154 Adaptations of cardiac muscle function during mammalian hibernation

Abstract

Entry into torpor during hibernation has major consequences for mammalian heart. Heart must continue to beat at cold body temperatures and although heart rate is much slower, contractile force must increase to deal with the higher viscosity of blood. Muscle remodelling is needed. We examined responses of the GATA-4 and Nkx2–5 transcription factors that are essential regulators of cardiac hypertrophic growth in thirteen-lined ground squirrels (Spermophilus tridecemlineatus) over a time course of torpor-arousal. Immunoblot analysis showed that both GATA4 and phosphorylated-GATA4(S262) increased significantly during entrance into torpor and GATA4 also showed enhanced binding to DNA. However, Nkx2–5 did not change. Two downstream targets of GATA-4, Troponin C and Troponin I, also showed increased protein levels during entrance into torpor. The data indicate that GATA-4 regulates expression of downstream genes that are important to heart function during transition periods. However, unlike other situations, GATA-4 and Nkx2–5 do not cooperate during torpor.