Respiratory inhibitors and uncouplers prevent the aeration-induced increase in mitochondrial anion conductivity

Hydrogen peroxide generated from cardiac myocytes impacts metabolic dilation in coronary arterioles

During oxidative cardiac metabolism, the myocardium produces reactive oxygen species, such as superoxide and hydrogen peroxide (H(2)O(2)). We hypothesized H(2)O(2) is a coronary metabolic dilator linking regulation of coronary tone with myocardium metabolism. Dilation of isolated, pressurized coronary arterioles (76 +/- 10 microm, diameter) in reaction to supernatant collected from enzymatically isolated cardiac myocytes was measured. Isolated rat myocytes were stimulated electrically [unpaced or stimulated at 200, 400 beats/min (bpm)]. H(2)O(2) was significantly generated by pacing (400 bpm n = 11, 9.3 +/- 0.4 microM P < 0.01, versus unpaced) and the addition of this supernatant caused vasodilation (500 microL to 2 mL bath, 14.6 +/- 0.7%, P < 0.01 versus unpaced). Supernatant from unpaced myocytes was not vasoactive. To clarify the source of H(2)O(2), myocytes were also stimulated at 400 bpm following treatment with Mn-TBAP (25 microM), which mimics the action of Mn-SOD, and apocynin (3 mM), an NADPH oxidase inhibitor (n = 11, each). Mn-TBAP increased H(2)O(2) generation in myocyte supernatant stimulated at 400 bpm (12.2 +/- 0.8 microM, P < 0.01 versus 400 bpm stimulation only). Treatment of the myocytes with Mn-TBAP augmented vasodilation by the stimulated myocyte supernatant (19.6 +/- 1.1%, P < 0.01 versus untreated myocyte supernatant). Apocynin did not alter vasodilation to myocyte supernatant. These results suggest that the main source of superoxide by metabolic stimuli is cardiac myocytes and Mn-SOD is a scavenger from superoxide to H(2)O(2). We conclude that H(2)O(2) is a key metabolic vasodilator produced by myocardium.

Inorganic phosphate is the major component of the thermostable cytoplasmic fraction which stimulates mitochondrial anion uniport

A low molecular weight thermostable cytoplasmic fraction isolated from rat liver homogenate when pre-incubated with mitochondria increases the rate at which anions enter mitochondria via the pH-dependent anion-conducting channel in the inner membrane. The crude fraction obtained by centrifuging and heating the liver homogenate was purified by gel filtration and chromatography on DEAE-cellulose. The resulting factor is stable to heating at 100 degrees C, freeze-drying and extremes of pH. Inorganic phosphate co-purified with activity and activity was lost when the phosphate was removed by barium salt precipitation. A pure sample of KH2PO4 produced stimulation of anion conductivity. These results show that the major portion of the activity which stimulates anion uniport can be accounted for by the presence of phosphate in the crude and purified fractions. Mersalyl blocks stimulation when added before, but not when added after, incubation with phosphate which shows that the stimulation is produced by phosphate in the mitochondrial matrix. The proposed role of this factor in thyroid hormone action is discussed in the light of its identification as inorganic phosphate.

Effect of buffers and osmolality on anion uniport across the mitochondrial inner membrane

The effects of buffers and osmolality of the suspending medium on the pH-dependent anion uniport across the inner membrane of isolated rat liver mitochondria have been studied using the light scattering technique to measure passive osmotic swelling. In contrast to some other transport processes the rates of entry of chloride and other anions via the anion-conducting channel decreased steeply with increasing solute concentration. This effect appears to be a result of increased osmolality or decreased matrix volume rather than inhibition by the anion since it was also produced by increasing the osmolality by addition of non-penetrant solutes. The effects of some pH buffers on the mitochondrial anion-conducting channel were also investigated. Some zwitterionic buffers had little effect other than that produced by increasing osmolality but Tricine, Popso and Caps produced marked additional inhibition of anion uniport and several other zwitterionic buffers were also inhibitory. The correlation between increased anion conductivity and increased matrix volume supports the proposal that this channel functions in regulation of the volume of the mitochondrial matrix.

Calcium ion activation of the anion-conducting channel in the rat liver mitochondrial inner membrane

Stimulation of the rat liver mitochondrial inner-membrane anion-conducting channel by aeration is dependent on the concentration of Ca2+ ions in the assay medium. Ca2+ activates anion conduction in both aerated and non-energised mitochondria but acts over a wider concentration range and produces a greater increase in anion-conductivity in aerated mitochondria. EGTA reverses Ca2+ stimulation but takes several seconds to act, indicating slow release of Ca2+ from the activation site possibly on the matrix side of the inner mitochondrial membrane. It is suggested that this channel may respond to hormone-induced changes in cytosolic Ca2+ concentration.