Anion channels in giant liposomes made of endoplasmic reticulum vesicles from rat exocrine pancreas

Abstract

Using the method of dehydration and rehydration, rough endoplasmic reticulum (RER) vesicles, isolated by differential centrifugation, can be enlarged to giant liposomes with diameters ranging from 5 to 200 micron. Patch-clamp studies on these giant RER liposomes revealed the existence of a channel with a mean conductance of 260 +/- 7 pS (n = 23; 140 mmol/liter KCl on both sides). The channel is about four times more permeable for Cl- than for K+. Its activity is strongly voltage regulated. At low potentials (+/- 20 mV) the channel is predominantly in its open state with an open probability near 1.0, whereas it closes permanently at high positive and negative voltages (+/- 70 mV). The channel activity is not influenced by changing the free Ca2+ concentration from 1 mmol/liter to less than 10(-9) mol/liter on either side, and is also not affected by typical Cl- -channel blockers like diphenylamine-2-carboxylate (DPC, 1 mmol/liter) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS, 1 mmol/liter). Another chloride channel with a single-channel conductance of 79 +/- 6 pS (n = 4) was less frequently observed. In the potential range of -80 to +40 mV this channel displayed no voltage-dependent gating. We assume that these anion channels are involved in the maintenance of electroneutrality during Ca2+ uptake in the RER.

Effect of cell volume on Acridine Orange fluorescence in hepatocytes

“…Accordingly, barium has been shown previously to mimic the effects of hypotonic extracellular fluid and glutamine on both cell volume and proteolysis (Haussinger et al, 1991;Hallbrucker et al, 1991b). The effect of different glutamine concentrations parallels dosedependent cell swelling and anti-proteolytic activity (Wettstein et al, 1990;Hallbrucker et al, 1991b (Schneider, 1983;Lake et al, 1987) or Clchannels (Schmid et al, 1988(Schmid et al, , 1989, or it could somehow lead to a base shunt across the cell membrane, similar to the action of NH3/NH4+. Interestingly, net formation of NH3/NH4' from amino acids is enhanced in hepatocytes exposed to hypotonic extracellular fluid Haussinger et al, 1990b).…”

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

“…In other tissues, mast cells [106], keratinocytes [107], osteogenic cells [108], cultured glomus cells of the carotid body [109], PC12 pheochromocytoma cells [110], pavement cells from the gills of the trout [111] and mammary gland C127 cells [8,11,112] have also been shown to possess this channel. Patch-clamping the intracellular organelles revealed the maxi-anion channel activity in sarcoplasmic reticulum ''sarcoballs'' [113], whereas the presence of this channel in endoplasmic reticulum [114] and in Golgi complex [115] was demonstrated by reconstituting these membranes into liposomes and lipid bilayers, respectively. Thus, it is likely that the maxi-anion channel is widely expressed in almost every part of the body.…”

“…5) as well as DIDS and DPC [2,5,6,8,12,30,54,93,96,103,114]. However, maxi-anion channels were found to be completely insensitive to phloretin, which is a relatively specific blocker for VSOR [121], in mouse mammary C127 cells [8,112] and mouse astrocytes [5,6,8,112], and to glibenclamide (Fig.…”

Rat Brain p64H1, Expression of a New Member of the p64 Chloride Channel Protein Family in Endoplasmic Reticulum

“…Most recordings are from non-neural tissues (3,5,7), and many of these channels may be protein translocation pores rather than conventional ion channels. Anion channels reconstituted from rat brain microsomes (2,45) and cardiac mitoplast membranes (11) are poorly selective for anions versus cations, like p64H1-related channels, but they appear to display different conductances and have different single-channel gating and substate behavior.…”