Length Dependent Activation in Synchronous Flight Muscle from Manduca Sexta

Abstract

In most striated muscles, the amount of force generated at a given concentration of activating calcium is greater at long sarcomere length (SL) than at shorter SL, referred to as “length dependent activation” (LDA). LDA is prominent in mammalian cardiac muscle and underlies the so-called Frank-Starling Law of the Heart” allowing cardiac output to be adjusted on a beat-to-beat basis. The molecular basis for LDA is not yet understood. The dorsal longitudinal flight muscle (DLM) of the Hawkmoth Manduca sexta is an emerging model system for structural and functional studies of muscle. It is a synchronous muscle, requiring a neural impulse for every muscle twitch, as in mammalian skeletal and cardiac striated muscle, but it is structurally similar to the more widely studied asynchronous insect flight muscles of Drosophila and Lethocerus. Its force-length curve has been shown to be remarkably similar to mammalian cardiac muscle (Tu & Daniel, J Exp Biol 207: 2455. 2004) indicating that Manduca flight muscle might be a useful model system to elucidate various aspects of cardiac function in comparative studies. The present studies were undertaken to characterize LDA in Manduca flight muscle. Conditions were found that allowed chemical skinning of the muscles while maintaining good structural order as assessed by by light and X-ray diffraction. Force-pCa curves were collected as a function of SL. Dorsally located DLMs (cooler in vivo) were compared to ventrally located DLM's (warmer in vivo). We found that both dorsal and ventral DLMs show length-dependent activation. Our study also showed that ventrally located DLM's are less cooperative (Hill coefficient nH ∼1-1.2) than the dorsally located DLM's (nH ∼1.8–1.9) which may be related to their different functions in vivo. Supported by NSF IOS 1022058 and NIH RR08630.