The effects of octopamine on contraction kinetics and power output of a locust flight muscle

Abstract

1. Perfusion with saline containing octopamine resulted in an increase in mechanical power output, and an increase in twitch and tetanic tension, from a flight muscle (metathoracic, second tergocoxal muscle) of the desert locust,Schistocerca americana gregaria. 2. Mechanical power output was measured under conditions approximating those of flight. Power output increased upon exposure to 10−6M DL-octopamine by about 20% when the muscle was activated by 1 stimulus per cycle and by about 8% when the muscle was activated by paired stimuli. 3. The increase in twitch tension was approximately proportional to the log of the octopamine concentration from 10−8M to 10−5M. At 10−6M, the twitch tension was about 19% greater than control values and the tetanic tension was about 8% above control values. The average twitch tension in control muscles was 17.0 N/cm2, and the average control tetanic tension was 27.9 N/cm2. 4. The maximum shortening velocity was little affected by octopamine. In control muscles, the maximum shortening velocity was 8.3 muscle lengths/s (L/s) for slack-test measurements and 5.7 L/s by extrapolation of force-velocity curves. In octopamine, the corresponding maximum shortening velocities were 8.0 and 5.9 L/s, respectively. 5. Octopamine perfusion resulted in a small increase in twitch duration (control mean = 40.5 ms at 30 °C). Both contraction and relaxation times increased by about 4%.