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Biomedical subjects

B Crabtree

Publications and source records attributed to B Crabtree.

48 records · Page 3Linked to original sources
PubMed1976

Substrate cycles in metabolic regulation and in heat generation.

1. The presence of substrate cycles in tissues has been demonstrated by direct isotope methods in recent years. This demonstration has provided the impetus for a reappraisal of the roles of substrate cycling in metabolic regulation and in heat production. These aspects of substrate cycling are discussed in this paper. The relationship between near-equilibrium reactions and substrate cycles is emphasized, since this provides a basis for the derivation of a function describing in precise quantitative terms the factors governing the amplification provided by substrate cycles in metabolic regulation. Some examples of the roles of substrate cycles in providing sensitivity in metabolic regulation are described. The importance of substrate cycling in heat generation in the flight muscle of the bumble-bee and in brown adipose tissue is discussed in detail. 2. We point out that the two possible roles of cycling, heat production and amplification, are intimately linked so that they must be discussed together. It is proposed that variable rates of substrate cycling may be possible so that, for short periods of time. sensitivity can be maximal without excessive heat generation. Variable rates over the long term may be involved in weight control, and the control of such variability in cycling rates may be impaired in obese subjects. Finally, the possibilities that substrate cycles provide explanations for the specific dynamic action of food and for alcoholic and accidental hypothermia are raised.

Adipose Tissue↗
PubMed1972

The activities of phosphorylase, hexokinase, phosphofructokinase, lactate dehydrogenase and the glycerol 3-phosphate dehydrogenases in muscles from vertebrates and invertebrates.

1. The maximum activities of hexokinase, phosphorylase and phosphofructokinase have been measured in extracts from a variety of muscles and they have been used to estimate the maximum rates of operation of glycolysis in muscle. These estimated rates of glycolysis are compared with those calculated for the intact muscle from such information as oxygen uptake, glycogen degradation and lactate formation. Reasonable agreement between these determinations is observed, and this suggests that such enzyme activity measurements may provide a useful method for comparative investigations into quantitative aspects of maximum glycolytic flux in muscle. 2. The enzyme activities from insect flight muscle confirm and extend much of the earlier work and indicate the type of fuel that can support insect flight. The maximum activity of hexokinase in some insect flight muscles is about tenfold higher than that in vertebrate muscles. The activity of phosphorylase is greater, in general, in vertebrate muscle (particularly white muscle) than in insect flight muscle. This is probably related to the role of glycogen breakdown in vertebrate muscle (particularly white muscle) for the provision of ATP from anaerobic glycolysis and not from complete oxidation of the glucose residues. The activity of hexokinase was found to be higher in red than in white vertebrate muscle, thus confirming and extending earlier reports. 3. The maximum activity of the mitochondrial glycerophosphate dehydrogenase was always much lower than that of the cytoplasmic enzyme, indicating that the former enzyme is rate-limiting for the glycerol 3-phosphate cycle. From the maximum activity of the mitochondrial enzyme it can be calculated that the operation of this cycle would account for the reoxidation of all the glycolytically produced NADH in insect flight muscle but it could account for only a small amount in vertebrate muscle. Other mechanisms for this NADH reoxidation in vertebrate muscle are discussed briefly.

Animals↗
PubMed1972

The activities of fructose diphosphatase in flight muscles from the bumble-bee and the role of this enzyme in heat generation.

