Transport and metabolism of calcium ions in nerve

Divalent cations as charge carriers in excitable membranes

Pharmacological characterization of different types of GABA and glutamate receptors in vertebrates and invertebrates.

1. Characteristics of the transmembrane ionic currents under controlled changes in ionic composition of extra- and intracellular medium were studied in isolated neurones from the ganglia of molluscs, Helix pomatia, Limnea stagnalis and Planorbis corneus. The neurones were investigated by a new technique which allows for dialysis of their interior and for clamping of the potential at the surface membrane without using micro-electrodes.



2. Replacement of K ions by Tris inside the neurones eliminated the outward K current so that the actual time course of the inward current could be measured. The latter was separated into two additive components, one of which was carried by Na ions and the other one by Ca ions.



3. Both inward currents were unaltered by tetrodotoxin (TTX); however, Ca current could be separately blocked by externally applied Cd ions (Kd = 7·2 × 10−5 M) and by the use of fluoride as an intracellular anion.



4. No reversal of Na inward current could be achieved in neurones dialysed with Na-free solution, indicating the absence of outward current carrying ions through the corresponding channels. With 5 m M-Na inside the cell, the equilibrium potential was close to the value predicted by the Nernst equilibrium.



5. A non-specific outward current could be detected in K-free cells at membrane potentials exceeding 20-40 mV. Its time course was proportional to 1 — exp (—t/τns). Cd ions depressed this current. The presence of the non-specific outward current made an exact measurement of the equilibrium potential for the Ca inward current impossible.



6. The kinetics of Na inward currents could be described by m3h and those of the Ca current by m2h law. The corresponding values for Vm = 0 are: τm(Na) = 1·1 ± 0·5 msec, τm(Ca) = 2·4 ± 1·0 msec, τh(Na) = 7·9 ± 2·0 msec. The inactivation of Ca current included two first-order kinetic processes with τh1 = 50 ± 10 msec and τh = 320 ± 30 msec.



7. The data presented are considered to be a proof of the existence of separate systems of Na and Ca ion-conducting channels in the nerve cell membrane.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1977.sp011968

Separation of sodium and calcium currents in the somatic membrane of mollusc neurones. With an Appendix by Yu A. Shakhovalov

1. Those features of a transmembrane current correlated with spike initiation were examined in Aplysia neurones using a Gaussian white-noise stimulus. This stimulus has the advantages that it presents numerous wave forms in random order without prejudgement as to their efficacies, and that it allows straightforward statistical calculations. 2. Stimulation with a repeating segment of Gaussian white-noise current revealed remarkable invariance in the firing times of the tested neurones and indicated a high degree of reliability of their response. 3. Frequencies (less than 5 Hz) involved in spike triggering propagated faithfully for up to several millimetres, justifying intrasomatic current injection to examine spike initiation at the trigger locus. 4. Examination of current wave forms preceding spikes indicated that a wide variety could be effective. Hence, a statistical analysis was performed, including computation of probability densities, averages, standard deviations and correlation coefficients of pairs of current values. Each statistic was displayed as a function of time before the spike. 5. The average current trajectory preceding a spike was multiphasic and depended on the presence and polarity of a d.c. bias. An early relatively small inward- or outward-going phase was followed by a large outward phase before the spike. The early phase tended to oppose the polarity of the d.c. bias. 6. The late outward phase of the average current trajectory reached a maximum 40--75 msec before triggering the action potential (AP) and returned to near zero values at the moment of triggering. The fact that the current peak occurs in advance of the AP may be partially explained by a phase delay between the transmembrane current and potential. The failure of the average current trajectory to return to control values immediately following the peak argues for a positive role of the declining phase in spike triggering. 7. Probability densities preceding spikes were Gaussian, indicating that the average was also the most probable value. Although the densities were broad, confirming that spikes were preceded by a wide variety of current wave forms, their standard deviations were reduced significantly with respect to controls, suggesting preferred status of the average current trajectory in spike triggering. 8. The matrix of correlation coefficients between current pairs suggested that spikes tended to be preceded by wave forms that in part kept close to the average current trajectory and in part preserved its shape. 9. The average first and second derivatives of spike-evoking epochs revealed that current slope and acceleration, respectively, were most crucial in the last 200 msec before spike triggering, and that these dynamic stimulus components were more important for a cell maintained under a depolarizing, rather than a hyperpolarizing bias. 10...

Spike initiation by transmembrane current: a white-noise analysis.

1. 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 drug has no effect on the calcium-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.

D. Junge

Papers

Sodium and calcium components of action potentials in Aplysia giant neurone