Warm-up and substrate cycling in flight muscles of male bumblebees, Bombus terrestris

Abstract

1. 1. Rates of substrate cycling between fructose 6-phosphate and fructose bis-phosphate were measured in the flight muscles of male bumblebees ( Bombus terrestris) in vivo during four denned activity phases. 2. 2. Shortly after the injection of double-labelled glucose thermocouples were fixed to the thorax to record surface temperature continuously and to determine the phase of activity every individual bee was in, when finally the cycling rate was measured. 3. 3. Based upon the 3H/ 14C-ratios, measured in the sugars and hexose-phosphates involved, the following relative cycling rates were determined: torpor: 0.24 ± 0.11; warm-up: 9.14 ± 3.51; “activated” rest: 1.94 ± 0.59, and flight: 0.22 ± 0.11. Using the data of the glycolytic fluxes, determined in the flight muscles of these bumblebees in previous studies, absolute cycling rates were evaluated as follows: torpor: 0.11 ± 0.04; warm-up: 249.91 ± 96.05; “activated” rest: 17.42 ± 5.29, and flight: 5.94 ± 2.95 μmol/min × g muscle. 4. 4. Conversion of these rates of substrate cycling (= rates of ATP-hydrolysis) into rates of heat production demonstrates that in the flight muscles of Bombus terrestris during phases of warming-up without any detectable movements of wings or thorax substrate cycling between fructose 6-phosphate and fructose-bis-phosphate very probably is the predominant mechanism of non-shivering heat production. 5. 5. Possible mechanisms for the regulation of the intensity of this fructose 6-phosphate/fructose bis-phosphate substrate cycle are discussed.