CYTOCHROME REDOX POTENTIAL DEPENDENCE ON SUBSTRATE IN RAT CEREBRAL CORTEX SLICES: IMPORTANCE OF CYTOPLASMIC NAD(P)H AND POTASSIUM

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Abstract
—Using dual‐wavelength absorbance spectrophotometry, the ability of various substrates to produce and maintain a redox potential in the cytochrome chain of rat cerebral cortex slices was studied. In general, the ability to reduce the cytochromes parallels previously reported capabilities of the substrates to support metabolic responses to stimulation. The steady‐state kinetics of cytochrome reduction by glucose or lactate displayed a very sharp dependency upon concentration in the regions of 1 or 3 mm, respectively. This was in contrast to a near linear reduction of the cytochromes with concentrations of pyruvate over a range of 1–10 mm. The production and maintenance of a cytochrome redox potential was found to be at least partially dependent upon the presence of potassium (3 mm in the incubation media). Reduction of the cytochromes attributable to potassium was inhibited by ouabain, indicating that intracellular potassium was the important variable.Addition of glucose or lactate to 'starved’ tissues was found to result in a complex cycle of oxidation and reduction of tissue NAD(P)H. A small initial reduction of NAD(P) was followed by an oxidation of NAD(P)H which occurred in an all‐or‐none fashion with reduction of the cytochromes. The oxidation of NAD(P)H and reduction of cytochrome b appeared to occur with a similar time course.Respiratory changes following addition of glucose were complex in time course, but established a new steady‐state rate 0.41 μmol/g per min above the preaddition rate in 10–12 min. Despite a similar level of reduction in the cytochrome chain, stimulation of respiration by pyruvate was only about 50% of the rate observed with addition of glucose. However, stimulation of respiration by addition of equim concentrations of pyruvate and lactate was found to be additive, producing a 0.48 μmol/g per min increase in the steady‐state rate of oxygen consumption.These data seem to support the conclusion that the cytoplasmic reducing equivalent derived from the initial oxidation of glucose or lactate plays an important, perhaps regulatory, role in the respiration of cerebral tissues.