The dopamine-D2-agonist pramipexole (PPX) was tested for blocking mitochondrial permeability transition (PT) in order to give a possible explanation for its neuroprotective effect seen in PPX-treated Parkinson's disease patients. Patch-clamp techniques for studying single-channel currents in the inner mitochondrial membrane and large-amplitude swelling of energized mitochondria were used to study PPX action on the permeability transition pore (PTP), a key player in the mitochondrial route of the apoptotic cascade. Identity of the PTP was proven by measuring the concentration-response relation for cyclosporin A-blockade (IC50=26 nM). PPX inhibits the PTP reversibly with an IC50 of 500 nM, which is close to the values determined earlier as plasma concentrations after PPX medication in patients. Interaction of PPX with the PTP is further supported by demonstrating that it abolished Ca2+-triggered swelling in functionally intact mitochondria. Blockade of the PTP by PPX was attenuated by increasing concentrations of inorganic phosphate and by acidification. We suggest that PPX could exert part of its neuroprotective effect by inhibition of the PTP and thus, probably, blocking of the mitochondrial pathway of the apoptosis cascade.

Single-channel currents were recorded from inner mitochondrial membranes of HepG2 hepatoma cells and of normal rat liver cells by means of patch-clamp techniques. The current events showed variable amplitudes of up to 1.1 ± 0.2 nS (n = 35) at room temperature (24 °C) and of up to 1.5 ± 0.2 nS (n = 10) at 34 °C including large numbers of subconductance states. Voltages of -40 mV and below closed the channels usually with a delay of about 2 min. Increasing Ca²⁺ concentrations activated the channels, whereas cyclosporin A (100 nM) blocked. The concentration-response relation for the Ca²⁺-effect could be fitted best using an EC₅₀ of 10 µM and a Hill coefficient of 1.5. Taken together the results indicate that we recorded from the permeability transition pore (PTP). As PTP activity may be related to apoptosis we tested if lysate from differently treated T-lymphocytes (Jurkat cells) was able to induce PTP activity in HepG2 cells. Lysate of untreated cells completely abolished the activity at a Ca²⁺ concentration of 18 nM (buffered by EGTA), i.e. three orders of magnitude below the EC₅₀. Under these conditions the lysate is not likely to contain stable factors that could open the PTP. Preliminary experiments show PTP activity in CD95-activated lysate.