Perfused skeletal muscle -- an experimental preparation for many questions!

Perfused mammalian skeletal muscle preparations either in vitro or in situ are one of the options to be considered when planning a physiological research program or project. Such preparations have been and continue to be used to investigate research questions as diverse as skeletal muscle function and metabolism, peripheral vascular function, and an approximation of exercise. When selecting a perfused muscle preparation, both anatomical and physiological organization must be evaluated in the context of the planned experiment.

Mechanical and metabolic determination of VO2 and fatigue during repetitive isometric contractions in situ.

Repetitive isometric tetanic contractions (1/s) of the canine gastrocnemius-plantaris muscle were studied either at optimal length (Lo) or short length (Ls; approximately 0.9 . Lo), to determine the effects of initial length on mechanical and metabolic performance in situ.

Muscle blood flow and distribution determine maximal VO2 of contracting muscle.

During repetitive contractions, the VO2 of the dog gastrocnemius-plantaris muscle rose with the contraction frequency up to a maximal value and then decreased as contraction frequency was increased further. PVO2 was constant over most of the contraction frequency range. Reducing perfusion pressure/blood flow reduced VO2max with a constant PVO2.

Blood flow and pressure relationships which determine VO2max.

The role of O2 delivery in regulating VO2max has been studied in an isolated gastrocnemius-plantaris muscle preparation contracting in situ; recent data addressing this issue are presented. VO2 increases nonlinearly with stimulation frequency reaching a peak at 5 twitches.s-1 or 1 tet.

Preload release increases blood flow and decreases fatigue during repetitive isotonic muscle contractions.

The effects of preload on blood flow (Q), O2 uptake (VO2), and fatigue were investigated in the canine gastrocnemius-plantaris muscle in situ. Repetitive (1 contraction/s, 200 ms duration) afterloaded (0.25-0.

Metabolic and work capacity of skeletal muscle of PFK-deficient dogs studied in situ.

Mechanical and metabolic relationships of muscle lacking phosphofructokinase (PFKD) activity were compared with muscle having normal phosphofructokinase (NORM) activity by using the gastrocnemius-plantaris muscle group with isolated circulation in situ. Muscle contractile properties were similar in both groups. Initial power output (W) during repetitive tetanic (200 ms, 50 impulses/s) isotonic contractions was similar in both groups; however, W declined significantly more (30-80%) in PFKD than in NORM muscle over time, with a constant O2 uptake (VO2)/W.

Lactate and acid-base exchange during brief intense contractions of skeletal muscle in situ.

Our goal was to design a stimulation-contraction paradigm using an isolated in situ dog gastrocnemius muscle preparation that would provide an experimental model for brief intense intermittent (IC) exercise in humans. Second, acid-base and ion exchanges across the muscle were investigated using four 30-s bouts of isotonic tetanic contractions (2 s-1, 100-ms train, 50 impulses/s) with 4 min of rest between bouts. During the bouts, peak power output (W) was 18.

Exercise and glucocorticoid-induced diaphragmatic myopathy.

We tested the hypothesis that a chronically active muscle, such as the rat diaphragm, would be more resistant to glucocorticoid-induced myopathy than a less active locomotor skeletal muscle (plantaris). Furthermore, we sought to determine whether endurance exercise could antagonize the glucocorticoid-induced atrophy in the diaphragm. Rats were assigned to one of seven experimental groups (n = 10 per group) and injected daily over a 10-day period with either a sham solution or prednisolone acetate: group 1: control; sedentary and sham injected; group 2: control; exercise trained and sham injected; group 3; sedentary; prednisolone (0.

Blood flow elevation increases VO2 maximum during repetitive tetanic contractions of dog muscle in situ.

Blood flow through the gastrocnemius-plantaris muscle of the dog in situ was increased by a pump in the arterial supply during a 30-min period of 1/s isotonic tetanic contractions. Compared with a control series of experiments with normoxemia and spontaneous flow, the pump increased flow 84%, from 1.51 +/- 0.

Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ.

The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.

Regulation of muscle lactate production.

It is not possible to make accurate measurements of muscle lactic acid net exchange during exercise by application of the Fick relationship. To make accurate measurements of lactic acid net exchange, preparations with isolated circulations have been used. Since such preparations utilize relatively small muscles or groups of muscles, the data apply to muscle contractions, not exercise.

Differences between VO2 maxima of twitch and tetanic contractions are related to blood flow.

The purpose of this investigation was to compare oxygen uptake (VO2) and fatigue characteristics of isotonic tetanic contractions with those observed during isotonic twitches in dog gastrocnemius-plantaris muscle. Tetanic contractions (1/s, 200-ms trains of 50 impulses/s) elicited a peak VO2 of 9.01 +/- 0.

Fatigue of mammalian skeletal muscle in situ during repetitive contractions.

When the gastrocnemius-plantaris muscle group of the dog is stimulated to contract repetitively for 30 min at frequencies high enough to generate VO2 levels at or near VO2 max, VO2 and mechanical performance decline with time. This decline with time is fatigue, and it occurs during twitch and tetanic contractions that are isometric or isotonic. There is oxidation of the mitochondrial electron transport system, and net lactic acid output is transient, ending after 20 min of contractions.

