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

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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 μm. 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 singlechannel 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.