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Intracellular pressure responses were recorded from squid giant axons after the axoplasm was removed by the intracellular perfusion technique. A glass tube was inserted into the axon and the movement of the air-water interface formed on the end of the tube was observed with a Y-shaped fibrescope. The intracellular pressure increased and decreased rapidly when an action potential was induced by electrical stimulation. The amplitude of the response was about 10 mPa (or 1 X 10(-3) mmH2O), which was very large in comparison with that observed in unperfused axons. It was sensitive to extracellular Ca2+. The pressure response appeared in an all-or-none manner and could be suppressed by tetrodotoxin. This excluded physicochemical processes on the stimulating electrode or current-supplying electrode as sources of the response. Various other sources of artifacts were also excluded. An extensive removal of the axoplasm by intracellular perfusion with a protease-containing solution and a KCl solution did not eliminate the pressure response. The intracellular pressure was membrane potential dependent, increasing upon depolarization and decreasing upon hyperpolarization of the membrane. Under voltage clamp, the relationship between the membrane potential and the pressure response was parabolic with a maximum at +109 mV (in reference to the resting level). The response did not depend on the membrane current. A much slower response due to electro-osmotic water flow was also detected. The pressure response induced by hyperpolarization of the membrane was suppressed by extracellular application of a lidocaine-containing solution, but not by a tetrodotoxin-containing solution. These results suggest that the pressure responses arise either from a change in electrostriction across the axolemma or from a change in charge-dependent tension along the axolemma.

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