Matrix volume measurements challenge the existence of diazoxide/glibencamide-sensitive KATP channels in rat mitochondria

A mitochondrial sulphonylurea-sensitive, ATP-sensitive K+ channel (mitoK(ATP)) that is selectively inhibited by 5-hydroxydecanoate (5-HD) and activated by diazoxide has been implicated in ischaemic preconditioning. Here we re-evaluate the evidence for the existence of this mitoK(ATP) by measuring changes in light scattering (A(520)) in parallel with direct determination of mitochondrial matrix volumes using (H2O)-H-3 and [C-14]sucrose. Incubation of rat liver and heart mitochondria in KCI medium containing Mg2+ and inorganic phosphate caused a decrease in light scattering over 5 min, which was accompanied by a small (15-30%) increase in matrix volume. The presence of ATP or ADP in the buffer from the start greatly inhibited the decline in A(520) whilst addition after a period of incubation (1-5 min) induced a rapid increase in A(520), especially in heart mitochondria. Neither response was accompanied by a change in matrix volume, as measured isotopically. However, the effects of ATP and ADP on A(520) were abolished by carboxyatractyloside and bongkrekic acid, inhibitors of the adenine nucleotide translocase (ANT) that lock the transporter in two discrete conformations and cause distinct changes in A(520) in their own right. These data suggest that rather than matrix volume changes, the effects of ATP and ADP on A(520) reflect changes in mitochondrial shape induced by conformational changes in the ANT. Furthermore, we were unable to demonstrate either a decrease in A(520) or increase in matrix volume with a range of ATP-sensitive K+ channel openers such as diazoxide. Nor did glibencamide or 5-HD cause any reduction of matrix volume, whereas the K+ ionophore valinomycin (0.2 nm), produced a 10-20% increase in matrix volume that was readily detectable by both techniques. Our data argue against the existence of a sulphonylurea-inhibitable mitoK(ATP) channel.