PubMed Health⌕ Search

Biomedical subjects

J B Chapman

Publications and source records attributed to J B Chapman.

At least 37 records · Page 2Linked to original sources
PubMed1979

Cardiac heat production.

The energy production (heat + work) of cardiac muscle must be interpreted in terms of the major ATPases underwriting cardiac contraction; these are the Ca2+ and Na+-K+ transport ATPases and actomyosin ATPase. It is possible to apply the classical phenomenological subdivisions to cardiac energy production; when this is done, certain properties immediately distinguish cardiac muscle from skeletal muscle. Little or no temporal distinction exists between initial (anaerobic) and recovery (oxidative) metabolism. Even at temperatures as low as 20 degrees C most of the recovery heat is released within the time course of a single contraction. Cardiac muscle is characterized by a high resting heat rate, the magnitude of which varies between species and depends on the metabolic substrate. In isometric contractions there is a slightly curvilinear relationship between developed force and heat production. There is a tension-independent or activation component, the magnitude of which reflects the prevailing level of contractility and is probably associated with calcium release and retrieval. In isotonic contractions energy production is maximal when the muscle is heavily loaded but falls steeply when the size of the load is reduced. The enthalpy:load relation is probably similar to that found in twitch contractions of skeletal muscle working at room temperature or above; but, unlike for skeletal muscle, there are families of such curves: At any instant of time the relation depends upon the prevailing physiological conditions (e.g. stimulus rate, substrate supply, humoral agents, extracellular ionic concentrations, initial length). Cardiac energy production can be estimated by a variety of other techniques (such as high-energy phosphate utilization, oxygen consumption, and changes in tissue fluorescence related to pyridine nucleotide oxidation levels). At the present time there is considerable agreement between heat measurements and results obtained with these different techniques. We should like to conclude on a cautionary note. First, there is considerable variability in the properties of cardiac muscle from different species. Significant variations occur at nearly all levels of cellular function--e.g. shape of action potential, electrical and mechanical dependence upon stimulus history, mechanisms of excitation-contraction coupling, actomyosin ATPase activity, metabolic regulation, and differential sensitivity to anoxia or ischemia. Second, the types of contractions readily studied in isolated papillary muscles (i.e. isometric or isotonic twitches) may not necessarily be the best mechanical paradigms for understanding myocardial energetics in vivo. The particular geometric demands of individual research techniques require the use of a wide variety of myocardial preparations from a wide variety of species. This necessarily produces a pastiche view of cardiac muscle rather than an integrated picture of some hypothetically typical mammalian myocardium.

Adenosine Triphosphatases↗
PubMed1976

Fluorometric studies of recovery metabolism of rat fast- and slow-twitch muscles.

Recovery metabolism of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of the rat has been investigated using fluorometric monitoring of reduction of nicotinamide adenine dinucleotide (NAD). In both EDL and SOL, groups of twitch contractions produced a decrease in fluorescence (oxidation of NADH) which returned to the resting base line after contraction ceased. These responses proceeded more quickly in EDL than SOL and were abolished by anoxia. A 1-s tetanus of SOL produced an initial reduction which could be abolished with iodoacetate followed by a prolonged oxidation which could be blocked by anoxia. The fluorescence of EDL was decreased immediately following a 1-s tetanus but then rapidly increased well beyond the resting level of reduction and persisted throughout the recovery period. This reduction was largely depressed by iodoacetate. The results indicate marked differences in the recovery metabolism of these muscles, consistent with predominantly mitochondrial oxidative activity in the slow-twitch muscles and predominantly glycolytic activity in the fast-twitch muscles.

Aerobiosis↗
PubMed1975

Fluorometric and myothermic studies of cardiac energetics.

Energy production during isometric and isotonic contractions of cardiac muscle has been studied using the isolated papillary muscle of the rabbit at room temperature. Muscles were mounted on thermopiles to record heat production associated with small numbers of contractions and then were mounted in an organ bath for fluorometric recording of the level of reduction of mitochondrial NAD. After each twitch there was a decrease in the level of fluorescence followed by an exponential return to the resting level, or further decreases after subsequent twitches. The fluorescence-time integral (integral delta Fl-dt), or area enclosed by a fluorometric transient during and after a series of twitches, was found to vary in magnitude in a manner similar to the magnitude of the total heat production when the mechanical conditions of contraction were altered by varying the load. The influence of the metabolic state of the muscle was also studied by using either glucose or sodium pyruvate as the exogenous substrates in the bathing medium. When pyruvate was substituted for glucose, maximum external work, maximum isometric force, and maximum mechanical efficiency were all increased. The kinetics of oxidative recovery metabolism, either measured fluorometrically or derived from the heat production, was accelerated when pyruvate replaced glucose.

