Calcium‐mediated inactivation of the calcium conductance in caesium‐loaded giant neurones of Aplysia californica.

Abstract

SUMMARY1. The intracellular potassium in giant neurones ofAplysia californica was replaced with caesium by a method utilizing the ionophore nystatin. Because caesium ions have low permeability through potassium channels, outward currents during voltageclamp depolarization were strongly curtailed after the caesium loading procedure and the subsequent wash-out of the ionophore.2. The calcium current elicited by a test voltage-clamp depolarization (pulse 2) was depressed following the entry of calcium elicited by a prior depolarization (pulse 1).3. The percentage depression of the test current was a linear function of the pulse 1 current-time integral, and thus appears to be related linearly to the amount of calcium carried into the cell during pulse 1. This linear relation was maintained when calcium entry was varied by changes in external calcium concentration, by altered pulse 1 amplitude and altered pulse 1 duration. Depression was substantially reduced by injection of EGTA, and by substitution of barium for extracellular calcium.4. The calcium current was unaffected by prior hyperpolarization ofthe membrane or by prior depolarizations to about Eca. Depression of the current was not altered by addition of extracellular 50 mm-TEA or by a strong hyperpolarization between the conditioning and test pulses. 5. The rate relaxation of the inward current during a given depolarization depended on the rate of entry and accumulation of free calcium. Relaxation under a given command potential became slower when calcium was partially replaced with magnesium so as to produce a smaller calcium current; or when accumulation of intracellular free calcium was retarded by injected EGTA or by barium substitution for extracellular calcium.6. Evidence is considered that accumulation of calcium ions at the cytoplasmic surface of the membrane leads to inactivation through an action upon the calcium conductance. Reduced driving force and intracellular surface-charge neutralization do not adequately account for the observed depression of the calcium current resulting from intracellular accumulation of calcium ions.

Calcium‐dependent inward current in Aplysia bursting pace‐maker neurones.

“…6) or Co2+. Substitution of external Ca2+ with Ba2+, which permeate well through Ca2+ channels (for review see Hagiwara & Byerly, 1981), but which only weakly mediate Ca2+-dependent ionic processes Eckert & Tillotson, 1981), causes an increase in the persistent inward current (Eckert & Lux, 1976). Substitution of Ba2+ for Ca2+, however, caused a decrease in the phase II tail current of 10-25 % in three cells tested (data not shown), as expected for a Ca2+-activated current.…”

“…cells, the largest peak phase II tail current occurred at pulse potentials between +20 and + 40 mV, and the amplitude of the current elicited by a pulse to + 100 mV ranged from 10 to 20 % of the largest current. Various neurophysiological processes which require Ca2+ influx also display an inverted U-shaped relationship with pulse potential; for example, the activation of IK(Ca) (Meech & Standen, 1975;Gorman & Thomas, 1980a), Ca2+-dependent inactivation of Ca2+ current (Brehm & Eckert, 1978;Eckert & Tillotson, 1981); and neurotransmitter release at squid central synapses (Katz & Miledi, 1967). Therefore, it seems likely that the inward current which predominates during phase II is Ca2+-activated.…”

“…One possible explanation for the great Na+ sensitivity of the phase II tail current is that Na+ removal leads to an accumulation of Ca2+ inside the cell, due to a block of the Na+-Ca2+ exchange pump. The increase of intracellular Ca2+ could result in a partial inactivation of the Ca2+ current during the depolarizing pulse (see Eckert & Tillotson, 1981;Plant, Standen & Ward, 1983), leading to a decrease of the Ca2+-dependent activation of the phase II tail current. In contrast, a blocked Na+-Ca2+ pump might enhance the effect of Ca2+ injection.…”

Ca(2+)‐dependent inactivation of Ca2+ current in Aplysia neurons: kinetic studies using photolabile Ca2+ chelators.

“…It was also unaffected by decreases in cyclic AMP induced by injecting 2',5'-dideoxyadenosine (1 mM) or bath application of the Rp isomer of cyclic adenosine 3',200 /uM). INTRODUCTION Inactivation of the macroscopic neuronal Ca2+ current (ICa) in Aplysia californica is predominantly mediated by an increase in the intracellular free Ca21 concentration ([Ca2+]1) and displays little voltage dependence (Eckert & Tillotson, 1981; Eckert & Ewald, 1983;. Inactivation is experimentally observed as either the decline of ICa during a long depolarizing pulse or as the loss of peak current in a test pulse following a prepulse.…”

Ca2+ channel currents of cortical neurons from pure and mixed cultures

“…was used instead of Ca 2? in the bath solution to reduce the influence of rundown (Ecckert and Tillotson 1981). Whole-cell Ba 2?…”