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Effect of stimulation rate, sarcomere length and Ca²⁺ on force generation by mouse cardiac muscle

Bruno D. Stuyvers, Andrew D. McCulloch *, Jiqing Guo, Henry J. Duff and Henk E. D. J. ter Keurs

The relations between stress, stimulation rate and sarcomere length (SL) were investigated in 24 cardiac trabeculae isolated from right ventricles of mice (CF-1 males, 25-30 g) and superfused with Hepes solution ([Ca²⁺]_o = 1 mM, pH 7.4, 25 °C). Stress and SL were measured by a strain gauge transducer and laser diffraction technique, respectively. Stress versus stimulation frequency formed a biphasic relation (25 °C, [Ca²⁺]_o = 2 mM) with a minimum at 0.7-1 Hz (~15 mN mm^-2), a 150 % decrease from 0.1 to 1 Hz (descending limb) and a 75 % increase from 1 to 5 Hz (ascending limb). Ryanodine (0.1 µM) inhibited specifically the descending limb, while nifedipine (0.1 µM) affected specifically the ascending limb. This result suggests two separate sources of Ca²⁺ for stress development: (1) net Ca²⁺ influx during action potentials (AP); and (2) Ca²⁺ entry into the cytosol from the extracellular space during diastolic intervals; Ca²⁺ from both (1) and (2) is sequestered by the SR between beats. Raising the temperature to 37 °C lowered the stress-frequency relation (SFR) by ~0-15 mN mm^-2 at each frequency. Because the amount of Ca²⁺ carried by I_Ca,L showed a ~3-fold increase under the same conditions, we conclude that reduced Ca²⁺ loading of the SR was probably responsible for this temperature effect. A simple model of Ca²⁺ fluxes addressed the mechanisms underlying the SFR. Simulation of the effect of inorganic phosphates (P_i) on force production was incorporated into the model. The results suggested that O₂ diffusion limits force production at stimulation rates >3 Hz. The stress-SL relations from slack length (~1.75 µm) to 2.25 µm showed that the passive stress-SL curve of mouse cardiac trabeculae is exponential with a steep increase at SL >2.1 µm. Active stress (at 1 Hz) increased with SL, following a curved relation with convexity toward the abscissa at [Ca²⁺] = 2 mM. At [Ca²⁺] from 4 to 12 mM, the stress-SL curves superimposed and the relation became linear, which revealed a saturation step in the activation of force production. EC coupling in mouse cardiac muscle is similar to that observed previously in the rat, although important differences exist in the Ca²⁺ dependence of force development. These results may suggest a lower capacity of the SR for buffering Ca²⁺, which makes the generation of force in mouse cardiac ventricle more dependent on Ca²⁺ entering during action potentials, particularly at high heart rate.

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