Calcium current‐dependent and voltage‐dependent inactivation of calcium channels in Helix aspersa

Abstract

SUMMARY1. Inactivation of the Ca channels has been examined in isolated nerve cell bodies of Helix aspersa using the suction pipette method for voltage clamp and internal perfusion.2. Satisfactory suppression of outward currents was essential. This was achieved over most of the voltage range by substitution of Cs ion for K ion and by the use of TEA intra-and extracellularly and 4-AP extracellularly. A small time-and voltage-dependent non-specific current remained at potentials above + 60 mV.3. In these solutions, Ca current approaches Eca but cannot be detected in the outward direction. The Ca channel appears to be impermeable to Cs and Tris ions.4. Inactivation of Ca currents occurs as a bi-exponential process. The faster rate is 10-20 times the slower rate and is about one twentieth the rate of activation. The development of inactivation during a single voltage-clamp step and the onset of inactivation produced by prepulses followed after brief intervals by a test pulse, have roughly similar time courses.5. The rates of inactivation increase monotonically at potentials more positive than about -25 mV. The amount of steady-state inactivation increases with membrane depolarizations to potentials ofabout + 50 mV. At more positive potentials, steady-state inactivation is reduced.6. Intracellular EGTA slows the faster rate of inactivation Of ICa and reduces the amount of steady-state inactivation measured with a standard two pulse protocol. The effect is specifically related to Ca chelation and hydrogen ions are not involved. This component of inactivation is referred to as Ca current-dependent inactivation and is consistent with observations that increased Cai inactivates the Ca channel. The process does not depend upon current flow alone since Ba currents of comparable or greater magnitude have smaller initial rates of inactivation. Furthermore, application of Ba ion intracellularly in large concentrations has no effect on steady-state inactivation.7. The bi-exponential inactivation process that persists in the presence of EGTA1 is similar to that occurring when extracellular Ba ion carries current through the Ca channel. Steady-state inactivation also persists and is similar in the two cases. Therefore it is concluded that inactivation is voltage-dependent as well as Ca current-dependent.

Accelerating effects of pentobarbitone on the inactivation process of the calcium current in Helix neurones

“…As to calcium current (ICa), the inactivation process (the decay phase of ICa) has been analysed in various tissues, and ICa after reaching its peak, is inactivated in a voltage-and current-dependent manner (Kostyuk, Krishtal & Shakhovalov, 1977;Akaike, Lee & Brown, 1978;Brehm & Eckert, 1978;Tillotson, 1979;Brown, Morimoto, Tsuda & Wilson, 1981;Plant & Standen, 1981). So far as we know, no drug has been found to accelerate the decay of ICa* We show here that pentobarbitone accelerates the decay of ICa in snail neurones.…”

“…In the control solution, the onset of ICa occurred without a detectable delay and the rising phase (acti- It has been reported elsewhere that the decay phase of ICa may be fitted by a least, two exponential functions (Kostyuk, et al, 1977;Brown et al, 1981;Byerly & Hagiwara, 1982;Oyama, Nishi & Akaike, unpublished observation). At this moment, however, the slower components of the decay phase of ICa have yet to be characterized in more detail.…”

Sodium and calcium channels in bovine chromaffin cells

“…5) is flowing through Ca2+ channels and not through K+ channels or through a voltage-dependent leakage conductance (Tillotson, 1979;Brown, Morimoto, Tsuda & Wilson, 1981;Byerly & Hagiwara, 1982 (Fig. 17).…”

Inactivation kinetics of calcium current of acutely dissociated CA1 pyramidal cells of the mature guinea‐pig hippocampus.

“…Furthermore Ba2+, which flows through Ca2+ channels, seems to be less effective than Ca2+ in bringing about inactivation. In a few cases both voltage and Ca2+ may simultaneously affect the inactivation of some channels (Brown, Morimoto, Tsuda & Wilson, 1981;Lee, Marban & Tsien, 1985).…”