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R H Fitts

Publications and source records attributed to R H Fitts.

85 records · Page 5Linked to original sources
PubMed1979

Effects of thyrotoxicosis on mitochondrial enzymes of rat soleus.

Cytochrome oxidase, glycerol-3-phosphate dehydrogenase, and succinate dehydrogenase were measured in mitochondrial fractions obtained from rat soleus muscle of control and 8 week T3 + T4 treated animals. Under these conditions of prolonged treatment, there is a five-fold increase in the specific activities of both cytochrome oxidase and glycerole-3-phosphate dehydrogenase. Significant increases in total cellular mitochondrial content and enzyme activities were observed in T3 + T4 treated animals as compared to controls. These results indicate that thyrotoxicosis can induce selective changes in mitochondrial enzymes in slow twitch red (Type I) muscle fibers.

Animals↗
PubMed1978

Effects of fatigue and recovery on contractile properties of frog muscle.

The contractile properties of frog sartorius muscles were evaluated during development of fatigue and recovery. Muscles were stimulated under anaerobic conditions at a rate of 30 stimuli/min. Twitch tension (Pt) decreased 59% and tetanic tension (Po) decreased 52% during 15 min of stimulation. Contraction time and one-half relaxation time increased markedly during the first 6 min and then leveld off. The peak rate of twitch tension development (dP/dt) progressively declined to 33% of its initial value. Maximum shortening velocity did not change significantly. Large increases in Pt and twitch dP/dt occurred during the first 15 s of recovery while Po did not increase significantly in this time. Thereafter, Pt, Po, and, dP/dt all progressively returned towards normal over 30 min. These results show that the initial rapid recovery in Pt is not due to an increase in the capacity to generate force. Po is a better indicator of the force-generating capacity of a muscle because, unlike Pt, it is not affected by changes in the rate of tension development.

Action Potentials↗
PubMed1977

Contractile properties of rat soleus muscle: effects of training and fatique.

The effects of exercise training and fatique on the contractile properties of rat soleus muscles have been investigated. Prolonged exercise, consisting of 2 h or daily treadmill running, induced small but significant decrease in contraction time, one-half relaxation time, and maximum tetanic tension (Po), and increase in the peak rate of tension development (dP/dt) during a twitch, and an increase in maximum shortening velocity (Vmax). The 20% increase in Vmax was proportional to the previously reported increase in actomyosin ATPase induced by 2 h of daily running. These results indicate that prolonged training can induce modifications of the neurally determined contractile properties of skeletal muscle. To investigate the effects of fatigue, soleus muscles were stimulated in situ with 250-ms trains of 100 Hz at a rate of 110 trains per minute for 30 min. This resulted in a 32% decrease in Po, a 48% decline in peak tetanic dP/dt, and a 12% decrease in Vmax in muscles of untrained animals. Muscles that had adapted to exercise were significantly protected against the decrease in Po (only an 8% decrease) and Vmax (no significant decrease) but not against the decline in peak dP/dt.

Animals↗
PubMed1976

Lactate and contractile force in frog muscle during development of fatigue and recovery.

The relationship between lactic acid concentration and twitch tension was reevaluated in electrically stimulated frog sartorius muscle. In muscles stimulated under anaerobic conditions at a rate of 30 stimuli/MIN CONTRACTILE FORCE DECREASED TO 36% OF THE INITIAL VALUE IN 15 MIN, Concomitantly lactate increased from 3.3 to 18.7 mumol/g of muscle. The correlaiton between the increase in lactate and the decrease in contractile force was significant (r = -0.99, P less than 0.000001). Recovery occurred in two phases. A rapid increase in contractile force, which represented 20% of the total recovery, took place during the first 15 s and occurred concomitantly with an increase in ATP from 3.9 to 4.6 mumol/g. Lactate concentration did not change significantly during this period. The second phase of recovery of contractile force was complete in 50 min. Lactate concentration and contractile force were significatly correlated during recovery (r = -0;92, P less than 0.00001). However, recovery of contractile force lagged behind the decrease in lactate; a given concentration of muscle lactate was associated with a higher contractile force early during development of fatigue than late during recovery.

Adenosine Triphosphate↗
PubMed1975

Depletion of muscle and liver glycogen during exercise. Protective effect of training.

Carbohydrate depletion during exercise was measured in the liver, in the three different types of skeletal muscle, and in the blood of exercise-trained and untrained rats. The acute exercise test consisted of 45 min of treadmill running of progressively increasing intensity. The training program consisted of 6 hrs of swimming per day, 5 days per week for 14 weeks; the training induced an increase of approximately 35 percent in the respiratory capacity of gastrocnemius muscle, and a 14 percent incrase in heart weight. Glycogen stores in fast-twitch red, fast-twitch white, and slow-twitch red types of skeletal muscle, were depleted significantly more slowly in the trained than in the untrained animals during the treadmill exercise test. Resting glycogen stores in the liver were higher and were depleted more slowly during exercise in the trained than the untrained animals. Blood lactate concentration was significantly lower in the trained than in the untrained rats at the end of the exercise test. These results provide evidence that endurance exercise training induces adaptation which protect against the depletion of glycogen from the liver and from the tree types of skeletal muscle during prolonged exercise.

