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Ca-Activation and Stretch-Activation in Insect Flight Muscle

Marco Linari ^*, Michael K. Reedy , Mary C. Reedy , Vincenzo Lombardi ^* and Gabriella Piazzesi ^*

^* Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze and Istituto Nazionale di Fisica della Materia, Firenze, Italy; and Duke University Medical Center, Department of Cellular Biology, Durham, North Carolina

Correspondence: Address reprint requests to Gabriella Piazzesi, Laboratorio di Fisiologia del Dipartimento di Biologia Animale e Genetica, c/o Dipartimento di Fisica, Via G. Sansone 1, I-50019 Sesto Fiorentino (Fl), Italy. Tel.: 39-055-457-2385; Fax: 39-055-457-2121; E-mail: gabriella.piazzesi{at}unifi.it.

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 12 nm per half-sarcomere (F_SA). 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 (F_0,max). The amplitude of F_SA depends only on the preceding baseline level of isometric force, which must reach at least 0.05 F_0,max to enable stretch-activation. F_SA rises very steeply to its maximum as F₀ reaches 0.2 F_0,max, then decreases as F₀ increases so as to produce a constant sum (F₀ + F_SA) = F_max. 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⁺].

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