Rabbit papillary muscle has been exposed to calcium concentrations ranging from 2.5 mM to zero. Its mechanical and electrical activity has been monitored and its heat production measured using a myothermic technique. Calcium depletion decreased the magnitude of the tension-independent heat per contraction from a mean of 0.45 mcal/g muscle to 0.31 mcal/g muscle at room temperature (18 degrees to 22 degrees C). Calcium-chelating agents did not abolish action potential conduction under the experimental conditions used but they further reduced the magnitude of the tension-independent heat. Raising the temperature from room level to 32 degrees C decreased the tension-independent heat from a mean of 0.52 to a mean of 0.24 mcal/g muscle. Calcium depletion at 32 degrees C further decreased this heat and it was calculated that the energy now liberated in activating the muscle was about 2% of the total energy normally liberated in the working heart. The results are interpreted in terms of current biochemical and myothermic data.

Action Potentials↗

PubMed1967

Changes in cardiac heat production with agents that alter contractility.

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Animals↗

PubMed1967

Energetics of cardiac contractions.

1. The average resting heat production of a muscle under zero tension is 24.8 mcal/g muscle .min at 20 degrees C. In the majority of muscles examined the resting heat production increases when the resting tension and muscle length are increased.2. The relation between actively developed tension and heat produced is similar to that existing in skeletal muscle. The plot of heat against developed tension can be obtained either by altering muscle length or by varying the stimulus frequency.3. The mean maximum total efficiency work/(work + heat) in the work experiments was 11.6%. The total energy produced (work + heat) correlates with the load rather than with the work done.4. In isotonic contractions more heat is liberated than the heat versus tension plot predicts. This extra heat is load-dependent.

Animals↗

PubMed1966

Activation heat in frog sartorius muscle.

Upon excitation of a muscle with two stimuli and variation of the interval between them up to the end of the period of full mechanical fusion, an increment of the isometric heat over that found in a single twitch is obtained. This is a good approximation to the activation heat, directly at 0 degrees C, or after certain corrections which become important at higher temperature. The activation heat so found is independent of the muscle length and nearly independent of temperature. It is increased by nitrate and caffeine.

Animals↗

PubMed1965

Heat production in a cardiac contraction.

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Animals↗

PubMedDate not supplied

Stress as an index of metabolic cost in papillary muscle of the cat.

Active stress, stress-time integral (STI) and total heat production of cat right ventricular papillary muscles were recorded during brief trains of isometric twitch contractions at muscle lengths less than or equal to optimal length. Individual muscles were subjected to a 10 degree C change in temperature, a change of stimulus frequency and the addition of isoprenaline sulphate (10(-7) mol. 1(-1). The STI-heat and stress-heat data were subjected respectively to linear and quadratic regression analyses. For both relations, the intercept (stress-independent heat) was unaffected by the frequency change, doubled by the temperature decrease and trebled by the addition of isoprenaline. None of the treatments had a significant effect on the first or second order coefficients of the stress-heat relation. The slope of the STI-heat relation was halved by lowering the temperature, increased 50% by the addition of isoprenaline and unaffected by stimulus frequency. Thus the energetic cost of a given stress increment was constant across conditions while that for a given STI increment was not. Stress is the better mechanical index of myocardial energy cost when the inotropic state is changing.

Animals↗

PubMedDate not supplied

Amrinone and energy output of rabbit papillary muscles.

We studied the effects of amrinone (300 micrograms X ml-1) on the mechanical and energetic outputs of nonfailing rabbit papillary muscles at 27 degrees C using a myothermic technique. Amrinone was found to markedly stimulate the basal (i.e., nonbeating) metabolism. In mechanical experiments, at a 0.5-Hz stimulus rate, amrinone increased the rate of stress development and peak active stress but shortened the time to peak stress and the contractile duration. The linear relationship between total (active + passive) stress and heat production was altered by amrinone such that there was a 68% increase in the activation heat component (intercept) and an 11% increase in the energy cost per unit stress development (slope). In afterloaded isotonic experiments amrinone increased the capacity of papillary muscles to accomplish external work (W) by approximately 50% on average; however, this effect was associated with an increased total energy usage per beat (ET) such that the overall mechanical efficiency (W/ET X 100%) was not significantly altered at any load level. We conclude that amrinone greatly increases calcium delivery to the myofilaments and slightly increases the cost of stress development. These effects are examined in the light of amrinone's postulated mode of action, and possible reasons for the clinical observation of a decrease in myocardial oxygen consumption after amrinone administration are considered.

Aminopyridines↗

PubMedDate not supplied

Acute energetic effects of daunomycin on rabbit heart muscle.

The acute effects of daunomycin on the mechanical and energetic outputs of rabbit papillary muscles have been examined at 27 degrees C using a myothermic technique. Contrary to expectations, daunomycin, in concentrations ranging from 10 to 160 micrograms/ml, acted as a positive inotropic agent in terms of peak stress development and work output. The rate of stress development was, however, slightly depressed, and the major mechanical effect of the drug was to prolong the relaxation phase so that there were large drug-dependent increments in the stress-time integral. Daunomycin at a concentration of 80 micrograms/ml produced a 22% increment in peak stress development and a 74% increment in stress-time integral. The linear relationship between total (active + passive) stress and heat production was altered such that there was a 73% increment in the activation heat component (intercept) and a 37% increase in the energy cost per unit stress development (slope). In afterloaded isotonic contractions, daunomycin increased the mean work output (W; averaged over all load levels) by 49%, but there was an even greater increment in the associated energy expenditure, ET, which rose by 66%. Consequently, the overall mechanical efficiency (W/ET X 100%) fell slightly. It is concluded that, in the rabbit, daunomycin in the acute situation increases total calcium delivery to the myofilaments and decreases the "apparent" transduction efficiency. These acute effects are the opposite of those reported in papillary muscles taken from rabbits in cardiac failure induced by chronic daunomycin administration. The short-term effects of daunomycin in the rabbit differ substantially from those seen in the rat and guinea pig.

Animals↗