Heat Sensing in Muscle Calorimetry

Abstract

Measuring the energetics of isolated cardiac muscle is important for improving the understanding of cardiac physiology in health and disease. Examining energetics requires the simultaneous measurement of force, length and rate of heat production. Until now these measurements have primarily been performed at sub-physiological temperatures, thereby requiring extrapolation of the observed energetic relations to body temperature (37 °C). Energetic measurements at 37 °C have been limited by the inability to resolve the meagre heat production (5 μW) of samples sufficiently small to prevent anoxia. The purpose of this thesis is to present work that culminated in the first simultaneous measurements of mechanics and heat production of adequately oxygenated isolated cardiac muscle at physiological temperature. Achieving these measurements required the development of new heat and temperature sensing methods, and their implementation in a novel calorimeter. Initial development focused on the creation of a vapour pressure thermometer that inferred the temperature of a solvent from measurements of the pressure of the solvent’s gas phase. The resulting vapour pressure thermometer was capable of higher temperature resolution than any previously reported vapour pressure thermometer. However, the thermometer’s resolution was not sufficiently high for application to the developed calorimeter and had limitations that made its use impractical for muscle calorimetry. A muscle calorimeter was subsequently developed that uses commercially available solid-state thermopiles. The calorimeter was capable of unprecedented power resolutions of 2.6 nW·Hz−1/2 at room temperature and 7.1 nW·Hz−1/2 at 37 °C. These power resolutions correspond to temperature resolutions of 0.37 μK·Hz−1/2 and 1.01 μK·Hz−1/2, respectively. The calorimeter was used to examine the effect of temperature on the heat-stress relationship of rat cardiac trabeculae and enabled the first measurements of the heat-stress relationship of isolated cardiac muscle at 37 °C. The heat-stress relationship was quantified at 27 °C and 37 °C and was found to be linear at both temperatures. The intercept decreased at the higher temperature, reflecting the smaller magnitude of the calcium transient at this temperature. The gradient of the relationship was independent of temperature, likely reflecting the underlying metabolic cost of crossbridge cycling. The major contributions to knowledge of this thesis are: • The development of a novel vapour pressure thermometer with a four-fold greater resolution than was previously achieved with this sensing technology; • The construction of the first flow-through muscle calorimeter to use robust solidstate thermoelectric modules as sensors. Characterisation of the calorimeter demonstrated unprecedented resolutions of 0.37 μK·Hz−1/2 and 2.6 nW·Hz−1/2 at room temperature, and 1.01 μK·Hz−1/2 and 7.1 nW·Hz−1/2 at 37 °C; • The first measurements of the heat-stress relationship of isolated cardiac muscle at physiological temperature.