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Changes in tissue blood flow and β‐receptor density of skeletal muscle in rats treated with the β 2 ‐adrenoceptor agonist clenbuterol
Author(s) -
Rothwell Nancy J.,
Stock Michael J.,
Sudera Deepak K.
Publication year - 1987
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1987.tb11211.x
Subject(s) - clenbuterol , endocrinology , medicine , skeletal muscle , blood flow , adipose tissue , agonist , receptor , chemistry
1 Rats injected with the β 2 ‐adrenoceptor agonist clenbuterol (2 mg kg −1 per day) for 18 days gained significantly more weight than controls. 2 Tissue blood flow assessed 24 h after the last injection from the distribution of radiolabelled microspheres was increased in white (5 fold) and brown (3 fold) adipose tissue of clenbuterol‐treated rats but was unaffected in kidney, brain and diaphragm, and was reduced by about 80% in skeletal muscle. 3 Acute injection of clenbuterol one hour before measuring blood flow, increased blood flow to brown fat (20 fold) in both treated and control groups. Blood flow to skeletal muscle increased more in the rats treated chronically with clenbuterol (6 fold increase) than in control rats (2 fold increase), but absolute flow rates were still significantly lower in the rats treated chronically with clenbuterol. 4 Skeletal muscle β‐adrenoceptor density and subtype were assessed from ligand binding and displacement studies using [ 3 H]‐dihydroalprenolol. Rats treated with clenbuterol for 18 days showed a 50% reduction in β‐receptor density, but the ratio of β 1 /β 2 ‐receptors was unaffected (15% β 1 /85% β 2 ). 5 The results indicate that, although clenbuterol produces acute increases in muscle blood flow, chronic treatment results in lower flow rates immediately (1 h) and 24 h after the previous injection. The attenuated response following chronic treatment is associated with a marked reduction in skeletal muscle β‐adrenoceptor density. 6 The data suggest that any anabolic effects of clenbuterol on muscle which may require, or may be mediated by increases in blood supply, cannot be sustained by chronic treatment. Conversely, blood flow to white and brown adipose tissue would appear to be potentiated by chronic treatment, possibly reflecting increases in lipolytic and/or thermogenic activity.