mediated transport. Glu influx under 0-trans conditions obeys simple Michaelis-Menten kinetics and shows simple competition with aspartate. Influx is strictly dependent on external Na. Trans-Na reduces influx, whereas trans-Glu has a small stimulatory effect. The preparation may serve as a general model for muscle Glu transport and can be used for a thorough study of the kinetic mechanism.