1. The maximum catalytic activities of fructose diphosphatase from flight muscles of bumble-bees (Bombus spp.) are at least 30-fold those reported for the enzyme from other tissues. The maximum activity of fructose diphosphatase in the flight muscle of any particular bee is similar to that of phosphofructokinase in the same muscle, and the activity of hexokinase is similar to or greater than the activity of phosphofructokinase. There is no detectable activity of glucose 6-phosphatase and only a very low activity of glucose 6-phosphate dehydrogenase in these muscles. The activities of both fructose diphosphatase and phosphofructokinase vary inversely with the body weight of the bee, whereas that of hexokinase is relatively constant. 2. There is no significant hydrolysis of fructose 1-phosphate, fructose 6-phosphate, glucose 1,6-diphosphate and glycerol 3-phosphate by extracts of bumble-bee flight muscle. 3. Fructose 1,6-diphosphatase from bumble-bee flight muscle and from other muscles is inhibited by Mn(2+) and univalent cations; the potency of inhibition by the latter varies in the order Li(+)>Na(+)>K(+). However, the fructose diphosphatase from bumble-bee flight muscle is different from the enzyme from other tissues in that it is not inhibited by AMP. 4. The contents of ATP, hexose monophosphates, fructose diphosphate and triose phosphates in bumble-bee flight muscle showed no significant changes between rest and flight. 5. It is proposed that both fructose diphosphatase and phosphofructokinase are simultaneously active and catalyse a cycle between fructose 6-phosphate and fructose diphosphate in resting bumble-bee flight muscle. Such a cycle would produce continuous hydrolysis of ATP, with the release of energy as heat, which would help to maintain the thoracic temperature during rest periods at a level adequate for flight.

Adenosine Monophosphate↗
PubMed1972

The activities of pyruvate carboxylase, phosphoenolpyruvate carboxylase and fructose diphosphatase in muscles from vertebrates and invertebrates.

1. The activities of pyruvate carboxylase, phosphoenolpyruvate carboxylase and fructose diphosphatase in crude homogenates of vertebrate and invertebrate muscles are reported. 2. Pyruvate carboxylase activity was present in all insect flight muscles that were investigated: in homogenates of bumble-bee flight muscle the activity was inhibited by ADP and activated by acetyl-CoA, and it was distributed mainly in the mitochondrial fraction. This is the first demonstration of pyruvate carboxylase activity in muscle. However, the activity appears to be restricted to insect flight muscle, since it was not found in other invertebrate or vertebrate muscles. 3. Since the three enzymes were never found together in the same muscle, it is concluded that these enzymes cannot provide a pathway for the synthesis of glycogen from lactate or pyruvate in muscle. Other roles for these enzymes in muscle are suggested. In particular, pyruvate carboxylase may be present in insect flight muscle for the provision of oxaloacetate to support the large increase in activity of the tricarboxylic acid cycle which occurs when an insect takes flight.

Acetyl Coenzyme A↗
PubMed1972

The activities of lipases and carnitine palmitoyltransferase in muscles from vertebrates and invertebrates.

1. The activities of tri-, di- and mono-glyceride lipase and carnitine palmitoyltransferase were measured in homogenates of a variety of muscles. These activities were used to estimate the rate of utilization of glycerides and fatty acids by muscle. In muscles whose estimated rates of fat utilization can be compared with rates calculated for the intact muscle from such information as O(2) uptake, there is reasonable agreement between the estimated and calculated rates. 2. In all muscles investigated the maximum rates of hydrolysis of glycerides increase in the order triglyceride, diglyceride, monoglyceride. The activity of diglyceride lipase is highest in the flight muscles of insects such as the locust, waterbug and some moths and is lowest in the flight muscles of flies, bees and the wasp. These results are consistent with the utilization of diglyceride as a fuel for some insect flight muscles. 3. In many muscles from both vertebrates and invertebrates the activity of glycerol kinase is similar to that of lipase. It is concluded that in these muscles the metabolic role of glycerol kinase is the removal of glycerol produced during lipolysis. However, in some insect flight muscles the activity of glycerol kinase is much greater than that of lipase, which suggests a different role for glycerol kinase in these muscles.

Animals↗
PubMedDate not supplied

Using practice genograms to understand and describe practice configurations.

BACKGROUND: Demands for change in medical practices are coming from multiple sources. Since interventions to change clinical practice continue to have limited success, understanding the functional structure of primary care practices and the dynamics of providing care have become increasingly important. METHODS: To portray and understand the primary care office system, we developed "practice genograms" that describe practice participants and their relationships with each other. Formal organizational structure is evaluated using family systems theory and family of origin genogram techniques. RESULTS: Practice genograms provided a more dynamic, relational model than the organizational chart and promoted identification of relationship strengths and weaknesses within a practice the same way that family genograms identify these characteristics in a family system. CONCLUSIONS: Research implications for the use of the practice genogram include enhanced data gathering, increased understanding of the complexity of practices as adaptive systems, and increased understanding of current and potential approaches to changing practices.

Family Practice↗