Force generation upon hydrostatic pressure release in tetanized intact frog muscle fibres

Single intact muscle fibres isolated from the tibialis anterior muscle of t he frog were exposed to hydrostatic pressures of 1-10 MPa, at 2-4 degrees C and sarcomere length of 2.1-2.2 mu m. The pressure was rapidly released (c a. 1 ms) to atmospheric level (0.1 MPa) during the plateau of a tetanic con traction (P-o) and the resultant tension ( =force) transient examined. The pressure release induced tension transient consisted of an initial tension drop coincident with pressure release (ca. 4% P-o per MPa, Phase 1), follow ed by a rapid recovery (Phase 2a) and a slower rise of tension (Phase 2b). Phase 1 was partly due to a length release at fibre ends (ca. 0.1 nm per ha lf-sarcomere per MPa) induced by pressure-release effects on the steel cham ber and fibre attachments, and partly due to 'expansion' upon pressure rele ase within muscle fibre (ca. 0.2 nm per half-sarcomere per MPa), probably i n the myofilaments and cross-bridges. The rate of tension recovery during p hase 2a (ca. 600/s) was comparable to that of the quick tension recovery (T -1-T-2 transition) reported from moderately fast small length releases; the time course of Phase 2b (rate ca. 40/s) was similar to the late phase of t ension rise in a tetanus, and hence compared with Phase 4 (T-4) of a length release tension transient. Results are compared with the previously report ed findings from analogous experiments on Ca2+-activated skinned (rabbit) m uscle fibres.