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PubMed1973

Isometric contractions of motor units in a fast twitch muscle of the cat.

1. Isosmetric contractions of cat flexor digitorum longus whole muscles and of functionally isolated motor units have been measured under conditions similar to those used by Buller & Lewis (1965a).2. Motor unit twitch time to peak was inversely related to axonal conduction velocity. The logarithm of tetanic tension was directly related to conduction velocity. These relationships suggest that each motoneurone has an influence on the muscle fibres which it innervates.3. The ratio of twitch to tetanic tension was directly related to the time to peak of the motor unit. This fact might be explained by variation between motor units of the duration of ;active state'.4. The muscle length at which tension was maximal varied between motor units and the optima were found over the range of muscle lengths which could occur in the body. Slow motor units had longer optimal lengths.5. The sample of motor units was considered to be unbiased because the distribution of axon conduction velocities was compatible with reported motor fibre diameter spectra of the muscle nerve. The mean motor unit tetanic tension gave a reasonable estimate of the number of alpha-motor axons in the muscle nerve. Twitch tensions gave a value that was 40% higher.6. Motor unit and whole muscle data were in good agreement for length-tetanus tension curves, for times to peak and for twitch-tetanus ratios at long muscle lengths.

Action Potentials↗

PubMed1975

Heat production and chemical changes during isometric contractions of the human quadriceps muscle.

1. Development of a new thermal probe and use in conjunction with chemical analysis of needle biopsy samples, has made possible a thermodynamic study of the energetics of muscular contraction in the human quadriceps. 2. The observed rate of muscle temperature rise was proportional to the force of the contraction. During maximal contractions the rate of heat production was 54 +/-8-5 W/kg wet muscle (mean +/- s.d.). 3. The observed rates of muscle temperature rise agreed well with the rates calculated from the measured metabolite changes when standard values for the enthalpy changes of the reactions involved were used. 4. During prolonged stimulation of the quadriceps at 15/sec via the femoral nerve, the rate of heat production per unit force fell to nearly half the initial value. It is estimated that this represented a two- to fourfold increased in economy of ATP turn-over required to maintain a given force. 5. Relaxation becomes progressively slower during prolonged contractions and it is suggested that the slowing of relaxation and the increased economy of force maintenance may both be due to an increased cross-bridge cycle time in the fatigued muscle.

Adenosine Triphosphate↗

PubMed1976

Heat production and chemical change during isometric contraction of rat soleus muscle.

1. Methods are described whereby the soleus muscle of the rat may be used for the investigation of initial processes in the absence of oxidative recovery. 2. The anaerobic conditions employed had no effect on the concentration of phosphocreatine in resting muscle or the mechanical response during contraction. 3. Muscles were stimulated tetanically for 10 s at 17-18 degrees C. Measurements were made of the heat production and metabolic changes that occurred during a 13 s period following the first stimulus. 4. There was no detectable change in the concentration of ATP. Neither was there detectable activity of adenylate kinase or adenylate deaminase. The changes in the concentration of glycolytic intermediaries were undetectable or very small. 5. The change in the concentration of phosphocreatine was large and amounted to -127 +/- 11-4 mumol/mmol Ct (mean and S.E. of the mean, negative sign indicates break-down, Ct = free creatine + phosphocreatine) which is equivalent to about -2-13 mumol/g wet weight of muscle. The heat production was 6549 +/- 408 mJ/mmol Ct (mean and S.E. of mean) which is equivalent to about 110 mJ/g. 6. About 30% of the observed energy output is unaccounted for by measured metabolic changes. 7. The ratio of heat production (corrected for small amounts of glycolytic activity) to phosphocreatine hydrolysis was -49-7 +/- 5-6 kJ/mol (mean and S.E. of mean), in agreement with previous results using comparable contractions of frog muscle, but different from the enthalpy change associated with phosphocreatine hydrolysis under in vivo conditions (-34 kJ/mol). 8. The results support the notion that the discrepancy between energy output and metabolism is an indication of an unidentified process of substantial energetic significance that is common to a number of species.

AMP Deaminase↗

PubMed1983

Temperature-dependent transitions in isometric contractions of rat muscle.

The effect of temperature on tetanic tension development was examined in extensor digitorum longus (fast-twitch) and soleus (slow-twitch) muscles of the rat, in vitro and with direct stimulation. The temperature range was from 35 to 10 degrees C. 2. The maximum tetanic tension decreased slightly on cooling from 35 to 25 degrees C. Cooling below 20 degrees C resulted in a marked depression of tetanic tension. The results were similar in the two muscles. 3. Analysis (in the form of Arrhenius plots) of the rate of tetanic tension development and relaxation clearly showed the occurrence of two phases in their temperature dependence, due to an increased temperature sensitivity below about 25 degrees C. Arrhenius activation energy estimates for temperatures lower than 21 degrees C were around twice as high as those for temperatures higher than 24 degrees C in both muscles.

Animals↗

PubMed1991

Cadmium block of isometric contractions of isolated bullfrog atrial cells.

We studied the effect of cadmium, verapamil, and quinacrine on the force of contraction (Fp) of isolated, single, field-stimulated bullfrog atrial cells. All agents were applied or removed rapidly (t1/2 approximately 15 ms) to minimize intracellular concentration changes other than intracellular calcium concentration. Two components of twitch force were observed, one blocked by micromolar Cd2+ and the other by millimolar Cd2+. The two contributed about equally to the activation of the twitch. The "cadmium-sensitive" portion of force (that affected by [Cd] less than or equal to 100 microM) had a K1/2 approximately 1 microM, was identical in magnitude to, and not additive with, a "verapamil-sensitive" (10 microM) component of force, was most strongly affected by 50-ms pulses of Cd2+ when they were applied in the mechanical latent period, and was potentiated by catecholamines. The cadmium-insensitive portion of force was abolished by the removal of extracellular calcium and was greatly potentiated by quinacrine (3 or 10 microM), a blocker of Na-Ca exchange. The results are consistent with the idea that activating calcium enters the cell via both an inactivating cadmium-sensitive L-type channel and a noninactivating cadmium-insensitive mechanism that is not Na-Ca exchange and leaves the cell via Na-Ca exchange.

Animals↗

PubMed1977