Meeting Abstract

20.5  Monday, Jan. 4  Mechanical energy gradients arise as a consequence of temperature gradients in the flight muscles of Manduca sexta GEORGE, N.T.*; DANIEL, T.L.; Univ of Washington, Seattle; Univ of Washington, Seattle ntgeorge@u.washington.edu

The inherent inefficiency of muscle contraction leads to significant heat production. Many insects use this energy byproduct to raise their thoracic temperature to achieve near maximal contraction rates. However, an elevated core temperature combined with surface heat loss invariably leads to a temperature gradient throughout the muscle. In the dominant flight muscles of Manduca sexta, (dorsolongitudinal muscles: DLMs), we measured a strong gradient in the dorso-ventral direction with a mean difference of 6.4 °C. Together with the temperature dependence of muscle force generation, this thermal gradient suggests the energy output of more dorsal subunits will differ from deeper and warmer muscle subunits. To test this hypothesis, we isolated the cooler dorsal subunits and the warmer ventral subunits and recorded both single and 25 Hz (wing beat frequency) isometric contractions at a range of temperatures. At 25 °C, the dorsal subunits had a rise time 1.5 ms longer than the ventral subunits at 35 °C. In addition, at 25 Hz, the cooler dorsal subunits contracted in an unfused tetanus, whereas the warmer ventral subunits produced isolated twitches. Furthermore, EMG recordings of the DLM subunits showed no separate activation that would align contraction peaks. The presence of an unfused tetanus in the dorsal subunits indicates that work output varies significantly throughout the DLMs. To test this hypothesis, we used temperature controlled work-loop methods. Results show the total amount of work lost by the system decreased as temperature increased; net work differed by at least a factor of 2 from 25 °C to 35 °C. Thus, the existence of a temperature gradient produces a mechanical energy gradient in muscles.