Voltage-independent barium-permeable channel activated inLymnaea neurons by internal perfusion or patch excision

Abstract

Isolated nerve cells from Lymnaea stagnalis were studied using the internal-perfusion and patch-clamp techniques. Patch excision frequently activated a voltage-independent Ba2+-permeable channel with a slope conductance of 27 pS at negative potentials (50 mM Ba2+). This channel is not seen in patches on healthy cells and, unlike the voltage-dependent Ca channel, is not labile in isolated patches. The activity of the channel in inside-out patches is unaffected by intracellular ATP, Ca2+ below 1 mM or the catalytic subunit of cAMP-dependent protein kinase but is reversibly blocked by millimolar intracellular Ca2+ or Ba2+. The channel can be activated in on-cell patches by either internal perfusion with high Ca2+ or the long-term internal perfusion of low Ca2+ solutions not containing ATP. These channels may carry the inward Ca2+ current which causes a regenerative increase in intracellular Ca+ when snail neurons are perfused with high Ca2+ solutions. High internal Ca2+, or long periods of internal perfusion with ATP-free solutions, induces an increase in a resting (-50 mV) whole-cell Ba2+ conductance. This conductance can be turned off by returning the intracellular perfusate to a low Ca2+ solution containing ATP and Mg2+. The activity of this channel appears to have an opposite dependence on intracellular conditions to that of the voltage-dependent Ca channel.

Nonselective cation channel activated by patch excision from lobster olfactory receptor neurons

“…Our demonstration of Na + permeability confirms recordings of macroscopic currents, which suggested that Na + could permeate this type of channel (Yazejian & Byerly, 1989). We show that K + and Cs + are also permeable, although less so than Na +.…”

“…Strong et al (1987) only observed a similar channel in cellattached patches from Aplysia neurons following injury to the cell's neurites. Yazejian and Byerly (1989) found that a similar channel in Lymnaea stagnalis neurons could be activated by sustained intracellular perfusion with high Ca 2+ solutions or low Ca 2+ solutions lacking ATP, or by a low Ca 2+, high K + bath solution. Since our data come from cells bathed in low divalent saline after being stripped of their neurites, it is possible that this species of cation channel is somehow involved in responses to trauma in the spiny lobster.…”

“…They have a relatively high conductance for divalent cations, are voltage independent or only weakly voltage dependent, and have conductances and kinetic properties dependent upon the types and quantities of permeant cations present. Patch excision activates these channels by mechanisms which remain unknown, though Ca 2+ and ATP may play a role (Yazejian & Byerly, 1989;Strong et al, 1987).…”

“…A third and smaller group consists of nonselective cation channels activated by patch excision. These channels have previously been de-* Present address: Section of Molecular Neurobiology, Yale University School of Medicine, 333 Cedar St., New Haven, CT 06510. scribed only in molluscan neurons (Chesnoy-Marchais, 1985;Strong et al, 1987;Yazejian & Byerly, 1989). They have a relatively high conductance for divalent cations, are voltage independent or only weakly voltage dependent, and have conductances and kinetic properties dependent upon the types and quantities of permeant cations present.…”

Voltage gated calcium channels in molluscs: classification, Ca2+ dependent inactivation, modulation and functional roles

“…Chesnoy-Marchais (1985) reported calcium channels in outside out patches of Aplysia neurons that were inhibited by internal Cs + but persisted in a solution with Na + ions facing the inside of the membrane. These channels are largely similar to calcium channels described as excision activated channels by Yazejian and Byerly (1989) and Strong and Scott (1992) Studies on Lymnaea neurons showed that the channel is regulated by intracellullar ligands. High internal Ca 2+, up to micromolar levels, activates the channel, but it is blocked by millimolar Ca 2+, Ba 2+ or Mg 2+ (Yazejian and Byerly, 1989;Strong and Scott, 1992).…”