Cross-bridge dependent cooperativity determines the cardiac force–length relationship

Opinions vary as to whether the dominant cooperativity mechanism, modulating the cardiac force–length–calcium relationships, depends on sarcomere length (SL), force, or calcium. We hypothesize that separately characterizing the dependence of the delay in the force response to length oscillations on SL, force and calcium allows testing these hypotheses; Oscillations imposed at the same SL, utilizing various calcium concentrations, entail similar delay if the cooperativity is exclusively length dependent. Oscillations at constant tetanic force, utilizing various SLs and calcium levels, entail similar delay when the cooperativity is exclusively force dependent. Force responses to large (102 ± 13 nm/sarcomere) oscillations, at different SLs, extracellular calcium concentrations ([Ca2+]os) and force, were studied in intact tetanized trabeculae, dissected from rats' right ventricles (N = 11). Tetanii (3.5 s) were obtained by utilizing cyclopiazonic acid (K–H, 25 °C). SL was measured by laser diffraction. Force responses to length oscillations lagged behind the length oscillations at short SL and low [Ca2+]o, yielding counterclockwise hystereses in the force–length plane: the force was higher during shortening than during lengthening. The area within the counterclockwise hysteresis represents the external work that originates from XB recruitment, and it increases as the phase delay increases. The phase decreases by 25.7 ± 17.0° for 0.1 μm sarcomere lengthening, at constant [Ca2+]o (e.g. 45 ± 9° and 22 ± 13° for SL = 1.89 ± 0.01 and 1.99 ± 0.01 μm, respectively, [Ca2+]o = 3 mM). The phase decrease by 15 ± 5.7° for an increase of 1.5 mM in [Ca2+]o, at constant SL. However, maintaining the same force with different pairs of [Ca2+]o and SL yields identical phase. The direct dependence of the phase on force indicates that XB recruitment is determined directly by the force and only indirectly by SL or calcium. The dominant cardiac cooperativity is determined by the number of strong XBs.