An oral administration of a single dose of beta-adrenoceptor agonist clenbuterol (15 mg/kg body weight) to mice resulted in an increased collagen distribution in the subendocardium and myocardium of the left ventricle. Abundant collagen accumulation is characteristic in myonecrotic regions and around blood vessels. Hydroxyproline assay and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of pepsin insoluble collagen confirmed this stimulated collagen proliferation. An MMP-activity assay of tissue extract by gelatin in gel zymography demonstrated a significant inhibition of MMP-9 activity in the beta-agonist-treated group. The results suggest that clenbuterol treatment is capable of inducing structural and functional remodeling of the extracellular matrix by down-regulating MMP-9 activity and thereby causing an impairment of collagen turnover. This may lead to changes in the different hemodynamic properties of the tissue, including ventricular compliance.

The aim of this study was to analyze the effects of chronic administration of the beta-adrenoceptor agonist clenbuterol (2 mg/kg body weight/day for a period of 30 days) on the major contractile protein (myosin) in the left ventricular muscle of the adult mouse heart. Separation of myosin heavy chain (MHC) isoforms on 7.5 % glycerol SDS-PAGE and subsequent quantification of the gels by laser densitometry showed a 6.5-fold increase in the beta-isoform of MHC in the clenbuterol-treated group. The alpha : beta ratio of these two isoforms in the control group was 98.16+/-0.14 %: 1.83+/-0.14 %, whereas in the treated group it was 88.05+/-1.15 % : 11.95+/-1.15 %. Actomyosin ATPase activity assay demonstrated a significant (20 %) decline in ATPase activity of the tissue in the beta-agonist-treated group. These results suggest that chronic clenbuterol treatment is capable to induced the transformation of MHC isoforms increasing the slow beta-MHC isoform, which may contribute to the altered contractile mechanics of clenbuterol-treated hearts.