Exposure of energy-depleted rat trabeculae to low pH improves contractile recovery: role of calcium

A mathematical model of cardiocyte Ca(2+) dynamics with a novel representation of sarcoplasmic reticular Ca(2+) control

Cardiac contraction and relaxation dynamics result from a set of simultaneously interacting Ca(2+) regulatory mechanisms. In this study, cardiocyte Ca(2+) dynamics were modeled using a set of six differential equations that were based on theories, equations, and parameters described in previous studies. Among the unique features of the model was the inclusion of bidirectional modulatory interplay between the sarcoplasmic reticular Ca(2+) release channel (SRRC) and calsequestrin (CSQ) in the SR lumen, where CSQ acted as a dynamic rather than simple Ca(2+) buffer, and acted as a Ca(2+) sensor in the SR lumen as well. The inclusion of this control mechanism was central in overcoming a number of assumptions that would otherwise have to be made about SRRC kinetics, SR Ca(2+) release rates, and SR Ca(2+) release termination when the SR lumen is assumed to act as a simple, buffered Ca(2+) sink. The model was sufficient to reproduce a graded Ca(2+)-induced Ca(2+) release (CICR) response, CICR with high gain, and a system with reasonable stability. As constructed, the model successfully replicated the results of several previously published experiments that dealt with the Ca(2+) dependence of the SRRC (, J. Gen. Physiol. 85:247-289), the refractoriness of the SRRC (, Am. J. Physiol. 270:C148-C159), the SR Ca(2+) load dependence of SR Ca(2+) release (, Am. J. Physiol. 268:C1313-C1329;, J. Biol. Chem. 267:20850-20856), SR Ca(2+) leak (, J. Physiol. (Lond.). 474:463-471;, Biophys. J. 68:2015-2022), SR Ca(2+) load regulation by leak and uptake (, J. Gen. Physiol. 111:491-504), the effect of Ca(2+) trigger duration on SR Ca(2+) release (, Am. J. Physiol. 258:C944-C954), the apparent relationship that exists between sarcoplasmic and sarcoplasmic reticular calcium concentrations (, Biophys. J. 73:1524-1531), and a variety of contraction frequency-dependent alterations in sarcoplasmic [Ca(2+)] dynamics that are normally observed in the laboratory, including rest potentiation, a negative frequency-[Ca(2+)] relationship, and extrasystolic potentiation. Furthermore, under the condition of a simulated Ca(2+) overload, an alternans-like state was produced. In summary, the current model of cardiocyte Ca(2+) dynamics provides an integrated theoretical framework of fundamental cellular Ca(2+) regulatory processes that is sufficient to predict a broad array of observable experimental outcomes.

Effects of hypoxia, simulated ischemia and reoxygenation on the contractile function of human atrial trabeculae

Hypoxia, ischemia and reoxygenation cause contractile dysfunction which will be characterized by the time course of isometric contraction of human atrial trabeculae. Post-rest potentiation (PRP) and postextrastimulatory potentiation (PEP) were elicited to obtain indirect information about the the role of the sarcoplasmic reticulum (SR) in excitation-contraction coupling. As lipid peroxidation could cause SR dysfunction, thiobarbituric acid reactive substances (TBARS) were measured. After 30 min of hypoxia (H) or simulated ischemia (H combined with acidosis-SI), contractile force decreased to 15% and 6%, respectively, of control (p < or = 0.05), whereas the normalized rate of both contraction and relaxation increased. In group H, rapid reoxygenation produced a recovery of contractile force to about 60%. After post-hypoxic reoxygenation the TBARS concentration was increased. In group SI, rapid reoxygenation and a rather gradual correction of acidosis produced complete recovery of contractile force. PRP and PEP were maintained during H and SI. Particularly post-ischemic reoxygenation caused a marked depression of PRP and partly of PEP. Thus, alteration of SR Ca2+ handling occurs predominantly during reoxygenation rather than during H or SI, probably associated with the damaging effect of increased oxygen radicals. The depression of potentiation occurred along with delayed relaxation, temporary increased resting force, mechanical alternans, and spontaneous activity which are further characteristics for SR dysfunction. Thus, for a possibly beneficial effect of low pH during SI combined with its gradual correction during reoxygenation on the recovery of contractile function, developed force should not be the only index.

Preconditioning rabbit cardiomyocytes: role of pH, vacuolar proton ATPase, and apoptosis

Ischemic preconditioning signals through protein kinase C (PKC) to protect against myocardial infarction. This protection is characterized by diminished intracellular acidification. Acidification is also a feature of apoptosis, and several agents act to prevent apoptosis by preventing acidification through activation of ion channels and pumps to promote cytoplasmic alkalinization. We characterized metabolic inhibition, recovery, and preconditioning through a PKC-dependent pathway in cardiomyocytes isolated from adult rabbit hearts. Preconditioning reduced loss of viability assessed by morphology and reduced DNA nicking. Blockade of the vacuolar proton ATPase (VPATPase) prevented the effect of preconditioning to reduce metabolic inhibition-induced acidosis, loss of viability, and DNA nicking. The beneficial effect of Na+/H+ exchange inhibition, which is thought to be effective through reduced intracellular Na+ and Ca++, was also abrogated by VPATPase blockade, suggesting that acidification even in the absence of Na+/H+ exchange may lead to cell death. We conclude that a target of PKC in mediating preconditioning is activation of the VPATPase with resultant attenuation of intracellular acidification during metabolic inhibition. Inhibition of the "death protease," interleukin-1-beta converting enzyme or related enzymes, also protected against the injury that followed metabolic inhibition. This observation, coupled with the detection of DNA nicking in cells subjected to metabolic inhibition, suggests that apoptotic cell death may be preventable in this model of ischemia/reperfusion injury.