SINGLE ANION CHANNELS RECONSTITUTED FROM CARDIAC MITOPLASTS

Ion channels from sheep cardiac mitoplast (inverted inner mitochondria l membrane vesicle) preparations were incorporated into voltage-clampe d planar lipid bilayers. The appearance of anion rather than cation ch annels could be promoted by exposing the bilayers to osmotic gradients formed by Cl- salts of large, relatively impermeant, cations at a pH of 8.8. Two distinct activities were identified. These comprised a mul ti-substate anion channel of intermediate conductance (approximately 6 0 pS in 300 vs. 50 mm choline Cl, approximately 100 pS in symmetric 15 0 mm KCl), and a lower-conductance anion channel (approximately 25 or approximately 50 pS in similar conditions), which only displayed two w ell-defined substates, at approximately 25 and approximately 50% of th e fully open state. The larger channels were not simple multiples of t he lower-conductance channels, but both discriminated poorly, and to a similar extent, between anions and cations (P(Cl-)/P(choline+) approx imately 12, P(Cl-)/PK+ approximately 8). The lower-conductance channel was only minimally selective between different anions (P(NO3-)(1.0) = P(CI)>P(Br-)>P(I-)>P(SCN-)(0.8)), and its conductance failed to satur ate even in high (>1.0 M) activities of KCl. The channels were not obv iously voltage dependent, and they were unaffected by 0.5 mM SITS, H2O 2, propranolol, quinine or amitriptyline, or by 2 mM ATP, or by variat ions in pH (5.5-8.8). Ca2+ and Mg2+ did not alter single channel activ ity, but did modify single current amplitudes in the lower-conductance channel. This effect, together with voltage-dependent substate behavi or, is described in the following paper.