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

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.