Ca²⁺ waves during triggered propagated contractions in intact trabeculae: Determinants of the velocity of propagation

During triggered propagated contractions, Ca²⁺ waves travel along cardiac trabeculae with a constant velocity (V(prop)) ranging from 0.34 to 5.47 mm/s. To explore the determinants of V(prop), we studied (1) the relationship between [Ca²⁺](i) and V(prop) and (2) the effect of low concentrations of caffeine on V(prop). Trabeculae were dissected from the right ventricle of rat hearts. [Ca²⁺](i) was measured using electrophoretically injected fura-2 and an image-intensified CCD camera. Force was measured using a silicon strain gauge, and sarcomere length was measured using laser diffraction techniques. After induction of reproducible Ca²⁺ waves by trains of electrical stimuli (2.5 Hz) at 21.9±0.2°C, the number of stimuli or [Ca²⁺](o) was varied in 9 trabeculae. In 5 trabeculae, the effects of caffeine (0.1 to 1.0 mmol/L) at [Ca²⁺](o) of 2.2±0.3 mmol/L were determined. All images were recorded under stable conditions of wave propagation. The increment in [Ca²⁺](i) during the last electrically stimulated transient (ΔCa(T)) and [Ca²⁺](i) just before onset of the Ca²⁺ waves (Ca(D)) were used to estimate the Ca²⁺ loading of the sarcoplasmic reticulum (SR) and the myoplasm, respectively. The ratio (ΔCa(w)/ΔCa(T)) of the [Ca²⁺](i) increment during the waves (ΔCa(w)) to ΔCa(T) was used to estimate the probability of opening of the SR-Ca²⁺ release channel during wave propagation. As a result of an increase of the number of stimuli or [Ca²⁺](o), V(prop) increased in proportion to (1) ΔCa(T) (r=0.82); (2) Ca(D) (r=0.88); (3) ΔCa(w) (r=0.85); and (4) ΔCa(w)/ΔCa(T) (r=0.74). The addition of caffeine (≤0.3 mmol/L) increased V(prop) for any ΔCa(T) and any ΔCa(w), revealing an increased sensitivity of V(prop) to ΔCa(T) and ΔCa(w). In contrast, caffeine had little effect on the relationship between V(prop) and Ca(D) and no effect on that between V(prop) and ΔCa(w)/δCa(T). These results suggest that both the cellular Ca²⁺ loading and open probability of the SR-Ca²⁺ release channels determine the velocity of propagation of Ca²⁺ waves.