Mechanical activity and force-frequency relationship of isolated mouse papillary muscle: effects of extracellular calcium concentration, temperature and contraction type.

Cardiac physiology of the mouse is becoming increasingly important because the mouse is the mammalian model animal of choice for genetic modifications. However, mouse cardiac muscle is still poorly characterized under physiological conditions and inconsistent results have been published in the literature regarding mechanical activity especially the force-frequency relationship in isolated mouse muscle preparations. In this study we investigated systematically several mechanical parameters of isolated mouse papillary muscle such as force-frequency relation, twitch force, time to peak tension, relaxation time and post-rest potentiation at different experimental conditions. Extracellular calcium concentration ([Ca2+]) was varied between 1.0 and 5.0 mM, temperature between 27 degrees C and 37 degrees C and force measurements were performed under isometric as well as auxotonic conditions. The mechanical activity of muscle preparations was found to be strongly dependent on [Ca2+] and temperature and slightly on contraction type. At low temperature and low [Ca2+] the force-frequency relation was strongly positive whereas at high temperature and high calcium it turned negative. The results of this study demonstrate a flat force-frequency relation in mouse papillary muscle at physiological conditions (37 degrees C, [Ca2+] of 1.5 mM) and provide a reliable experimental basis for comparative studies with genetically altered mice.