Animals↗
PubMed1972

Energetics of isometric and isotonic twitches in toad sartorius.

Contractile energetics have been studied in twitches of toad sartorius muscle at 6-7 degrees C. Isometric and isotonic energy production has been measured and plotted against a wide range of developed tensions and tension-time integrals. These parameters were varied by altering the isotonic load or by changing the preset isometric length. The isometric tension-independent heat was 1.12 +/-0.18 (SD) mcal/g. The isometric heat coefficient Pl(0)/H was 12.0 +/-1.4 in muscles having twitch to tetanus ratios ranging from 0.4 to 0.6. Isometric enthalpy increased monotonically with tension or tension-time integral but the correlation between isometric heat and these parameters was poor. Isotonic enthalpy consumption was always higher than isometric enthalpy for any given tension or tension-time integral; however, isotonic heat production was consistently less than isometric heat production. The isotonic heat for the highest load (3 g) was not significantly different from the isometric tension-independent heat. Thus isotonic heat production first decreased and then increased with increasing tension or tension-time integral. In the discussion it is shown that the results conflict with all current interpretations of muscle energetics.

Animals↗
PubMed1972

An energetic model of muscle contraction.

Initial energy utilization in the twitch is visualized as the result of the activity of two distinct processes. The first is the calcium-pumping activity of the sarcoplasmic reticulum, which has a constant energy requirement under normal conditions. The second is the chemomechanical transduction process consisting of a variable number of quantal contractile events, each with a fixed enthalpy equal to the molecular enthalpy of adenosine triphosphate (ATP) hydrolysis in vivo. This enthalpy appears either as heat or as contractile element work. Total enthalpy varies according to the number of quantal contractile events that occur in the twitch cycle. The basis of the variation is suggested to be velocity-dependent activity of the actomyosin ATPase, allowing more quantal events to occur in a contraction cycle when shortening occurs. The classical designation "activation heat" is held to be appropriate for the first process. The partition of the enthalpy of the second process that is currently in vogue is held to be misleading and a new formulation is suggested in which the properties of the quantal contractile event are reflected in general terms. The formulation of the proposed transduction model represents a conceptual return to the viscoelastic theory, but at a quantal level. The model can explain the results of the preceding paper and is adaptable to different muscles without having to postulate fundamental differences in energy utilization.

Actomyosin↗
PubMed1972

Fluorometric studies of oxidative metabolism in isolated papillary muscle of the rabbit.

The fluorometric technique for measuring the levels of reduced pyridine nucleotides was used to study oxidative metabolism in isolated rabbit papillary muscle at 23 degrees C. The 100% standard level of tissue fluorescence was defined as that measured for muscles resting in oxygenated 10 mM pyruvate solution. This level increased 15% with anoxia and decreased 45% with stimulation in substrate-free solution. Thus, about one-half of the standard tissue fluorescence was metabolically labile and this labile fraction is suggested to be mitochondrial in origin. Decreased tissue fluorescence following mechanical activity was identified with increased oxidation of mitochondrial reduced nicotinamide adenine dinucleotide (NADH) owing to stimulation by adenosine diphosphate (ADP), released during activity, of mitochondrial respiration. The kinetics of the fluorescence transients were slowed fourfold by removal of pyruvate. This effect was not significantly reversed by addition of 10 mM glucose. The time integrals of the fluorescence transients were linearly related to the amounts of mechanical activity in the presence, but not in the absence, of pyruvate. A positive correlation was observed between the steady-state peak tension at constant stimulus rate and the resting level of reduction of pyridine nucleotides in various media. The fluorometric results are interpreted to be indicative of the steady and transient states established by the substrate dehydrogenases and the respiratory chain during oxidative phosphorylation in mitochondria.

Animals↗