Unstimulated force during hypoxia of rat cardiac muscle: Stiffness and calcium dependence

The stiffness of rat cardiac trabeculae was measured in vitro to distinguish between an increase in unstimulated force (F(u)) caused by rapid cycling of cross bridges or caused by rigor bridges during hypoxia. The force was measured with a strain gauge, the sarcomere length was determined by laser diffraction techniques, and muscle length was controlled by means of a motor. Stiffness was analyzed by using small (< 1% of muscle length) sinusoidal length perturbations of 1 and 100 Hz. The stiffness at 100 Hz increased linearly with force during tetani at a varied [Sr²⁺] (0.25-10 mM) in the Krebs-Henseleit (K-H) buffer, but remained virtually unchanged at 1 Hz. In contrast, the stiffness of both the passive muscle and the muscle exposed to either CN^- or to PO₂ < 1.5 mmHg up to development of maximal F(u) (F(u)(max)) was similar at 1- and 100-Hz perturbations. Less profound hypoxia (PO₂ 6-10 mmHg) resulted in spontaneous sarcomere activity during the rise in F(u), and an increase in the ratio of stiffness at 100 Hz to stiffness at 1 Hz was detected. When oxidative phosphorylation was inhibited by CN^- (2 mM) while the muscle was stimulated in the absence of both Ca²⁺ and Na⁺ (choline + substituted), the addition of Na⁺ at the time at which F(u) had reached 30-40% of F(u)(max) did not affect the rate of rise of F(u). These results show that the development of F(u) during more complete anoxia in rat trabeculae is completely due to the formation of rigor links and that Ca²⁺-dependent cross-bridge activation can contribute to the rise in F(u) during less severe hypoxia.