Heat-stress relationships of rat cardiac trabeculae determined using a micromechanocalorimeter

Abstract

In order to study the physiology and pathophysiology of the heart, it is insightful to employ isolated cardiac muscle preparations. To that end, we have recently constructed a micromechanocalorimeter for measuring simultaneously the heat rate and force production of superfused cardiac trabeculae. In the microcalorimeter component of the micromechanocalorimeter, two arrays of non-contact thermopile sensors measure the temperature of superfusate upstream and downstream of a centrally-located respiring trabecula. The increment in temperature of superfusate downstream, relative to that upstream, is proportional to the heat liberated by the trabecula. Using the micromechanocalorimeter, we have determined the relationship between heat and force production of rat cardiac trabeculae (n = 10), in 1 mM and 2 mM extracellular calcium concentration ([Ca2+]o) at room temperature (20-22oC). Muscle force was varied by reducing muscle length below optimal value (Lo). Muscle force and rate of heat production were measured simultaneously at two stimulus frequencies: 0.2 Hz and 2.0 Hz. In healthy superfused cardiac trabeculae, we found that heat production was linearly correlated with stress (normalized force) production. The heat-stress regression lines were not significantly different between 0.2 Hz and 2.0 Hz at either value of [Ca2+]. Activation heat values, which are the heat extrapolated to the zero-stress intercepts of the heat-stress regression lines, did not differ between stimulus frequencies, but were significantly higher in 2 mM than in 1 mM [Ca2+]o. We conclude that the heat-stress relationships are independent of stimulus frequency, but dependent on [Ca2+]o. To our knowledge, our micromechanocalorimeter is unique, and has allowed the first determination of the heat-stress relationships of cardiac trabeculae.