Animals↗
PubMed1975

Skeletal muscle respiratory capacity, endurance, and glycogen utilization.

This study was undertaken to evaluate the relationship between physical performance capacity and the mitochondrial content of skeletal muscle. Four groups of rats were trained by means of treadmill running 5 days/wk for 13 wk. One group ran 10 min/day, a second group ran 30 min/day, a third group ran 60 min/day, and a fourth group ran 120 min/day. The magnitude of the exercise-induced adaptive increase in gastrocnemius muscle respiratory capacity varied over a twofold range in the four groups. There were significant correlations between the levels of three mitochondrial markers (cytochrome c, citrate synthase, respiratory capacity) in the animals' gastrocnemius muscles and the duration of a run to exhaustion. There was also a significant correlation between the amounts of glycogen remaining in liver and skeletal muscle after a 30-min-long exercise test and the respiratory capacity of the animal's leg muscles. These findings are compatible with the interpretation that a close relationshiop exists between skeletal muscle mitochondrial content and the capacity to perform endurance exercise.

Adenosine Diphosphate↗
PubMed1975

Phosphorylated beta-guanidinopropionate as a substitute for phosphocreatine in rat muscle.

To evaluate phosphorylated beta-guanidinopropionate (beta-GPAP) as a substitute for phosphocreatine (PC), hypoxic tibialis anterior muscles were stimulated to contract isometrically in situ until twitch tension fell to 25 percent of the peak value. Muscles from rats fed beta-guanidinopropionic acid (beta-GPA) failed to exhibit the staircase phenomenon, and they developed 28 percent less tension than control muscles. In control muscles lactate increased from 0.75 to 20.99, ADP increased from 0.89 to 1.20, ATP decreased from 5.09 to 2.73, and PC decreased from 15.78 to 1.52 mumol/g. In muscles from rats fed beta-GPA, lactate increased from 0.85 to 14.31, ADP increased from 0.86 to 1.05, ATP decreased from 2.69 to 1.71, PC decreased from 0.73 to 0.30, and beta-GPAP decreased from 30.34 to 19.45 mumol/g. From these measurements, the use of high-energy phosphate was calculated to be reduced 32 percent in muscles from rats fed beta-GPA. The relationships between the use of high-energy phosphate was calculated to be reduced 32 percent in muscles from rats fed beta-GPA. The relationships between the use of high-energy phosphate and tension development confirm experimentally the ability of beta-GPAP to substitute for PC as a source of energy to sustain muscle contraction.

Adenosine Diphosphate↗
PubMed1975

Effect of exercise on response of liver lipogenic enzymes to a high fructose diet.

Meal-fed Long-Evans rats fed a high fructose diet and exercised for 2-hr daily on a treadmill for three days had lower levels of several hepatic lipogenic enzymes (acetyl CoA carboxylase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, malic enzyme and citrate cleavage enzyme) than did sedentary rats pair-fed the diet. Accumulation of triglycerides in plasma following ingestion of a fat-free, high fructose meal and injection of Triton WR-1339, an inhibitor of plasma triglyceride clearance, was not significantly different in the two groups of animals. All of the hepatic lipogenic enzymes measured, with the exception of citrate cleavage enzyme, attained similar levels in the runners as in the controls after 5 days on the high fructose diet. Thus the exercise appeared to affect the time course of the increase in the levels of activity of most of the lipogenic enzymes but not the final steady state levels attained.

Acetyl-CoA Carboxylase↗
PubMedDate not supplied

Contractile and fatigue properties of thyrotoxic rat skeletal muscle.

The effects of thyrotoxicosis on the contractile properties and development of muscle fatigue in the slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles were examined in rats given 3 mg of L-thyroxine and 1 mg of L-triiodothyronine per kilogram of diet for 6 weeks. The hormone treatment produced significant decreases in the contraction time, one-half relaxation time, and twitch tension in the SOL, while the peak rate of tension development (+ dP/dt) and decline (- dP/dt) in this muscle were elevated. Additionally, the force-frequency curve was shifted to the right and, thus, resembled the curve of a normal fast-twitch muscle. In contrast, the contractile properties of the fast EDL were relatively unaltered by the hormone administration. Thyrotoxicosis also changed the SOL response to contractile activity as twitch tension, + dP/dt, and - dP/dt remained high, and a faster decline in muscle glycogen and an increase in lactate occurred compared to control muscles. These results clearly demonstrate a preferential effect of thyroid hormone on slow compared to fast skeletal muscle.

Adenosine Triphosphate↗