The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism.

Relation between mitochondrial calcium transport and control of energy metabolism.

Summary

1
To answer the question ‘What controls the rate of respiration?’ requires a clear definition of control and an explicit description of the limits of the system to be considered. In this review we use a neutral definition of control in which A controls B if changes in A cause changes in B. A useful system to define when discussing the control of respiration consists of the electron transport chain, the H+-ATPase, the adenine nucleotide carrier, the intramitochondrial adenine nucleotide and phosphate pools, δp and the proton leak across the mitochondrial inner membrane.

2
Controls operating within this system are designated internal controls and many of them are fairly well characterized. Several models have been advanced to describe the rates of these internal processes in isolated mitochondria, including control of respiration rate by cytochrome oxidase with all other steps near to equilibrium, control by the adenine nucleotide carrier or control by the extent of displacement of individual reactions from equilibrium. More recently, analysis using control theory has shown that in the resting state (state 4) most control over flux is exerted by the leak of protons through the inner membrane, whereas in more active, phosphorylating states (up to state 3) control is distributed between a number of steps, including the proton leak, the adenine nucleotide carrier and cytochrome oxidase. This approach seems a very useful framework within which to pose further questions.

3
This system may be treated as a ‘black box’ interacting with its environment (the rest of the mitochondrion and the experimental cuvette or living cell) through the redox states of NAD, Q and O2 and through the phosphorylation state of the extramitochondrial adenine nucleotides. Very few external effectors cross the system boundary; the only well-characterized ones are long-chain fatty acyl-CoA, which inhibits the adenine nucleotide carrier, and fatty acids, which activate a specific uncoupling protein found only in the inner membrane of mitochondria from brown adipose tissue. At this level respiration rate is determined only by the internal properties of the ‘black box’, by the redox states of NAD, Q and O2, and by the phosphorylation status of the extramitochondrial adenine nucleotide pool.

4
Within a cell the rate of respiration is controlled primarily by the rates of reactions feeding electrons to the electron transport chain (through their effects on NADH/NAD and QH2/Q ratios) and by the rates of reactions consuming or producing ATP (through the cytosolic phosphorylation potential or ATP/ADP ratio). Control of reducing equivalent supply occurs through availability of oxidizable substrates (determined by diet and hormonal status), through regulation of pathways such as glycolysis or fatty acid catabolism and, importantly, through Ca2+ activation of intramitochondrial dehydrogenases.

5
Hormonal control over respiration can occur at all the levels mentioned. Hormones may alter the kinetic properties of the oxidative phosphorylation system by altering the concentrations of individual proteins or by altering their kinetic properties either by affecting the lipid environment or, possibly, more directly. Important controls by hormones occur through changes in ATP demand altering the cytoplasmic adenine nucleotide pool and by changes in free Ca2+ concentration in the mitochondrial matrix, altering the activity of dehydrogenases and the supply of electrons to NAD and Q. Hormones also affect the supply of reducing equivalents to the mitochondria by their catabolic or anabolic effects on other pathways.

Control of electron flux through the respiratory chain in mitochondria and cells

Surgical procedures evoke an endocrine response which results in substrate mobilization, a change in metabolism towards catabolism with a negative nitrogen balance and retention of salt and water. The magnitude of this response is proportional to the severity of the operative trauma. Intraabdominal procedures are associated with a much greater response than body surface surgery (Clarke, 1970; Clarke, Johnston and Sheridan, 1970) and cardiac surgery with cardiopulmonary bypass induces profound hormonal and biochemical changes (Stanley et al., 1979). There is a vast and often conflicting literature on the hormonal and metabolic response to surgery. The purpose of this short article is not to present a complete review of the subject but rather to provide the necessary background information to enable the results described in more detailed studies to be readily interpreted.

Endocrine and metabolic changes during surgery: anaesthetic implications

Major thermal injury is associated with extreme hypermetabolism and catabolism as the principal metabolic manifestations encountered following successful resuscitation from the shock phase of the burn injury. Substrate and hormonal measurements, indirect calorimetry, and nitrogen balance are biochemical metabolic parameters which are useful and more readily available biochemical parameters worthy of serial assessment for the metabolic management of burn patients. However, the application of stable isotopes with gas chromatography/mass spectroscopy and more recently, new immunoassays for growth factors and cytokines has increased our understanding of the metabolic manifestations of severe trauma. The metabolic response to injury in burn patients is biphasic wherein the initialebb phase is followed by a hypermetabolic and catabolicflow phase of injury. The increased oxygen consumption/metabolic rate is in part fuelled by evaporative heat loss from wounds of trauma victims, but likely also by a direct central effect of inflammation upon the hypothalamus. Although carbohydrates in the form of glucose appear to be an important fuel source following injury, a maximum of 5–6 mg/kg/min only is beneficial. Burn patients have accelerated gluconeogenesis, glucose oxidation, and plasma clearance of glucose. Additionally, considerable futile cycling of carbohydrate intermediates occurs which includes anaerobic lactate metabolism and Cori cycle activity arising from wound metabolism of glucose and other substrates. Similarly, accelerated lipolysis and futile fatty acid cycling occurs following burn injury. However, recent evidence suggests that lipids in the diet of burned and other injured patients serve not only as an energy source, but also as an important immunomodulator of prostaglandin metabolism and other immune responses. Amino acid metabolism in burn patients is characterized by increased oxidation, urea synthesis, and protein breakdown which is prolonged and difficult to reduce with current nutritional therapy. However, the current goal of nutritional support is to optimize protein synthesis. Specific unique requirements may exist for supplemental glutamine and arginine following burn injury but further research is needed before enhanced branched chain amino acids supplements can be recommended for burn patients. Recent research investigations have revealed the importance of enteral feeding to enhance mucosal defense against gut bacteria and endotoxin. Similarly, research has demonstrated that many of the metabolic perturbations of burns and sepsis may be due, at least in part, to inflammatory cytokines. Investigation of their pathogenesis and mechanism of action both at a tissue and a cellular level offer important prospects for improved understanding and therapeutic control of the metabolic disorders of burn patients.

