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

Abstract

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.