SUIMMARY1. Action potentials resulting from direct stimulation can be recorded from the soma of the Aplysia giant neurone (located in the visceral ganglion) in sodium-free and in calcium-free external solutions. The neurones were impaled by internal micro-electrodes throughout the change of external solutions.2. Complete replacement of either sodium or calcium in the bathing medium with Tris results in only a partial reduction of spike overshoot. Simultaneous replacement of both sodium and calcium reversibly and quickly abolishes the spike.3. The sodium component of the spike in a calcium-free medium is blocked by tetrodotoxin; the dg e e n -dependent spike in sodium-free medium. Externally applied cobalt chloride blocks only the calcium-dependent component.4. In calcium-free media, the overshoot value varies with sodium concentration in the manner predicted for a sodium electrode. In sodium-free media, the membrane behaves like a calcium electrode.5. These results suggest that, during the normal action potential, both sodium and calcium act as carriers of the inward-directed current.
SUMMARY1. The membrane properties of the Aplysia giant neurone were studied under controlled voltage conditions. Emphasis was placed on the early transient currents resulting from step polarizations applied while the ganglion was immersed in different test solutions.2. Early inward-going currents were observed when the neurone was bathed in normal saline (containing both Na and Ca), in Ca-free (Nacontaining) saline, in Na-free (Ca-containing) saline, and in the normal saline to which tetrodotoxin 10-5 g/ml. was added. When both Na and Ca are absent from the bathing solution no evidence for early inward-going current could be found.3. When tetrodotoxin is added to the normal saline, the maximum inward-going current is reduced, and no further reduction of this current is observed when the external Na-concentration is subsequently halved in the presence of the drug. When the external Ca-concentration is increased fivefold in the presence of the drug, the maximum transient current increases significantly.4. Hyperpolarizing prepulses result in a membrane inactivation in the presence of tetrodotoxin or in the absence of Na. In the presence of Na (and absence of tetrodotoxin) no such voltage-dependent inactivation occurs, and for this case, inactivation results only from depolarizing prepulses.
Squid giant axon possesses a hyperpolarizing electrogenic sodium pump which is stimulated by internal sodium and by external potassium. This conclusion is based on the following observations: strophanthidin depolarizes the membrane and enhances the depolarizing effect of 5 or 10 millimolar external potassium; the magnitude of these effects is directly related to the internal sodium concentration; both effects are abolished by cyanide.
SUMMARY1. Changes in membrane conductance and potential of sodium-loaded frog muscle fibres were found when the external recovery solution was changed: from cold to warm, to warm plus ouabain, to cold plus ouabain. Comparisons of these measurements for different external solutions were made by leaving the electrodes implanted in the same fibre during all solution changes. (The recovery solutions contained 10 mM-K and 82 mM-Cl.)2. The membrane potential became more negative on warming, less negative when ouabain was added, and still less negative when the ouabain-containing recovery solution was cooled. The membrane conductance increased on warming, increased further on addition of ouabain, and decreased when the ouabain-containing recovery solution was cooled.3. The increase of conductance which occurred on warming decreased with increasing periods in cold recovery. The increase of conductance which occurred on addition of ouabain decreased if the ouabain was added to the recovery solutions of muscles which were more fully recovered.4. The ouabain-sensitivity of the membrane conductance may be dependent upon the sodium-pump rate, or the extent of recovery of the sodium-loaded muscle fibre in the potassium-and chloride-containing recovery solutions.5. It is suggested that if the potassium conductance of the membrane increases with decreasing sodium-pump rates, then during the initial part of the recovery period a non-electrogenic mechanism must be producing a substantial part of the early net potassium influx.
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