Patch-clamping mitoplasts, we have observed a complex pattern of conductance transitions. This report discusses primarily the 45, 120-150, 350, and 1,000 pS transitions.

The mitochondrial benzodiazepine receptor (mBzR) binds a subset of benzodiazepines and isoquinoline carboxamides with nanomolar affinity and consists of the voltage-dependent anion channel, the adenine nucleotide translocator, and an 18-kDa protein. The effect of ligands of the mBzR on two inner mitochondrial membrane channel activities was determined with patch-clamp techniques. The relative inhibitory potencies of the drugs resemble their binding affinities for the mBzR. RoS-4864 and protoporphyrin IX inhibit activity of the multiple conductance channel (MCC) and the mitochondrial centum-picosiemen (mCtS) channel activities at nanomolar concentrations. PK11195 inhibits mCtS activity at similar levels. Higher concentrations of protoporphyrin IX induce MCC but possibly not mCtS activity. Clonazepam, which has low affinity for mBzR, is at least 500 times less potent at both channel activities. Rol5-1788, which also has a low mBzR affinity, inhibits MCC at very high concentrations (16 FM). The fin indicate an association of these two channel activities with the proteins forming the mBzR complex and are consistent with an interaction of inner and outer membrane channels.Benzodiazepines exert their well-known antianxiety, anticonvulsant, and sedative effects through a receptor in nervous tissue that is associated with a chloride channel regulated by the inhibitory neurotransmitter y-aminobutyric acid (1). Benzodiazepines also bind with nanomolar affinity to a distinct site in many tissues that is selectively associated with mitochondria and designated the mitochondrial benzodiazepine receptor (mBzR). Drugs with high affinity for mBzR influence diverse biological processes, including mitochondrial respiration (2-4), neural excitability (5), leukocyte respiration (6), and adrenal steroid synthesis (7-9). These pleiotropic actions are consistent with influences on some fundamental aspect of mitochondrial function.Purification of mBzR in a form retaining reversible ligand binding yielded a complex containing the voltage-dependent anion channel (VDAC, or mitochondrial porin), the adenine nucleotide translocator (ANT), and an 18-kDa protein (10). Since VDAC and the 18-kDa protein are localized to the outer membrane and ANT is an inner membrane protein, the functional mBzR is most likely associated with "contact sites" where the inner and outer membranes adhere (11) and manifest a number ofionic conductance levels (12). While the multiple conductance channel activity (MCC, defined with patch-clamp studies as ranging in conductance between 40 and >1000 pS) is thought to be associated with contact sites (for a review see ref. 13), this has not been postulated for mCtS activity [mitochondrial centum-picosiemen, the '100-pS voltage-dependent activity originally described by Sorgato et al. (14)].We now demonstrate that nanomolar levels of mBzR ligands influence both MCC and mCtS activities with drug specificities appropriate for mBzR (15, 16) in patch-clamp studies of mitoplasts (mitochondria whose outer membrane is rem...

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