Effects of ischemic and hypoxic hypoxia on VO2 and lactic acid output during tetanic contractions.

We measured O2 uptake (VO2), CO2 output (VCO2), and net lactic acid output (L) during a 30-min period of repetitive 1/s isotonic tetanic contractions of the dog gastrocnemius-plantaris muscle group. The conditions were modest ischemic hypoxia (ischemia), hypoxia hypoxia (hypoxia), and free-flow normoxia (control). The major goal was to assess the effects of these perturbations on L during contractions.

Oxidation/reduction state of cytochrome oxidase during repetitive contractions.

There is disagreement regarding whether inadequate O2 determines maximal O2 uptake (VO2max) and lactic acid output (L) during muscular activity. Direct assessment of mitochondrial cytochrome oxidase (cytochrome a-a3) oxidation/reduction (O/R) state should provide an unequivocal answer for this issue. A new near-infrared spectrophotometric method was used to measure the O/R state of cytochrome a-a3 of dog gastrocnemius-plantaris muscle in situ during repetitive isotonic twitch and tetanic contractions.

Maximal blood flow and power output of dog muscle in situ.

The two goals of this study were: 1) to establish a stimulation pattern which would drive the gastrocnemius-plantaris muscle of the dog, in situ, to maximal work rate, W, and 2) to determine whether the surgical isolation of the circulation of this muscle, necessary to measure the muscle flow, Q, reduced the muscle's performance. The first goal was achieved in a preparation with minimal surgery, MS, to isolate the nerve and the tendon. The nerve was cut and connected to a stimulator, and the tendon was cut and connected to a pneumatic lever system.

Roles of CO2, O2, and acid in arteriovenous [H+] difference during muscle contractions.

To determine the origins of the arteriovenous [H+] difference of muscle during contractions, arterial and muscle venous blood sample pairs were taken before and after 0.5, 5.0, and 30.

A pictographic essay on blood and tissue oxygen transport.

This review describes the transport of oxygen from ambient air to mitochondria in the cells. Using simple equations and diagrams, the presentation illustrates the variables which determine the magnitude of each of the three major steps in the partial pressure of O2 along the passage, from: (i) ambient air to alveolar gas; (ii) arterial blood to venous blood; and (iii) capillary blood to tissue. The emphasis is on steps (ii) and (iii), and how they are modified from the normoxic case by ischemia, anemia, hypoxia, and increased VO2.

Effects of adrenergic agonists and antagonists on muscle O2 uptake and lactate metabolism.

To investigate adrenergic receptor-mediated responses in dog gastrocnemius-plantaris muscle, several catecholamine agonists, isoproterenol, epinephrine, norepinephrine, and phenylephrine, and two antagonists, propranolol and phenoxybenzamine, were given during repetitive, isotonic, tetanic contractions. The response variables that were measured were muscle blood flow, shortening during constant load contractions, and arterial and venous O2 and lactate concentrations. The calculated variables were O2 uptake (VO2), net lactic acid output (L), and power output.

Lactic acid output of cat gastrocnemius-plantaris during repetitive twitch contractions.

Because fatigable, white (FF) muscle fibers have been reported to have a greater glycolytic capacity and a lower oxidative capacity than fatigue-resistant, red (FR and SR) muscle fibers, it is generally supposed that FF muscle fibers produce and therefore release more lactic acid into the blood during contractions than FR and SR muscle fibers. To test this supposition, the net lactic acid output, L, and O2 uptake, VO2, were measured for the cat gastrocnemius-plantaris muscle during repetitive isometric contractions. The results obtained from this low metabolic capacity (FF, FR and SR muscles) were compared to results obtained previously in the high metabolic capacity (FR and SR muscles) of the same muscle group in the dog during similar contractions.

Biochemical and physiological bases for lactate production.

I have proposed an explanation of lactic acid production and output by red and white skeletal muscle during repetitive contractions. The proposal does not require O2 lack to slow oxidative phosphorylation. In this proposal lactic acid production is due to the fact that activation of glycolysis is more rapid than activation of oxidative phosphorylation.

Effects of catecholamines on lactic acid output during progressive working contractions.

Epinephrine and norepinephrine together (E + NE) and epinephrine (E) alone were infused intravenously in stepwise increasing doses during progressive isotonic tetanic contractions. The goal was to mimic, for in situ dog skeletal muscle, the concentrations of these catecholamines in the blood and the contractions during progressive exercise. The concentrations of lactate and O2 in arterial and muscle venous blood, the arterial plasma concentration of E and NE, PO2 in arterial and muscle venous blood, and the venous outflow were measured.

Plasma catecholamines and their effect on blood lactate and muscle lactate output.

This study was designed to test the hypothesis that epinephrine (E) and norepinephrine (NE) increase net muscle lactate output (L) of in situ gastrocnemius-plantaris muscle group during contractions. Plasma [E] and [NE] were measured before and after the surgical isolation of the muscle and at 10-min intervals during the 60-min experiments. Plasma [E] and [NE] were increased threefold by intravenous infusions of E (n = 3) or NE (n = 3) at a rate of 1.