Transgenic mice can be created to serve as models of human cardiac disease. Despite the technology available to manipulate the cardiovascular system of the mouse, there is relatively little information available concerning the normal physiology of the mouse heart. Therefore, we have characterized the response of the adult mouse to chronic physical conditioning by swimming. Adult female C57/B16 mice were conditioned by swimming up to 90 min twice daily for 4 wk, resulting in a 10% increase in heart weight and a 16% increase in heart weight-to-body weight ratios compared with sedentary controls. The heart rate response to a submaximal work load decreased > 20% with this conditioning program. Succinate dehydrogenase activity increased markedly in the soleus muscles of the conditioned animals, from 28 +/- 3 to 44 +/- 3 nmol.mg-1.min-1. In contrast to these changes, which also characterize the exercise model in the rat, no increase in cardiac tissue norepinephrine content or in cardiac myosin or myofibrillar adenosinetriphosphatase (ATPase) activities was observed, and no change in the V1 predominant myosin isoform or alpha-myosin heavy chain mRNA profiles was seen in the hearts of the swimmers. This study establishes that mice are able to develop cardiac hypertrophy in response to chronic conditioning which is not associated with changes in the ATPase activities of cardiac muscle. These data should be of use to investigators using murine models to define the molecular basis of adaptive cardiac hypertrophy in vivo.

We To address these issues, we asked whether an injected gene could be expressed in cardiac tissue and, if so, whether a gene coupled to a cellular promoter could be expressed at detectable levels and regulated appropriately. Such a finding would suggest an approach for targeting the expression of injected genes to specific cell types and for modulating their expression. The cellular promoter chosen for this study was derived from the rat a-cardiac myosin heavy chain (a-MHC) gene. In vivo, the expression of this gene is restricted to the heart (9) and its activity has been shown both in vivo (10) and in vitro (11, 12) to be positively regulated by thyroid hormone. By co-injecting rat cardiac and skeletal muscles with reporter genes linked to the a-MHC promoter and to a second viral promoter as an internal control, we demonstrated that the heart can be transfected in vivo with greater efficiency than skeletal muscle. Furthermore, we showed that the directly injected a-MHC promoter is active in heart, but not in skeletal muscle, and that its activity is regulated by the thyroid hormone status of the animal. MATERIALS AND METHODSPlasmids. pa-MHCluc contains the firefly luciferase coding region coupled to the rat a-MHC 5' flanking sequence, base pairs -613 to +32 (13). The latter, which includes a putative thyroid hormone response element (12), had been subcloned into ptz19R (United States Biochemical) as an Xba I-blunted EcoRI fragment. A HindIII-Kpn I fragment of this plasmid was subcloned into pXP2 (14), a promoterless luciferase vector, to construct pa-MHCluc. pRSVCAT, in which the coding sequence of the chloramphenicol acetyltransferase (CAT) gene is spliced to the long terminal repeat of the Rous sarcoma virus (RSV), has been described (15). pO-CAT (pCAT-Basic Vector; Promega) is a promoterless CAT construct.Animal Models. Adult female Wistar rats were anesthetized with inhaled methoxyfluorane and 0.1 ml/100g (body weight) of ketamine (50 mg/ml) and zylazine (10 mg/ml) intramuscularly. Animals receiving cardiac injections underwent exteriorization of the heart through a left thoracotomy (16) followed by injection of 50 /tl of a solution containing each plasmid at 2 ug/,l~as indicated in 20% (wt/vol) sucrose and 2% (vol/vol) Evans blue into the apex of the left ventricle through a 27-gauge needle. After expression of air from the chest, animals were ventilated briefly on a small-animal respirator and the incision was closed. In other rats, the belly of the adductor magnus muscle was injected under direct visualization.Two protocols were used in experiments involving thyroid hormone manipulations. (i) Rats were rendered hypothyroid by the administration of propylthiouracil (500 mg/liter in drinking water) (17) for 3 weeks, at which point their hearts were co-injected with 100 Ag of pRSVCAT and 100 pug of pa-MHCluc. The animals were then divided into two groups: one was continued on propylthiouracil and a second received 3,5,3'-triiodothyronine [T3; 200 ,ug/100 g (body weight), intraperitoneally] 2 hr aft...

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