The activating effects of calcium ions on the contractile systems of insect fibrillar flight muscle

Abstract

The Ca2+ induced activation of the ATP-ase and contraction of myofibrils and actomyosin from fibrillar insect flight muscle, the sensitivity to Ca2+ and conditions for the Ca2+ activation have been investigated. 1. Ca2+ activate the ATP-ase of the myofibrils and of the actomyosin but not that of mitochondria (in presence of Mg2+). 2. 50% activation of ATP-ase is produced by about 10−7 M Ca2+. 3. The degree of ATP-ase activation by Ca2+ (10−6 M) is five to ten times less than in non-oscillating insect muscle or in vertebrate-skeletal muscle. 4. The low ATP-ase activity of isolated myosin and the superprecipitation of an actomyosin gel in the relaxing medium suggest that the relativly high ATP-ase of myofibrils and actomyosin gels is not a myosin ATP-ase but an actomyosin ATP-ase or an additional ATP-ase. 5. The relaxation of glycerinated fibres and isolated actomyosin by removal of Ca2+ requires the presence of Mg2+. But the inhibitory effect on the ATP-ase activity produced by removal of Ca2+ is decreased by addition of Mg2+. 6. Contraction induced by addition of Ca2+ is associated with an increased ATP-ase activity, contraction induced by removal of Mg2+ in the absence of Ca2+ is associated with a decreased ATP-ase activity. This difference may suggest that both types of contraction depend on different mechanisms.