Regulation of pyruvate dehydrogenase activity and citric acid cycle intermediates during high cardiac power generation

Abstract

A high rate of cardiac work increases citric acid cycle (CAC) turnover and flux through pyruvate dehydrogenase (PDH); however, the mechanisms for these effects are poorly understood. We tested the hypotheses that an increase in cardiac energy expenditure: (1) activates PDH and reduces the product/substrate ratios ([NADH]/[NAD(+)] and [acetyl-CoA]/[CoA-SH]); and (2) increases the content of CAC intermediates. Measurements were made in anaesthetized pigs under control conditions and during 15 min of a high cardiac workload induced by dobutamine (Dob). A third group was made hyperglycaemic (14 mm) to stimulate flux through PDH during the high work state (Dob + Glu). Glucose and fatty acid oxidation were measured with (14)C-glucose and (3)H-oleate. Compared with control, the high workload groups had a similar increase in myocardial oxygen consumption ( and cardiac power. Dob increased PDH activity and glucose oxidation above control, but did not reduce the [NADH]/[NAD(+)] and [acetyl-CoA]/[CoA-SH] ratios, and there were no differences between the Dob and Dob + Glu groups. An additional group was treated with Dob + Glu and oxfenicine (Oxf) to inhibit fatty acid oxidation: this increased [CoA-SH] and glucose oxidation compared with Dob; however, there was no further activation of PDH or decrease in the [NADH]/[NAD(+)] ratio. Content of the 4-carbon CAC intermediates succinate, fumarate and malate increased 3-fold with Dob, but there was no change in citrate content, and the Dob + Glu and Dob + Glu + Oxf groups were not different from Dob. In conclusion, compared with normal conditions, at high myocardial energy expenditure (1) the increase in flux through PDH is regulated by activation of the enzyme complex and continues to be partially controlled through inhibition by fatty acid oxidation, and (2) there is expansion of the CAC pool size at the level of 4-carbon intermediates that is largely independent of myocardial fatty acid oxidation.

Figure 1. Experimental protocol

See text for abbreviations.

Figure 2. Myocardial oxygen consumption and anterior wall power (expressed as a per cent of pretreatment values during the final 5 min of the protocol

*P < 0.05, significantly different from control group. There were no other statistical differences among Dob, Dob + Glu and Dob + Glu + Oxf groups.

Figure 3. Pyruvate dehydrogenase activity expressed as a per cent of total activity measured in myocardial biopsies taken at the end of the protocol

*P < 0.001 compared with the control group. There were no other statistical differences among Dob, Dob + Glu and Dob + Glu + Oxf groups.

Figure 4. Molar ratios of ATP/ADP, NADH/NAD⁺ and acetyl-CoA/free CoA measured in myocardial biopsies taken at the end of the protocol

*P < 0.0005 compared with the control group. There were no other statistical differences among Dob, Dob + Glu and Dob + Glu + Oxf groups.

Figure 5. Rates of glucose and fatty acid oxidation

*P < 0.05 compared with the control group; †P < 0.05 compared with the Dob group during increased cardiac power.

Figure 6. Myocardial content

The myocardial content of citrate (CIT), α-ketoglutarate (α-KG), succinate (SUC), fumarate (FUM) and malate (MAL) in the Dob, Dob + Glu, and Dob + Glu + Oxf groups expressed as a fraction of the mean value in the control group. *Significantly different from the Dob + Glu group.