Myocardial hypertrophy in transgenic mice overexpressing the bovine growthhormone (bGH) gene

Objectives. The main purpose of the present study was to characterize cardi ac muscle hypertrophy using both qualitative and quantitative microscopy in mice overexpressing the bovine growth hormone. Results. Measurements of 30 fibres from each group revealed that fibre diam eter in transgenic hearts was significantly larger than in control hearts. There was a significant decrease in interfibrillar space in transgenic hear ts as compared with control hearts. The enlarged transgenic hearts displaye d unchanged organelles such as normal myofibrils and mitochondria in a norm al pattern, suggesting balanced growth. Myelin structures were occasionally observed between normal myofibrils. Moreover, myocardial beta-adrenergic r eceptors and muscarinic receptors in the hearts of transgenic mice overprod ucing GH were studied to see whether they are involved in the hypertrophic process. It was shown that the density of muscarinic receptors had decrease d and the super-high affinity of muscarinic receptors was lost, without any significant changes in either the density or the affinity of beta-adrenerg ic receptors, as compared with controls. Conclusions. These results demonstrate that a GH excess was able to induce significant myocardial hypertrophy and that there was a downregulation of m uscarinic receptors.