Siberian hamsters (Phodopus sungorus) respond to short-day photoperiod by reducing metabolic rate, body temperature, and food intake. During this time they enter torpor for 4-8 h per day, 2-4 days per week, for up to 20 weeks, torpor reducing their daily energy expenditure by up to 70 % (Dark et al. 1999; Heldmaier et al. 1999). Body weight falls by a third in torpor, whereas the heart weight remains constant (Puchalski et al. 1986). Prior to torpor cardiac carbohydrate metabolism may be inhibited and fatty acid oxidation increased (Heldmaier et al. 1999; Buck et al. 2002). Little is known of cardiac function in Siberian hamsters, although contractility may increase as it does in hibernating ground squirrels (Milner et al. 1991).

Consequently, we measured cytoskeletal and GLUT 4 protein expression in Siberian hamster hearts during activity and torpor. Torpor was identified by a decrease in core body temperature to below 25 °C for at least 2 h, as measured by telemetry devices (Dataquest) previously implanted under anaesthesia. Animals were humanely killed by cervical dislocation, and hearts rapidly excised. Hearts (n = 7 for active, n = 4 for torpor) were homogenised, centrifuged, and the supernatant protein was separated using SDS-PAGE. After transferring to a nitrocellulose membrane, proteins were blotted using primary antibodies. Protein bands were detected using HRP-conjugated secondary antibodies, and visualised by enhanced chemiluminescence.

Hearts from hamsters in torpor had significant 62 ± 18.4% and 54 ± 12.4% increases in actin and muscle LIM protein expression, respectively, whereas syncoilin and desmin were decreased by 42 ± 15.0% and 48 ± 14.2%, indicating that myocyte structure was altered during torpor. A significant 255 ± 43.7% increase in β-myosin heavy chain (β-MHC) expression, and no change in total MHC expression occurred, suggesting a switch to the slow muscle fibre type during torpor. A significant reduction in GLUT 4 protein expression of 58 ± 14.6 % during torpor may have been related to a reduction in carbohydrate metabolism during torpor. Thus the hamster heart underwent changes during torpor that have been associated with cardiac hypertrophy in other species.

We thank the MRC, BHF and BBSRC for funding this work.