The short-chain phospholipid, diheptanoyl phosphatidylcholine, at 520 microM, reduced the maximum inward sodium current in voltage-clamped squid giant axons by greater than 50%. Analysis of these currents by means of the Hodgkin-Huxley equations showed this reduction to be mainly the result of a large depolarizing shift in the voltage dependence of the steady state activation parameter, m infinity. The voltage dependence of the steady state inactivation parameter, h infinity, was also moved in the depolarizing direction and the axonal membrane capacitance per unit area measured at 100 kHz was increased. A longer chain length derivative, didecanoyl phosphatidylcholine, had no significant effect on the axonal sodium current at concentrations of 3.7 and 18.5 microM. Dioctanoyl phosphatidylcholine was intermediate in its effects, 200 microM producing approximately the same current suppression as 520 microM diheptanoyl phosphatidylcholine, together with depolarizing shifts in m infinity and h infinity. These effects may be contrasted with those of the normal and cyclic alkanes (1-3), which tend to move both m infinity and h infinity in the hyperpolarizing direction and to reduce the capacitance per unit area at 100 kHz. The above results are all consistent with the hypothesis that small hydrocarbons thicken, while short-chain phospholipids thin, the axonal membrane. Thus membrane thickness changes may be of considerable importance in determining the behavior of the voltage-gated sodium channel.
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