It is assumed that calpain plays an important role in apoptotic cell death in several diseases e.g. in macular degeneration. The intrinsic signaling cascade in apoptosis includes mitochondria. Inhibition of this signaling chain within the mitochondria was thought to be a possibility to slow down the progress of the disease. Opening of the permeability transition pore (PTP) is thought to be the step in apoptosis by which the mitochondrial membrane potential is lost and equilibration of the smaller ions occurs. We used mitoplasts, i.e. vesicles of inner mitochondrial membrane, to record single-channel events of the PTP and of the maxi-K(Ca) channel (BK) by patch-clamp techniques. Mitoplasts were bathed in an isotonic KCl-solution containing additionally 200 mM Ca2+, pH 7.2. Capain and calpeptin were purchased from Calbiochem.

Single-channel recordings of the PTP demonstrated an insignificant tendency to an increased open probability by 50 mM calpain. In earlier experiments we found that the PTP may be inhibited by an open BK (Cheng et al., 2010, FEBS Lett. 584, 2005–2012). Therefore, we tested the effect of calpain also on the BK. As calpeptin is known as an antagonist of calpain, we tested additionally for the effect of calpeptin on both channels. 0.01% (w/v) calpeptin inhibited the PTP significantly by 35%. However, calpeptin inhibited the BK significantly. The effect on the BK was concentration-dependent and could not be reversed by wash-out in isotonic solution without calpeptin. Calpain is necessary for AIF release by truncated Bid, an effect precluded by calpeptin (Polster et al., 2005, J. Biol. Chem. 280, 6447–6454). Our results suggest that calpain has minor direct influence on the open probability of the PTP while its inhibitor calpeptin inhibits both channels.

Financial support by Deutsches Zentrum für Neurodegenerative Erkrankungen (DZNE)