Ca-activation and stretch-activation in insect flight muscle

Asynchronous insect flight muscle is specialized for myogenic oscillatory work, but can also produce isometric tetanic contraction. In skinned insect flight muscle fibers from Lethocerus, with sarcomere length monitored by a striation follower, we determined the relation between isometric force (F0) at serial increments of [Ca2+] and the additional active force recruited at each [Ca2+] by a stretch of 12nm per half-sarcomere (FSA). The isometric force-pCa relation shows that 1.5–2 units of pCa are necessary to raise isometric force from its threshold (pCa 6.5) to its maximum (F0,max). The amplitude of FSA depends only on the preceding baseline level of isometric force, which must reach at least 0.05 F0,max to enable stretch-activation. FSA rises very steeply to its maximum as F0 reaches 0.2 F0,max, then decreases as F0 increases so as to produce a constant sum (F0+FSA)=Fmax. Thus Ca- and stretch-activation are complementary pathways that trigger a common process of cross-bridge attachment and force production. We suggest that stretch-induced distortion of attached cross-bridges relieves the steric blocking by tropomyosin of additional binding sites on actin, thereby enabling maximum force even at low [Ca2+].