Leszek Budohoski

Characterization of the adenosine receptor modulating insulin action in rat skeletal muscle.

The pharmacological profile of adenosine receptors in rat soleus muscle has been investigated by studying the effects of A1-and A2-selective adenosine receptor agonists on glucose utilization and the system A amino acid transporter under conditions where adenosine has been reported to exert a modulatory action on these insulin-sensitive processes. In the presence of adenosine deaminase and a sub-maximally effective concentration of insulin (50 microU/ml), the A1-selective agonists N6-cyclopentyladenosine and R(-)-N6-(2-phenylisopropyl)adenosine (R(-)PIA) caused concentration-dependent inhibitions of 2-deoxy[3H]glucose 6-phosphate and alpha-[14C]methylaminoisobutyric acid accumulations, but had no effect on the rate of [14C]glucose incorporation into glycogen, in incubated soleus muscle strips. These effects on glucose transport/phosphorylation and system A amino acid transport could be antagonized by 8-cyclopentyl-1,3- dipropylxanthine and 8-phenyltheophylline. The A2-selective adenosine receptor agonists CGS 21680 and 2-(phenylamino)adenosine were much less potent in their inhibition of these metabolic processes. These data support the proposal that adenosine exerts a post-receptor insulin-modulatory action in skeletal muscle and strongly suggest that this action is mediated by A1 adenosine receptors: the possible intracellular signalling mechanism(s) for this hormone-modulatory effect of adenosine are discussed.

Effects of chronic administration of vanadate to the rat on the sensitivity of glycolysis and glycogen synthesis in skeletal muscle to insulin

Male Wistar rats were given sodium orthovanadate in their drinking water for at least 14 days. This treatment increased the hypoglycaemic effect of intravenously administered insulin and increased the sensitivity of isolated soleus muscle strips to insulin with respect to both glycolytic and glycogen synthetic rates. This effect of chronic vanadate administration was shown not to be a consequence of a change in the insulin binding characteristics of soleus muscle. It is suggested that these changes may be brought about by the interaction of vanadate with insulin-mediated alterations in tyrosine kinase/phosphotyrosyl phosphatase activities.

Effect of various types of acute exercise and exercise training on the insulin sensitivity of rat soleus muscle measured in vitro.

Effects of acute exercise varying in duration and intensity, as well as of two training regimes (endurance and sprint training) on the sensitivity of the soleus muscle of rat to insulin was measured in vitro and compared in rats. As an index of the muscle insulin sensitivity the hormone concentration in the incubation medium which would produce half maximum stimulation of lactate production (LA) and glycogen synthesis was determined. A single bout of moderate endurance exercise (60 min treadmill running at 20 m×min−1, 0° inclination) increased the rate of LA production at the hormone concentrations used and increased the sensitivity of the process to insulin at 0.25 and 2 h but not 24 h after termination of exercise. Similar though less pronounced effects were found after heavy endurance exercise (30 min at 25 m×min−1, 10°), but sprint exercise (6×10 s bouts at 43 m×min−1, 0°) had no influence on the insulin sensitivity of the soleus muscle. The rate of glycogen synthesis in vitro was accelerated after endurance exercise, but the sensitivity of this process to insulin was unaffected by the preceding exercise. Endurance training for 5 weeks caused marked enhancement of sensitivity of both LA production and glycogen synthesis to insulin, which persisted for at least 48 h after the last training session. No changes in the soleus muscle sensitivity to insulin were found after sprint training. It is concluded that the increased insulin sensitivity of glucose utilization by skeletal muscle which occurs after endurance exercise and particularly during endurance training can substantially contribute to improved carbohydrate tolerance. Sprint exercise does not produce any changes in muscle insulin sensitivity, at least in the soleus muscle of the rat.

Exercise-induced changes in lipoprotein lipase activity (LPLA) in skeletal muscles of the dog

