Alexandre Fabiato

CALCIUM‐INDUCED RELEASE OF CALCIUM FROM THE SARCOPLASMIC RETICULUM OF SKINNED CELLS FROM ADULT HUMAN, DOG, CAT, RABBIT, RAT, AND FROG HEARTS AND FROM FETAL AND NEW‐BORN RAT VENTRICLES

The small transsarcolemmal influx of Ca2+ during the action potential is by itself insufficient t o activate the myofilaments of mammalian cardiac muscle.’ It has been suggested that this small amount of Ca2+ might trigger the release of a larger amount of Ca2+ from the sarcoplasmic reticulum (SR) that would permit myofilament activation.2 Experiments supporting this hypothesis of a Ca2+-induced release of CaZ+ from the SR have been done in single cardiac cells of the adult rat ventricle from which the sarcolemma had been removed by microdissection (skinned cardiac cell^).^ The present report extends this study to skinned cardiac cells from atrial and ventricular tissues of animal species other than rat and to the fetal and new-born rat ventricles.

Appraisal of the physiological relevance of two hypotheses for the mechanism of calcium release from the mammalian cardiac sarcoplasmic reticulum: calcium-induced release versus charge-coupled release

Recent studies correlating the calcium current with, respectively, the clamp-imposed voltage and the calcium current in intact isolated mammalian cardiac myocytes are reviewed. The major findings are the following: [1] With the exception of one group, all investigators agree that a calcium transient is never observed in the absence of a calcium current. In addition, there is a good correlation between voltage dependence of the calcium current and that of the calcium transient, although this correlation may vary among the cardiac tissues from different animal species. [2] Repolarization clamp pulses from highly positive potentials produce a ‘tail current’ which is associated with a ‘tail calcium transient’. [3] The calcium transient is inhibited when the calcium current is blocked by calcium deprivation or substitution, or by the addition of calcium current antagonists, despite the fact that sarcoplasmic reticulum still contains calcium that can be released by caffeine (with inhibition of this release by ryanodine). These three findings are strongly in favor of a calcium-induced release of calcium and against the hypothesis of charge-movement-coupled release of calcium from the sarcoplasmic reticulum. [4] The only finding that would be more in favor of the latter hypothesis (although till reconciliable with the former) is that repolarization occurring before the rapid rise of calcium transient is complete curtails the calcium transient. Thus, the possibility that charge movement might somehow regulate calcium-induced release of calcium cannot be excluded.

Cyclic AMP-induced enhancement of calcium accumulation by the sarcoplasmic reticulum with no modification of the sensitivity of the myofilaments to calcium in skinned fibres from a fast skeletal muscle

In the presence of low concentrations of total EGTA (5 . 10(-4) M) and free Mg2+ (3.16 . 10(-5) M) and in the presence of caffeine (8 . 10(-3) M), cyclic AMP (5 . 10(-6) M) produces a relaxation of the tension developed by skinned fibres from cat caudo-femoralis. The relaxation can be attributed to an enhancement of the Ca2+ accumulation by the sarcoplasmic reticulum, since cyclic AMP does not modify the sensitivity of the myofilaments of Ca2+. These results are similar to those previously reported for the effect of cyclic AMP on skinned cardiac cells in the presence of a higher free Mg2+ concentration and in the absence of caffeine. This similarity suggests that the mode of action of cyclic AMP on the sarcoplasmic reticulum is not fundamentally different in cardiac and fast skeletal muscles.

Use of aequorin for the appraisal of the hypothesis of the release of calcium from the sarcoplasmic reticulum induced by a change of pH in skinned cardiac cells

A change of pH did not modify the sensitivity of aequorin to Ca2+, but an increase of pH enhanced the Ca2+ sensitivity of the myofilaments of a skinned canine cardiac Purkinje cell. The tension-pCa curve did not present any hysteresis when a given [free Ca2+] was reached from a higher versus from a lower [free Ca2+] in the presence of pH 6.60, 7.10 or 7.40. A rapid variation of pH in either direction failed to induce Ca2+ release from the sarcoplasmic reticulum (SR). The proton ionophores CCCP and gramicidin also failed to induce Ca2+ release from the SR. Increase of pH from 7.10 to 7.40 enhanced Ca2+ accumulation into the SR and, thereby, augmented the Ca2+ content of the SR. Consequently, the amplitude of a subsequent Ca2+ release triggered by a rapid increase of [free Ca2+] at the outer surface of the SR was increased. Conversely, a decrease of pH from 7.10 to 6.60 diminished the Ca2+ accumulation into the SR, the Ca2+ content of the SR and the amplitude of a subsequent Ca2+-induced release of Ca2+ from the SR. In addition, the optimum [free Ca2+] for triggering Ca2+-induced release of Ca2+ was shifted to higher [free Ca2+] values by a decrease of pH from 7.40 to 7.10 or 7.10 to 6.60. This may help to explain the enhancement of the aequorin light transient during acidosis in the intact cardiac muscle inasmuch as acidosis may increase the [free Ca2+] trigger at the outer surface of the SR by inhibiting Na+-Ca2+ exchange across the sarcolemma.

