Long-lasting inward current in snail neurons in barium solutions in voltage-clamp conditions

Abstract

Summary. The inward membrane current was recorded under voltage clamp from nonbursting neurons of the snail Helix pomatia in Na-free solutions containing Ba ions but no other divalent cations. The inward current was separated into two components: (i) an early fast inactivating component and (ii) a smaller long-lasting component. Both components were dependent on the external Ba concentration. It is concluded that both components of the inward current are carried by Ba ions. The activation of the early fast inactivating component of the inward current occurred at more positive membrane potential than that of the long-lasting component. The shape of the inactivation curve for the peak value of the inward current was similar to that for the long-lasting component. The potentials of half-inactivation for the peak value of the inward current and for its long-lasting component were -28 and -22mV, respectively. The blocking effect of Co ++ on the early fast inactivating component was substantially greater. In some neurons after treatment with 15 mg Co + § only the long-lasting component was recorded. The activation kinetics of the long-lasting component of the inward current were analyzed using the Hodgkin-Huxley equations. The results could be explained by assuming that two components of the inward current in Na-Ca-free solution with Ba ions flowed through the two different channels. The significance of the long-lasting inward current for the normal spike generation is discussed.

Calcium current inactivation in identified neurones of Helix aspersa.

“…When 25 mM-Sr was substituted for Ca the inward current declined much more slowly (Fig. 1 A) and in barium solution the currents declined more slowly still (see also Magura, 1977). The more rapid decline of inward current in Ca saline cannot therefore be attributed to depletion of extracellular Ca, as has been demonstrated in frog muscle (Almers, Fink & Palade, 1981) or to accumulation of intracellular Ca, since these effects should be at least as marked in Sr or Ba saline.…”

Intracellular calcium accumulation during depolarization in a molluscan neurone.

“…There is direct evidence that the Ca2+ current inactivates (Magura, 1977;Kostyuk & Krishtal, 1977;Akaike et al 1978b;Tillotson, 1979). The decline in the absorbance signal with time has been noted previously and could reflect a voltage dependent inactivation of Ca2+ influx.…”

“…The influx of Ca2+ ions, measured from changes in the Ca2+ current at different membrane potentials, has been widely studied in Aply8ia and in other molluscan neurones in recent years in intact cells (Geduldig & Gruener, 1970;Kostyuk, Krishtal & Doroshenko, 1974;Standen, 1975;Magura, 1977;Connor, 1979;Adams & Gage, INTRACELLULAR Ca2+ ACCUMULATION 1979a, b), in perfused cells (Kostyuk & Krishtal, 1977;Akaike et al 1978b) and in chemically altered cells (Tillotson & Horn, 1978;Tillotson, 1979) (Baker et al 1973). This channel is activated at about -45 to -40 mV (see and is only partially inactivated at all potentials more negative than 0 mV (50 % inactivation at about -14 mV).…”

Voltage‐dependent and calcium‐dependent inactivation of calcium channel current in identified snail neurones.

“…Temperature 30 'C. decrease in the amplitude of the inward tail currents measured upon repolarization following activating pulses of various duration (see also Brown et al 1981;Eckert & Tillotson, 1981 Effects of substituting barium for calcium As has previously been reported (Magura, 1977;Connor, 1979;Adams & Gage, 1980;Brown et al 1981), replacement of Ca21 with Ba2+ resulted in larger inward currents, a shift to the left of the I-V curve by about 10 mV, and slowing of inactivation (Fig. 2B).…”