Zusammenfassung

1) Role of the adrenergic system in the adaptive charges following physical training. Chronic treatment with beta-adrenergic agonists in animals induces biochemical, structural and physiological changes in skeletal muscle, heart muscle and adipose tissue, and an increase in insulin sensitivity in skeletal muscle that are comparable to those observed after physical training, whereas chronic treatment with beta-adrenergic antagonists reduces the metabolic adaptation to exercise training. 2) We want to characterize the beta-adrenergic receptor subtypes presentin the three different muscle fiber types in rats and in humans. We will also study the effects of endurance training on the density of beta-adrenergic receptor subtypes in rats. 3) Effects physical training on the response to beta-adrenergic stimulation in skeletal muscle. The role of possible skeletal muscle beta-adrenoceptor changes in the adaptations to exercise training ist still uncertain. It may be hypothesized that training, through chronic exposure to catecholamines, decreases the sympathetic response of skeletal muscle. This catecholamine - induced catecholamine resistance of skeletal muscle might reduce catecholamine - induced training effects. Characterization of the beta-adrenergic receptor subtypes mediating catecholamine effects in skeletal muscle and determination of the effects of physical training on these receptors might help design appropriate training programs.

(Zwischen)Ergebnisse

This study is an attempt to characterize the ß-adrenergic system of rat skeletal muscle (tibialis anterior, TA, gastrocnemius, GA, gluteus maximus, GM, soleus, SO, and tensor fascia latae, TFL). We used two different approaches to characterize the ß-adrenergic receptor (ß-AR) subtypes present in the three muscle fiber types: molecular biology and pharmacology. With the Northern blot technique we were able to show, in rat TA nad TFL muscles, the presence of ß2-AR mRNA. The mRNA of the other two subtypes (ß1 and ß3) were not detected. As the ß3-AR mRNA might be present in very low amount in muscle we used RNA from SO, TA and TFL to make a reverse transcription (RT) followed by a polymerase chain reaction (PCR) with ß3-AR primers. The amplified fragments hybridized to a ß3-AR DNA probe by the Southern blot technique. Furthermore, the amplified fragments from the three muscles were cloned and sequenced. Their sequence was identical to that of the ß3-AR found in rat brown adipose tissue. This result suggests that the ß3-AR is present in rat skeletal muscle at a very low level. However, we cannot rule out the possibility that the ß3-AR RNA amplified came from another type of cells adjacent to the muscle tissue used. We also tried to characterize the skeletal muscle ß-AR by binding studies on rat muscle crude membranes. With the help of two radioligands, [3H]CGP 12177 and [3H]SB 206606, we showed the presence of ß2-AR and two additional ß-adrenergic-like binding sites, which might represent atypical ß-ARs. The pharmacology of these receptors is different from that of the ß3-AR. The density of these in muscle membranes is ca. 2 times that of the ß2-AR. Furthermore, whereas the stimulation of the rat ß3-AR of brown adipose tissue with both CGP 12177 and BRL 37344 (the unlabeled form of SB 206606) induce a cAMP response, these two agents had no effect on skeletal muscle membranes. This suggests that these drugs are antagonists for their respective receptors. Thus, in rat skeletal muscle we found: - ß2-adrenergic and two atypical ß-adrenergic-like binding sites by binding studies, - ß2 and very low levels of ß3-adrenergic receptor mRNA by molecular biology studies. We are currently screening a cDNA library from rat skeletal muscle with three different DNA probes: ß1, ß2 and ß3-ARs. We are searching for the gene of a possible atypical ß-adrenergic receptor expressed in skeletal muscle.