Factors affecting aerobic recovery heat production and recovery ratio of frog sartorius.

1. Sartorius muscles of Rana temporaria, equilibrated at 20 degrees C in Ringer solution buffered with phosphates, were stimulated isometrically for 0.2 up to 0.75 s at lengths varying from 1.03 to 1.48 times rest length, L0. The aerobic recovery heat was measured for 10.5 min after contraction. 2. The recovery heat production had a complex time course, showing a variable delay to maximum, declining thereafter. In most cases, the rate of heat production did not decrease monotonically; attention was focused on the slow exponential decay which only persisted from 1.5-5 min after contraction. This latter part of the time course was considered as strictly aerobic and characterized by the time constant tau s. 3. Increasing the tetanus duration from 0.2 to 0.75 s increased initial heat Qi and recovery heat Qr in proportion, so that the recovery ratio R (Qr/Qi) did not change; it was equal to 1.29 +/- 0.03 (S.E.M.; n = 44) for muscles at about L0. The kinetics of heat production were modified with longer tetani; in particular, tau s was increased from 2.2 to 5.2 min. 4. When muscles were stretched beyond L0, as long as there was no increase of the resting heat rate (stretch response or 'Feng effect'), recovery heat production had a similar evolution to that in muscles at about L0; R was constant and equal to 1.21 +/- 0.03 (n = 46). 5. When muscles were sufficiently stretched to develop a stretch response, R increased proportionally to the stretch response. The effect seemed independent of the contractile machinery, as it vanished concomitantly with the stretch response, while force and Qi remained unchanged for the length considered. The kinetics were also modified--the delay to maximum was no longer detected and tau s most likely increased. 6. Substitution of 60% of the NaCl of the Ringer solution by NaI (mol/mol) produced a significant increase of R, mainly due to the increase of Qr. 7. The results show that neither the time course nor the amount of aerobic recovery heat Qr are strictly determined by the amount of initial heat Qi. The hypothesis is discussed that Qr might include a variable fraction due to processes which are not directly implicated in the actin-myosin interactions, possibly those involving the cytosolic Ca2+ concentration and the rate of resting metabolism.

Density of myosin filaments in the rat anococcygeus muscle, at rest and in contraction. II.

Rat anococcygeus muscles were fixed at rest or in contraction by conventional methods and prepared for electron microscopy. Myosin filaments were counted on cross sections and their density expressed per unit cytoplasmic area. In contracted muscles, the mean density increased from 86 to 168 filaments per micron 2 (1.95 times), while the density of intermediate (10 nm) filaments increased by 1.25 times. Cell cross sections from the same muscles were measured. Contraction produced a shrinkage which explains the apparent increased density of the 10 nm filaments; however an excess of 61 myosin filaments per micron 2 cannot be explained in this way. These findings provide the structural basis which quantitatively explains the birefringence changes observed in living contracted muscle (Godfraind-De Becker & Gillis, 1988). Our optical and electron optical results provide evidence for a reversible formation of myosin filaments during contraction of the rat anococcygeus muscle.

Analysis of the birefringence of the smooth muscle anococcygeus of the rat, at rest and in contraction. I.

The birefringence of the rat anococcygeus muscle was measured at rest and in contraction. A large increase (+30%) of the optical retardation was observed in muscles fully stimulated by Noradrenaline, in isometric conditions. This was associated with a reduction of the muscle thickness (-12%), so that the birefringence increased by 48%. These changes were reversed upon relaxation. The relationship between the birefringence increase and the mechanical response was investigated as a function of time and of Noradrenaline concentration. Possible origins for the birefringence increase are considered: an increased density of birefringent material, mainly filaments, seems the most likely explanation of the results.

Polarized light microscopy of the smooth muscle anococcygeus of the rat.

1. The birefringence of the rat anococcygeus muscle has been studied in the presence of solutions which dissolve myosin and actin filaments. About 80% of the muscle anisotropy disappear after this treatment. 2. The 'form' and 'intrinsic' components of the birefringence of muscles fixed either at rest or during maximal isometric contraction were studied by inhibition with media of different refractive indices. At rest, the 'form' birefringence accounts for about 67% of the total birefringence. In contraction, the increase of total birefringence is primarily (80%) due to an increase of the 'form' component, indicating that new filamentous material appears in this condition.

Heat production and fluorescence changes of toad sartorius muscle during aerobic recovery after a short tetanus.

1. The time course of the aerobic recovery following a 0.5 sec tetanus at 20 degrees C of the sartorius muscle of the toad Bufo bufo, equilibrated in bicarbonate-CO(2) Ringer solution, has been followed by recording simultaneously the heat production and the fluorescence excited by ultra-violet light at 366 nm.2. The fluorescence light emitted in these conditions in the region of 450 nm monitors the state of oxidation-reduction of the nicotinamideadenine dinucleotides (NAD(+)-NADH). After a short tetanus, the cycle evoked consists of an initial increase of the fluorescence (reduction of NAD(+)) followed by a long lasting phase of decreased light emission. This includes an early period of oxidation of NADH succeeded by a slow reduction of the NAD(+) formed in excess over the resting state. After iodoacetate, the initial reduction is suppressed.3. The time course of both fluorescence and heat production may be analysed into a rapid and a slow component by a double exponential model.4. The time courses of the aerobic recovery heat and of the fluorescence changes are similar after five minutes, but differ in their fast components. IAA significantly increases the rate constants of the fast terms of both monitors.5. The slow component is mainly related to aerobic processes while the fast one is due to both oxidative and glycolytic reactions occurring simultaneously.