Usefulness of decrease in oxygen uptake efficiency slope to identify myocardial perfusion defects in men undergoing myocardial ischemic evaluation.

Cardiopulmonary exercise testing (CPX) might aid in the diagnosis of coronary artery disease. However, a heterogeneous clinical population without previous workup bias has not been studied nor has a more extensive list of CPX variables. A total of 303 subjects (age 49.9 ± 11.6 years, 157 men) with symptoms suggestive of coronary artery disease underwent CPX and a single photon emission computed tomographic myocardial perfusion study (MPS). Ventilatory efficiency was calculated using the oxygen uptake efficiency slope (OUES). The change in the OUES was calculated by subtracting the OUES response during the first 50% of CPX from the OUES obtained during the last 25% of CPX. A negative change in the OUES (< 0) from the first 50% to the last 25% of CPX was predictive of positive MPS findings only in the male subjects. The diagnostic significance of the change in OUES in men was found for any level (including equivocal studies) of positive MPS findings (area under the curve 0.67, 95% confidence interval 0.59 to 0.76, p < 0.0001) and was even stronger in those with a more definitive (excluding equivocal studies) perfusion defect (area under the curve 0.76, 95% confidence interval 0.67 to 0.85; relative risk 5.4, 95% confidence interval 2.1 to 13.8, p < 0.0001). In conclusion, this is the first time that a change in ventilatory efficiency, assessed using the OUES, has been shown to be predictive of positive MPS findings However, the OUES change only provided diagnostic information for men, a finding that warrants additional analysis.

Overestimation of aerobic capacity with the bruce treadmill protocol in patients being assessed for suspected myocardial ischemia.

INTRODUCTION Peak oxygen uptake (VO₂) is prognostic for morbidity and mortality. Estimating aerobic capacity during traditional exercise stress testing is common as it has been shown that total treadmill time on the Bruce protocol predicts peak VO₂. However, the potential to overestimate peak VO2 exists and may have clinical implications regarding the interpretation of exercise test data. METHODS Subjects (N = 303) with symptoms suggestive of myocardial ischemia underwent a myocardial perfusion study and an exercise test with simultaneous ventilatory expired gas analysis. Estimated peak VO₂ from the Bruce treadmill protocol was compared with measured peak VO₂. The Duke Treadmill Score (DTS) was calculated with treadmill time (DTS(time)) and also with measured VO₂ (DTS(measured)),expressed as metabolic equivalents (METs), and converted to time. RESULTS Peak measured METs was significantly lower than peak estimated METs in the entire cohort (6.5 ± 1.9 vs 8.8 ± 2.8, P < .001) as well as in female (5.7 ± 1.4 and 7.8 ± 2.1, P < .001) and male (7.3 ± 2.0 and 9.7 ± 3.1, P < .001) subgroups. Calculation of the DTS with measured METs resulted in a significantly lower score compared with its calculation with treadmill time (2.7 ± 3.5 vs 5.8 ± 4.6, P < .001). Receiver operating characteristic curve analysis revealed that DTS(measured) produce a statistically significant model for diagnosing a perfusion defect in both men and women (P < .05), whereas DTS(time) was diagnostic only in men (P < .05). DISCUSSION This study demonstrates that estimates of aerobic capacity are significantly higher than measured values and this difference may result in a significant underestimation of morbidity/mortality risk.

Model of calcium-induced calcium release mechanism in cardiac cells

A model with which to elucidate the mechanism of Ca2+ release from, and Ca2+ loading in the sarcoplasmic reticulum (SR) by Ca2+ current (ICa) in cardiac cells is proposed. The SR is assumed to be comprised of three functional subcompartments: (1) the main calcium store (MCS), which contains most of the calcium (both free and bound); (2) the releasable terminal (RT), which contains the calcium readily available for release; and (3) the longitudinal network of the SR (LSR), which sequesters and the transfers the sarcoplasmic calcium to the RT. A rapid increase of the Ca2+ concentration at the outer surface of the SR (Cae) due to the fast component ofICa activates and inactivates this surface, inducing the release of Ca2+ from the RT to the sarcoplasmic space. The RT in turn is further activated and inactivated by a increase in the concentration of sarcoplasmic Ca2+. The Ca2+ in the sarcoplasmic space is then sequestered by the LSR, leading to the reactivation of the RT. Further increase of Cae due to the slow component ofICa enhances the entry of Ca2+ into the MCS to be bound by the binding substance. The free Ca2+ released from the Ca-binding substance complex is transferred to the RT for subsequent release. The activation, inactivation and reactivation are Ca2+-mediated and time-dependent. The proposed model yields simulation of the many events qualitatively similar to those observed experimentally in skinned cardiac cells.