Factors affecting the metabolism of resting rabbit papillary muscle

The rate of resting heat production of 12 right ventricular rabbit papillary muscles was measured myothermically. Resting heat rate was measured at 4 temperatures (15, 20, 25 and 30° C) in either 45% or 95% O₂ while the muscle was passively stretched with various pre-loads. The metabolic substrate was pyruvate (10 mmol · l^?1). The mean resting heat rate, averaged across all treatment conditions, was 2.88 mW/g with no significant difference between the two oxygen concentrations. The calculated Q₁₀ of the resting heat rate was surprisingly low — only about 1.4 — but is shown to be in general agreement with literature values from whole heart oxygen consumption studies when the time-dependent decline is taken into account. Stretching the muscle beyond its rest length increased the rate of resting heat production. This response appeared unrelated to muscle diameter. The results are discussed in terms of the possible diffusion limitation of isolated papillary muscle preparations.     

Effect of hypoxia on mechanical properties of hyperthyroid cat papillary muscle.

It has been previously established that hyperthyroid myocardium exhibits increased performance under well-oxygenated conditions. To date, it is not known whether hyperthyroid cardiac muscle can maintain this increased performance during hypoxia. The responses of isolated right ventricular papillary muscles from hyperthyroid and euthyroid kittens to hypoxia were compared under isometric conditions at 31 degrees C. Under well-oxygenated conditions, the hyperthyroid cardiac muscle exhibited both an increased contractility and an accelerated rate of relaxation. A similar degree of acute hypoxic stress for 15 min resulted in a greater decrease in contractility in the hyperthyroid compared with the euthyroid papillary muscle as indicated by a greater fall in both peak tension development (2.2 +/- 0.25 from 4.2 +/- 0.2 vs. 0.9 +/- 0.15 from 3.2 +/- 0.4 g/mm2, P less than 0.01) and +dT/dt (12.9 +/- 2.3 from 25 +/- 3 vs. 4.0 +/- 0.6 from 14 +/- 1 g-s-1-mm-2, P less than 0.01). In addition, compared with the euthyroid data, hypoxia resulted in impaired myocardial relaxation in the hyperthyroid cardiac muscle. Thus, the hyperthyroid compared with the euthyroid papillary muscle exhibits both a greater decrease in contractility and an impairment of myocardial relaxation during hypoxia, indicating a greater susceptibility to a given hypoxic stress.     

The rate of resting heat production of rat papillary muscle

The rate of resting heat production of quiescent rat left ventricular papillary muscles was measured myothermically. The effects of contractile activity, stretch, oxygen partial pressure, temperature, amino acids and time were examined. The rate of basal heat production was the same throughout the day whether or not muscles contracted isotonically under a small pre-load. Passive stretch increased the rate of resting heat production; the stretch-induced increment was highly variable from muscle to muscle. The resting heat rate per se was only moderately sensitive to oxygen partial pressure and temperature, and was insensitive to the presence of amino acids in the bathing medium. The stretch-induced increase in resting heat rate was independent of these three factors. The rate of resting heat production declined exponentially with time to reach a plateau about 4 h following cardiectomy.     

Dissociation between mechanical performance and the cost of isometric tension maintenance inLethocerus flight muscle

Summary A method for the continuous measurement of ATP hydrolysis (ATPase) by demembranated muscle fibres has been applied to isometrically held, glycerol-extracted flight muscle fibres from the water-bugLethocerus, under conditions of high MgATP, neutral pH, and varying ionic strength, Ca²⁺ and extension. These variables caused parallel changes in isometric tension and ATPase. The slope of ATPase upon tension (ATPase/Tension; incremental tension cost) remained the same upon extension at either different Ca²⁺ concentrations or different ionic strength. Isometric activation by Ca²⁺ gave a higher incremental tension cost. The calculated mechanical rate constant of the work-producing process, measured by the small-amplitude behaviour, was increased by either Ca²⁺ or ionic strength, and little changed by extension; there was therefore a dissociation between its value and that of the incremental tension cost. The results appear to exclude a two-state crossbridge model for fibrillar insect flight muscle.     