The metabolic effects of thermal injury

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2880%2980523-0

Stimulation of [1-14C]oleate oxidation to 14CO2 in isolated rat hepatocytes by vasopressin: effects of Ca2+.

1. Blood alanine was measured in six patients undergoing total hip replacement and in four normal subjects starved for 4 days. Hypoalaninaemia occurred in both groups and persisted in the surgical patients despite an adequate diet. The blood alanine was also low in four insulin-dependent diabetic patients and in four patients with muscular dystrophy; it was normal in four patients with cirrhotic liver disease. 2. The removal of an intravenous L-alanine load (12 g; 133 mmol) was significantly increased after surgery and in the diabetic patients, unaltered by starvation, and decreased in the cirrhotic patients. 3. Increases in blood glucose were observed when alanine infusion was performed 6 h after surgery and after 3 days' starvation. Increases in blood lactate and pyruvate always occurred after alanine infusion but were most marked 6 h after surgery. 4. These results show that the metabolic response to an alanine load and the ability of the body to remove it alter with change in physiological state, and that the hypoalaninaemia after surgery and in diabetes is related to an increased removal of intravenous alanine, whereas that during starvation is not.

Relationship between the basal blood alanine concentration and the removal of an alanine load in various clinical states in man.

1. To investigate the effects of starvation, elective surgery, accidental injury and other clinical conditions on the metabolism of branched-chain amino acids in man, we have measured the basal concentration of leucine and the removal and metabolic effects of infused l-leucine. 2. The blood concentration of leucine was significantly increased by surgery, starvation and accidental injury, and decreased in cirrhosis. It tended to increase in diabetes and was unaffected by muscular dystrophy. 3. The half-life of infused leucine was nearly doubled by 4 days of complete starvation, unaltered by surgery and decreased by severe accidental injury. Infusion with Intralipid, which increased free fatty acid and ketone-body concentrations, had no effect on the removal of a leucine load. The clearance rate of infused leucine was reduced in diabetes and muscular dystrophy and increased in cirrhosis. 4. The effects of infused leucine on blood glucose and ketone bodies differed according to the groups studied. 5. Since the traumatized patients were given sufficient energy and nitrogen and disposed of a leucine load at a different rate from the starved patients, the causes of the increase in blood concentration of leucine in these two conditions are different.

The removal of infused leucine after injury, starvation and other conditions in man.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2880%2981051-9

α-Adrenergic stimulation of [1-14C]oleate oxidation to 14CO2 in isolated rat hepatocytes

1. The increase in blood glucose after intravenous galactose was measured at 6−8 h and 14 days after hip replacement (14 patients) and accidental injury (14 patients). 2. After hip replacement there was a greater rise of glucose after galactose on the day of the operation than on recovery, despite basal hyperglycaemia. This earlier period was also associated with inappropriately low insulin concentrations for the prevailing glucose concentration, and hyperketonaemia (13 out of 14 patients). 3. After accidental injury patients with initial hyperketonaemia (nine out of 14) also had a greater rise of blood glucose after galactose than on recovery and had relative insulin deficiency; in contrast those who were initially normoketonaemic (five out of 14) showed a rise in glucose comparable with that after recovery and basal insulin concentrations more appropriate to the existing glucose concentration. 4. It is concluded that in most subjects after injury hepatic glucose release after galactose administration is not suppressed despite hyperglycaemia.

Vera Ilic

Papers

Stimulation of [1-14C]oleate oxidation to 14CO2 in isolated rat hepatocytes by vasopressin: effects of Ca2+.

Relationship between the basal blood alanine concentration and the removal of an alanine load in various clinical states in man.

The removal of infused leucine after injury, starvation and other conditions in man.

α-Adrenergic stimulation of [1-14C]oleate oxidation to 14CO2 in isolated rat hepatocytes

Changes in Blood Glucose and Plasma Insulin after Intravenous Galactose in Human Injury