Voltage activation of heart inner mitochondrial membrane channels

An ADP-sensitive cyclosporin-A-binding protein in rat liver mitochondria

Mitochondria contain a structure which forms a large aqueous pore in the inner membrane after Ca2+ overload in the presence of Pi. In the present study, pore activation in liver mitochondria was monitored using the collapse of the inner membrane potential (delta psi). Ca(2+)-induced pore opening (delta psi collapse) was prevented by the immunosuppressant cyclosporin A, but cyclosporin A did not reverse pore opening (i.e. allow delta psi regeneration) unless ADP was also added. At concentrations that produced substantial pore blockade, [3H]cyclosporin partitioned more or less equally between membrane and soluble fractions, but the distribution was shifted slightly to the membranes in the presence of ADP. ADP also increased the binding of [3H]cyclosporin A to membranes washed free of soluble components. The indication that cyclosporin A inhibition of the pore is mediated by an ADP-sensitive membrane component was examined using a tritiated photoactivable derivative of cyclosporin A. ADP selectively increased covalent binding of this derivative to a membrane component. This component eluted from molecular-sizing columns as a 13-17-kDa-protein in the presence of 0.5% Chaps as detergent and migrated as a 10-kDa (approximately) protein in SDS/PAGE. These findings provide the first evidence that a protein of approximately 10 kDa may be part of the cyclosporin-A receptor of the Ca(2+)-activated pore. The possible implications of these findings are discussed.

Mitochondrial oxidative phosphorylation in saponin-skinned human muscle fibers is stimulated by caffeine

The addition of 15 mM caffeine to saponin-skinned human muscle fibers from M. vastus lateralis caused in the presence of 2 mM ATP an approx. 2-fold stimulation of respiration with glutamate+malate. This effect can be abolished by either the addition of the Ca2+ chelator EGTA, the inhibitor of Ca2+ transport Ruthenium red and the inhibitor of the myosin ATPase vanadate. The caffeine concentration dependency of respiration of fibers coincided with the caffeine-caused stimulation of myosin ATPase activity. The activation of oxidative phosphorylation in saponin-skinned human muscle fibers by caffeine can be explained by a stimulation of myosin ATPase caused by Ca2+ release from sarcoplasmic reticulum.

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.

Multiple conductance levels in rat heart inner mitochondrial membranes studied by patch clamping

The behavior of the mitochondrial inner membrane multiple conductance channel (MCC) which has a peak conductance of 1-1.5 nS has been examined in rat heart mitochondria. MCC can display several unique characteristics: (a) prolonged open and closed times on the order of seconds to minutes, (b) a voltage dependence in which MCC opens (negative potential) or closes (positive potential) generally in steps, (c) a response to inhibitors such as amiodarone in steps corresponding at least approximately to those in (b), (d) a 'free-running mode' in which the current level rapidly fluctuates between a minimum of nine conductance levels but with a preferred occupation of the 0.5-0.7 nS levels, and (e) very large transitions (1-1.5 nS) resolved at 4 kHz bandwidth as single events with variable mean open time.

Modulation of inner mitochondrial membrane channel activity

Three classes of inner mitochondrial membrane (IMM) channel activities have been defined by direct measurement of conductance levels in membranes with patch clamp techniques in 150 mM KCl. The "107 pS activity" is slightly anion selective and voltage dependent (open with matrix positive potentials). "Multiple conductance channel" (MCC) activity includes several levels from about 40 to over 1000 pS and can be activated by voltage or Ca2+. MCC may be responsible for the Ca(2+)-induced permeability transition observed with mitochondrial suspensions. A "low conductance channel" (LCC) is activated by alkaline pH and inhibited by Mg2+. LCC has a unit conductance of about 15 pS and may correspond to the inner membrane anion channel, IMAC, which was proposed from the results obtained from suspension studies. All of the IMM channels defined thus far appear to be highly regulated and have a low open probability under physiological conditions. A summary of what is known about IMM channel regulation and pharmacology is presented and possible physiological roles of these channels are discussed.