Binding of calcium to myoplasmic buffers contributes to the frequency-dependent inotropy in heart ventricular cells

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 (Vprop) ranging from 0. 34 to 5.47 mm/s. To explore the determinants of Vprop, we studied (1) the relationship between [Ca2+]i and Vprop and (2) the effect of low concentrations of caffeine on Vprop. 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 degrees 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 (DeltaCaT) and [Ca2+]i just before onset of the Ca2+ waves (CaD) were used to estimate the Ca2+ loading of the sarcoplasmic reticulum (SR) and the myoplasm, respectively. The ratio (DeltaCaW/DeltaCaT) of the [Ca2+]i increment during the waves (DeltaCaW) to DeltaCaT 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, Vprop increased in proportion to (1) DeltaCaT (r=0.82); (2) CaD (r=0.88); (3) DeltaCaW (r=0.85); and (4) DeltaCaW/DeltaCaT (r=0.74). The addition of caffeine (</=0.3 mmol/L) increased Vprop for any DeltaCaT and any DeltaCaW, revealing an increased sensitivity of Vprop to DeltaCaT and DeltaCaW. In contrast, caffeine had little effect on the relationship between Vprop and CaD and no effect on that between Vprop and DeltaCaW/DeltaCaT. 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.

Control of cardiac performance by Ca-turnover

A quantitative model of Ca-turnover in cardiac cells that incorporates negative feedback modulation of sarcolemmal calcium transport (via Ca channels and Na/Ca exchange) has been designed. The Na/Ca exchange current was expressed as INaCa = INaCar + delta INaCa. The component INaCar reflects slow changes of Ca2+ and Na+ concentrations and depends on the Na/K pump. delta INaCa is the fast component related to the Ca2+ transient. The single input to the model is an arbitrary sequence of intervals between excitations; outputs are sequences of calcium amounts transferred among the compartments during individual intervals. The model operates with a combination of discrete variables (amounts of Ca transferred during contraction, relaxation and rest) and continuous variables - slow changes in ionic concentrations. Since the model is not formalistic but respects the nature of the underlying elements of the system, it enables us to stimulate the known effects of cardiotropic drugs or to predict their unknown mechanisms by visualizing the changes in individual Ca compartments. By altering the parameters, the model also stimulates the known species and tissue differences in rate-dependent phenomena.

Ca2+ load of guinea-pig ventricular myocytes determines efficacy of brief Ca2+ currents as trigger for Ca2+ release

1. In guinea-pig ventricular cells, the concentration of ionized cytosolic calcium ([Ca2+]o) was estimated from the fluorescence of 100 microM K5-indo-1. At 36 degrees C and 2 mM [Ca2+]o, the Ca2+ load of the cells was varied by 1 Hz trains of conditioning clamp pulses to -30 mV (low Ca2+ load), 0 mV (intermediate Ca2+ load) and paired pulses (high Ca2+ load). After seven pulses potentiation was steady and short test pulses to 0 mV were tested for their efficacy in triggering [Ca2+]c transients. The influx of trigger Ca2+ was graded by varying the test-pulse duration between 1 and 180 ms. 2. After a 3 min rest period, [Ca2+]c was 100 +/- 20 nM (mean +/- S.E.M.) and 2 ms test pulses were unable to induce [Ca2+]c transients. Test pulses of 2 ms duration, however, induced [Ca2+]c transients after potentiation with single or paired pulses. 3. At high cellular Ca2+ load, the amplitude of the [Ca2+]c transients (delta[Ca2+]c) gradually increased with pulse durations up to 8 ms. Pulse durations between 8 and 160 ms, however, did not further increase delta[Ca2+]c as if the largest part of the [Ca2+]c transient was due to regenerative contribution of Ca(2+)-induced Ca2+ release. 4. Pulses of 160 ms duration induced 'saturating' responses whose amplitudes delta[Ca2+]c, t = infinity decreased from 938 +/- 120 nM at high Ca2+ load, to 610 +/- 90 and 350 +/- 120 nM at intermediate and low Ca2+ loads, respectively. 5. Delta[Ca2+]c was more sensitive to the duration of Ca2+ influx at low or intermediate Ca2+ loads than at high Ca2+ load. When delta[Ca2+]c was plotted against the test-pulse duration, 50% of delta[Ca2+]c, t = infinity was found to be at 9 +/- 2 ms (low), 4.6 +/- 1 ms (intermediate) or 1.8 +/- 0.5 ms pulses (high Ca2+ load). Correspondingly, the efficacy of 2 ms test pulses in triggering [Ca2+]c transients increased with the Ca2+ load. 6. At high Ca2+ load, [Ca2+]c peaked nearly independently of pulse duration at 19 +/- 3 ms. At intermediate or low Ca2+ load, time to peak increased with pulse duration. 7. The results confirm the theory that sarcoplasmic reticulum (SR) Ca2+ release contributes an amount to the [Ca2+]c transient that increases with the cellular Ca2+ load. The results are compatible with the hypothesis that SR Ca2+ release can be activated by both Ca2+ influx and by SR Ca2+ release and that the latter mechanism constitutes a positive feedback, the amplification of which increases with the amount of releasable Ca2+.