Resting Tension Research Articles

Effect of substrate on hypoxic response of pulmonary artery.

Main pulmonary arteries (MPA) were isolated from young guinea pigs and incubated in a modified Krebs-Henseleit solution containing either glucose and sucrose, or sucrose alone. Experimental hypoxia (PO2=30 mmHg) was induced by aerating the muscle chamber with a gas mixture of 95% nitrogen and 5% carbon dioxide. In must experiments the MPA was electrically stimulated, whereas in some no electrical stimulation was used. The isometric tension of electrically stimulated MPA was analyzed by means of resting tension (RT), active tension (ATmax), and maximum rate of tension development (dT/dt). It was demonstrated that when MPA was exposed to acute hypoxia in a glucose-free medium a significant increase in resting tension was observed in both electrically stimulated and nonstimulated preparations. However, the significantly hypoxic response was inhibited by the presence of glucose in the experimental medium. This tonic response of MPA is suggestive of the in vivo hypoxic pressor response of pulmonary circulation. A lower oxygen tension (PO2=95 mmHg) exposure prior to hypoxia significantly potentiated this hypoxic response of the MPA. In a separate series of experiments, isolated aortas demonstrated a similar response when exposed to acute hypoxia in vitro. Similarly, as in experiments with MPA, the presence of glucose inhibited the hypoxic tonic response of isolated aorta.

The Effect of Bathing Solution Tonicity on Resting Tension in Frog Muscle Fibers

Resting tension and short-range elastic properties of isolated twitch muscle fibers of the frog have been studied while bathed by solutions of different tonicities. Resting tension in isotonic solution at 2.3-microm sarcomere spacing averaged 0.46 mN.mm(-2) and was proportional to the fiber cross-section area. Hypertonic solutions, containing 0.1-0.5 mM tetracaine to block contracture tension, caused a small sustained tension increase, which was proportional to the fiber cross-section area and which reached 0.9 mN.mm(-2) at two times normal tonicity (2T). Further increases in tonicity caused little increase in tension. Hypotonic solutions decreased tension. Thus, tension at 2.3 microm is a continuous, direct function of tonicity. The dependence of tension on tonicity lessened at greater sarcomere lengths. At 3.2 microm either a very small rise or, in some fibers, a fall in tension resulted from an increase in tonicity. Hypertonic solutions also decreased the tension of extended sarcolemma preparations. In constant-speed stretch experiments the elastic modulus, calculated from the initial part of the stretch response, rose steeply with tonicity over the whole range investigated (1-2.5T). The results show that tension and stiffness of the short-range elastic component do not increase in parallel in hypertonic solutions.

ISOM: A computer programme for on-line recording and analysis of isometric myogram of skeletal muscle

The paper describes the basic features of a software system, ISOM, developed for use in the study of isometric mechanical characteristics of skeletal muscle. The programme is written in PAL-D assembler language (DEC-D8-ASAB-D). The minimum hardware requirements include the Lab-8 computer system (PDP 8/I, AXO8) extended with option XR, an oscilloscope, and an external oscillator (e.g., Digitimer, Devices). The overall cycle period of the system is determined by the external oscillator. During each cycle the output volt-age from the strain gauge is sampled at a constant preset rate, and the samples are stored. The signal so stored is displayed against a calibrated time base. ISOM makes further analyses on the stored samples to provide such characteristics as resting tension (RT), active tension (AT), time to peak tension (TP), time to half relaxation (HR), and maximum rate of rise of tension (MR). These data may be displayed on a CRO screen and/or printed out on the teletype. Pattern of stimulation, sampling rate, and gain of the output from the strain gauge amplifier are under programme control, and can be set prior to a given cycle. Other options incorporated in the system include digital differentiation of the stored signal, display of the signal during sampling, and a few variations of the alpha-numeric displays on the CRO screen.

Coronary blood flow, oxygen delivery rate and cardiac performance

1. Studies have been made on the isolated blood-perfused heart of dogs in which isometric tension development at various settings of resting tension (RT) was measured at various levels of O(2) delivery rates controlled by altering (a) coronary blood flow (CBF), (b) O(2) capacity or (c) O(2) saturation of the perfusate. Measurements were also made of O(2) consumption and vascular perfusion resistance.2. The capacity of the left ventricle to develop tension at any given setting of resting tension was found to be directly correlated with changes in O(2) delivery rate using any of the above three methods of altering the latter.3. The slopes of the curves relating resting tension to developed tension are positively correlated with total O(2) delivery to the heart.4. The O(2)-dependent metabolic effect upon tension production was found to be slow in development, in contrast to the Frank-Starling effect, which reached full development in the first heart beat after a change in resting length and tension.5. The O(2) consumption of the isometrically contracting heart is strongly correlated with the O(2) delivery rate at all particular values of resting tension and related developed tension.6. The metabolic state of the myocardium as determined by the rate of O(2) delivery within physiological ranges is (a) a direct major determinant of the tension-producing capacity of the heart muscle and (b) determines the magnitude of adaptation via the Frank-Starling mechanism.

Influence of Resting Tension on Contractility and Morphology of the Papillary Muscle.

SummaryPapillary muscles of the cat subjected to various resting tensions exhibited morphologic changes, the severity of which could be correlated with degree of loading with loads of 3 g or greater. These changes were similar to those which occur post mortem in cardiac muscle and appeared whether the muscle was stimulated to contract or subjected to static tension. Morphologic alterations appeared in both isotonic and isometric preparations but appeared more severe in isotonic preparations. Rate and degree of failure of isotonically contracting muscles could be correlated with size of the resting load but such a correlation was not observed with muscles contracting under isometric conditions.