Analysis of mechanical alternans in rabbit papillary muscle

Isometric force and action potentials were recorded in rabbit papillary muscles. It was found that the monophasic decline of the contractile potentiation that was recorded after an extrasystole (ES) was replaced by transient mechanical alternans (TMA) when temperature and calcium concentration of the perfusion medium had been lowered (from 37 degrees to 27 degrees C and from 2.0 to 0.5 mM, respectively). TMA in response to ES was also seen when the preparation was exposed to 2mM 4-aminopyridine. Furthermore, TMA could be induced by a shortening step during activity. Mechanical restitution curves were recorded by relating max. rate of force development of a test contraction to the duration of the preceding stimulus interval. It was found that the alternating contractions during TMA were associated with shifts of the mechanical restitution curve along the force axis. The duration of the action potential was inversely related to force development during TMA. It is proposed that TMA is due to a reduced damping of a regulatory feedback system between inotropic state and intake of activator calcium during the action potential. The following sequence of events are suggested: The abbreviated action potential accompanying a potentiated contraction is associated with reduced intake of activator calcium. This leads to depression of the subsequent contraction. The latter contraction is associated with increased calcium intake due to prolongation of the action potential. This will lead to potentiation of the next beat and the sequence is repeated. It is proposed that recirculation of calcium between heart beats will act as a damping factor of this system.     

Quantifying the mechanical work of resting quadriceps muscle tone

Our laboratory has shown that resting muscle, commonly thought to be mechanically inert, is actually mechanically active. We report a study of the mechanics of resting quadriceps muscle in adult surgical patients that determines how much metabolic activity can be attributed to quadriceps resting-muscle mechanical work. This was calculated by studying the motion of relaxed supine subjects’ instrumented legs dropped onto a pillow before and after anesthesia with muscle paralysis. By subtracting the acceleration of the dropping leg of the conscious subject before the quadriceps is paralysed from that found after paralysis, resting muscle tensile force and power of the quadriceps muscles can be calculated. Mechanomyography was also recorded, using an accelerometer. Paralysis produced an increase in acceleration in all cases (pre-paralysis 6.99 ± 1.51 m s⁻²; post-paralysis 7.65 ± 1.51 m s⁻²; P = 0.00007) and a decrease in mechanomyographic mean absolute amplitude (pre-paralysis 10.6 ± 3.7 mm s⁻²; post-paralysis 4.5 ± 2.6 mm s⁻²; P = 0.00003). Calculated force exerted by resting quadriceps was 22.6 ± 16.8 N; power 0.34 ± 0.17 W, corresponding to a daily caloric expenditure of 7.0 ± 3.6 kcal day⁻¹. This corresponds to approximately 205 kcal day⁻¹ for all skeletal muscle. Knowledge of the phenomenon of resting muscle mechanical activity may be of clinically importance in the study and treatment of obesity and of disorders of muscle tone.     

Influence of osmotic swelling on cross section and resting tension in isolated skeletal muscle fibers

The causes of change in resting tension with hypotonic swelling of isolated living frog skeletal muscle fibers were studied by observing their cross-sectional shape. The cross-sectional area steadily increased, in an almost osmometer-like manner, to one-third the standard tonicity. When the cross-sectional shape became almost circular and the circumferential length began to extend, the resting tension began to decrease/increase prominently at sarcomere lengths shorter/longer than 2.8 microns. The coincidence of the occurrence of prominent changes in tension and in the cross-sectional shape indicates that tension changes are closely related to the circumferential extension of the sarcolemma. For reference purposes, we studied the effects of reduction in ionic strength and in osmotic compressive force on the resting tension of mechanically and chemically skinned fibers. We concluded that (1) the decrease in resting tension with hypotonic swelling was mediated by an elevation in intracellular hydrostatic pressure due to the circumferential extension of sarcolemma, and that (2) the increase in tension was due to longitudinal contraction of the two-dimensionally tense sarcolemma, arising from its circumferential extension.     

The effect of temperature on the resting tension of frog''s muscle in hypertonic solutions

1. It has long been known that the tension of a resting muscle rises when it is warmed and falls on cooling. It is shown here that treatment with hypertonic solution causes an alteration in this effect; the usual rise or fall of tension with warming or cooling is then preceded by a transient change in the opposite direction.2. The magnitude of the abnormal reversed phase does not depend in a simple, linear, manner on the temperature change. For instance, the effect for cooling from 20 to 0 degrees C is much greater than twice that for 10 to 0 degrees C.3. The results lead to the suggestion that the tension, in both normal and hypertonic muscle, comes from an active process. It is proposed that a temperature change operates by altering the concentration of an ;activating substance', and that the change of concentration becomes delayed by hypertonicity.