Regional differences in responsiveness to octopamine within a locust skeletal muscle

Regional differences in the physiological and biochemical responses to octopamine have been investigated in the extensor tibiae muscle of the hind leg of the locust. Octopamine increases the rate of relaxation of twitch tension generated by the slow motoneurone by different amounts in different regions of the muscle. It also increases the amplitude of twitch tension by the same amount in different regions of the muscle. The relaxation rate of fast twitch tension is increased by the same amount in all regions of the muscle innervated by the fast motoneurone. Octopamine also increases cyclic AMP levels by different amounts in different regions of both the extensor muscle and its antagonistic muscle, the flexor tibiae. In both muscles the maximal responses are obtained in the regions of the muscles containing the highest proportions of slow and intermediate muscle fibres as characterized by their contractile and ultrastructural properties. The results are discussed in terms of their functional significance and compared with examples of differential responses of different muscle fibre types in other invertebrate and vertebrate skeletal muscles.