The aim of this work was to study the effect of physical exercise on muscle lipoprotein lipase activity (LPL) in dogs. Existence of two forms of LPL: heparin releasable and unreleasable was demonstrated in skeletal muscles, and the changes in the activity of both forms were followed during 3 h treadmill running, using biopsy samples taken from m. biceps femoris.During the first two hours of exercie the heparin releasable form of LPL was progressively increasing, whereas the heparin unreleasable form of the enzyme was decreasing. Thus, a significant negative correlation between activities of the two forms was ascertained (r=0.72,P<0.01). In the final period of exercise, activity of the heparin releasable form of LPL tended to stabilize on the enhanced level, and activity of the heparin unreleasable form increased.In the further series of experiments a relationship between exercise intensity and activity of the heparin releasable form of LPL was studied during 1 h exercise bouts.A significant positive correlation (r=0.84,P<0.001) was ascertained between LPL activity and intensity of work. A comparison between LPL activity in the muscle engaged in exercise (m. biceps femoris) and nonactive muscle (m. coccygeus) revealed that the enhancement of the enzyme activity during physical work does not occur in the latter.In conclusion: it was found that physical exercise induces a marked intensity-dependent increase of LPL activity in working muscles, which is probably caused by an elevated transport of the enzyme molecules from the muscle cells to the intravascular space. The latter suggestion is based on the reciprocal changes of the heparin releasable and unreleasable (probably intracellular) forms of LPL.

Effect of endurance and sprint exercise on the sensitivity of glucose metabolism to insulin in the epitrochlearis muscle of sedentary and trained rats

SummaryThe effects of two types of acute exercise (1 h treadmill running at 20 m· min−1, or 6 × 10-s periods at 43 m · min−1, 0° inclination), as well as two training regimes (endurance and sprint) on the sensitivity of epitrochlearis muscle [fast twitch (FT) fibres] to insulin were measured in vitro in rats. The hormone concentration in the incubation medium producing the half maximal stimulation of lactate (la) production and glycogen synthesis was determined and used as an index of the muscle insulin sensitivity. A single period of moderate endurance as well as the sprint-type exercise increased the sensitivity of la production to insulin although the rate of la production enhanced markedly only after sprint exercise at 10 and 100 μU· ml−1 of insulin. These effects persisted for up to 2 h after the termination of exercise. Both types of exercise significantly decreased the muscle glycogen content, causing a moderate enhancement in the insulin-stimulated rates of glycogen synthesis in vitro for up to 2 h after exercise. However, a significant increase in the sensitivity of this process to insulin was found only in the muscle removed 0.25 h after the sprint effort. Training of the sprint and endurance types increased insulin-stimulated rates of glycolysis 24 h after the last period of exercise. The sensitivity of this process to insulin was also increased at this instant. Both types of training increased the basal and maximal rates of glycogen snythesis, as well as the sensitivity of this process to insulin at the 24th following the last training session. It was concluded that in the epitrochlearis muscle, containing mainly FT fibres, both moderate and intensive exercise (acute and repeated) were effective in increasing sensitivity of glucose utilization to insulin. Thus, the response in this muscle type to increased physical activity differs from that reported previously in the soleus muscle, representing the slow-twitch, oxidative fibres in which sprint exercise did not produce any changes in the muscle insulin sensitivity.

Changes in muscle lipoprotein lipase activity during exercise in dogs fed on a mixed fat-rich meal

Skeletal muscle lipoprotein lipase activity LPLA was compared in dogs performing prolonged treadmill exercise after 20–22 h fasting or 4 h following mixed fat-rich meal ingestion. In the fasting state muscle LPLA increased progressively during 2 h exercise. In fed dogs the resting value of the muscle LPLA was considerably lower than that in the fasting state, and no increase in the enzyme activity occurred during physical effort. The inhibition of the muscle LPLA by the meal ingestion can be attributed to the persisting effects of increased plasma insulin and/or FFA concentrations, found at the beginning of exercise.

Lipoprotein lipase activity in the skeletal muscle during physical exercise in dogs

Changes in lipoportein lipase activity/LPLA/ in the quadriceps femoris muscle were followed in ten dogs during 3-hour treadmill exercise and 2-hour post-exercise recovery period. During the first hour of exercise muscle LPLA steeply increased. Subsequently the enzyme activity tended to plateau. After exercise LPLA decreased to the pre-exercise value within 1 hour. It is concluded that exercise increases the ability of skeletal muscles to remove triglycerides from the circulation.

Electrical stimulation partly reverses the muscle insulin resistance caused by tenotomy

It was shown that 15‐min electrical stimulation of the rat sciatic nerve greatly increases the in vitro measured sensitivity of lactate formation, glucose transport, and glycogen synthesis to insulin, impaired by previous tenotomy. The insulin sensitivity of all these processes was, however, still below that found in the stimulated intact soleus muscle. Extending the stimulation up to 30 min did not cause any further changes in insulin sensitivity either in tenotomized or in intact muscles.