title: Energetics of Mouse Papillary Muscle
Creators: name: Widen, Cecilia
Publisher: name: Griffith University
Year: 2006
Subjects: subject: Mouse papillary muscle
subject: myothermic technique
subject: ischaemia and reperfusion damage

description: The overall aim of this Thesis was to characterise the energetic properties of the mouse papillary muscle as this preparation could become a useful model to study alterations of energetic aspects of cardiac pathologies and heart-focussed genetic changes. Measurements of resting and active metabolism of the papillary muscles were made in vitro using the myothermic technique. In the first study the mechanism underlying impaired contractility of post-ischaemic rat papillary muscle was investigated. The rat preparation is well established and was used to develop protocols and approaches that could later be used as the basis for studies with mouse papillary muscle. The muscles were exposed to simulated ischaemia for 60 min and change in energetics was studied 30 min into the reperfusion phase. The work output was reduced to 66 ± 3% of the pre-ischaemia value and the enthalpy output decreased to 71 ± 3% of pre-ischaemia value. However, there was no change in either initial, 19 ± 3%, or net mechanical efficiency, 9.0 ± 0.9%. These data, in combination with studies of Ca2+ handling, suggests that the reduced work output was caused by attachment of fewer cross-bridges in each twitch, but with no change in work generated by each cross-bridge. The following two studies involved characterisation of the energetics of the mouse papillary muscle and included measurements of resting and active metabolism. The resting metabolic rate varied with muscle size but the mean initial value was tilda 25 mW g-1 and the estimated steady value tilda 5 mW g-1 . The resting metabolic rate declined exponentially with time towards a steady value, with a time constant of 18 ± 2 min. There was no alteration in isometric force output during this time. The magnitude of resting metabolism depended inversely on muscle mass, more than doubled following a change in substrate from glucose to pyruvate and was increased 2.5-fold when the osmolarity of the bathing solution was increased by addition of 300 mM sucrose. Addition of 30 mM BDM affected neither the time course of the decline in metabolic rate nor the eventual steady value. The energy requirements associated with contractile activity were tilda7 mJ g-1 twitch-1 at a contraction frequency of 1 Hz. The enthalpy output was not affected by changing substrate from glucose to pyruvate but did decrease with an increase in temperature. The enthalpy output was partitioned into force-dependent and force-independent components using BDM to selectively inhibit cross-bridge cycling. The force-independent enthalpy output was 18.6 ± 1.9% of the initial enthalpy output. Muscle initial efficiency was &amp;tilda;32% and net efficiency tilda 17% when shortening at a realistic velocity. The enthalpy output decreased with increased contraction frequency but was independent of shortening velocity. On the basis of these values, it was calculated that the twitch energetics were consistent with ATP splitting by half the cross-bridges and the pumping of one Ca 2+ into the SR for every three cross-bridge cycles. The lack of influence of shortening velocity on energy cost supports the idea that the amount of energy to be used is determined early in a twitch and is not greatly influenced by events that occur during the contraction. The suitability of the mouse papillary muscle as a model to study ischaemia and reperfusion damage was also assessed. This preparation is excellent for studying muscle specific changes in work and enthalpy output; however, due to the long-term instability and variability amongst preparations, the suitability of this preparation in prolonged experiments remains uncertain.