Effects of sodium on the calcium paradox in rat hearts

Ca2+ paradox injury mediated through TRPC channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE The Ca(2+) paradox is an important phenomenon associated with Ca(2+) overload-mediated cellular injury in myocardium. The present study was undertaken to elucidate molecular and cellular mechanisms for the development of the Ca(2+) paradox. EXPERIMENTAL APPROACH Fluorescence imaging was performed on fluo-3 loaded quiescent mouse ventricular myocytes using confocal laser scanning microscope. KEY RESULTS The Ca(2+) paradox was readily evoked by restoration of the extracellular Ca(2+) following 10-20 min of nominally Ca(2+)-free superfusion. The Ca(2+) paradox was significantly reduced by blockers of transient receptor potential canonical (TRPC) channels (2-aminoethoxydiphenyl borate, Gd(3+), La(3+)) and anti-TRPC1 antibody. The sarcoplasmic reticulum (SR) Ca(2+) content, assessed by caffeine application, gradually declined during Ca(2+)-free superfusion, which was further accelerated by metabolic inhibition. Block of SR Ca(2+) leak by tetracaine prevented Ca(2+) paradox. The Na(+) /Ca(2+) exchange (NCX) blocker KB-R7943 significantly inhibited Ca(2+) paradox when applied throughout superfusion period, but had little effect when added for a period of 3 min before and during Ca(2+) restoration. The SR Ca(2+) content was better preserved during Ca(2+) depletion by KB-R7943. Immunocytochemistry confirmed the expression of TRPC1, in addition to TRPC3 and TRPC4, in mouse ventricular myocytes. CONCLUSIONS AND IMPLICATIONS These results provide evidence that (i) the Ca(2+) paradox is primarily mediated by Ca(2+) entry through TRPC (probably TRPC1) channels that are presumably activated by SR Ca(2+) depletion; and (ii) reverse mode NCX contributes little to the Ca(2+) paradox, whereas inhibition of NCX during Ca(2+) depletion improves SR Ca(2+) loading, and is associated with reduced incidence of Ca(2+) paradox in mouse ventricular myocytes.

Induction of peritoneal sepsis increases the susceptibility of isolated hearts to a calcium paradox-mediated injury

The present study was designed to test the hypothesis that induction of chronic peritoneal sepsis in rats would produce a more severe calcium paradox-mediated myocardial injury in isolated heart preparation than is seen in normal hearts, and that this would be inhibited by sucrose as in normal hearts. Male Sprague-Dawley rats were made septic using 200 mg of cecal material (obtained from a donor rat) suspended in 5 mL of 5% dextrose in sterile water D5 W/kg. In septic animals, the cecal material was injected in the peritoneum, while sham-septic animals received only D5 W/kg (5 mL/kg). A third group consisting of normal rats (no surgery) group was also included. Hearts were harvested from all three groups and were subjected to a calcium paradox-mediated injury in an isolated heart preparation. Hearts were perfused with Krebs-Henseleit (KH) medium and were allowed to stabilize, followed by a perfusion with Ca2+-free KH for 10 min. After this 10-min Ca2+-free KH perfusion, rats were reperfused with KH medium for 60 min. Ca2+-free KH medium was used in control experiments, while sucrose experiments were conducted with the same medium except that 150 mM sucrose replaced 75 mM NaCl. A marked decrease in ATP and phosphocreatine occurred during Ca2+ reperfusion in all hearts in absence of sucrose. In the presence of the disaccharide, no change in high-energy phosphate (HEP) levels was observed in normal hearts, while lower ATP concentrations were seen in sham and septic hearts. Thus, sucrose did not inhibit cellular injury in sham and septic hearts as it did in normal hearts, and this might be due to a smaller HEP availability. Control studies with normal, sham, and septic hearts exhibited cessation of contractions in the absence of Ca2+, and appearance of large amounts of cytosolic protein in the effluent perfusate during Ca2+ reperfusion. With normal hearts, perfusion with sucrose caused a 96% inhibition of the total creatine kinase (CK) release observed in control experiments. With sham hearts, 32% of CK release was inhibited by sucrose, while 68% of the CK release was attributed to stress associated with surgery performed in the sham-septic group. In septic hearts, only 8% of the CK release was inhibited by sucrose, suggesting that more severe myocardial injury occurs when septic hearts are subjected to a calcium paradox as compared to other groups. It is evident that sucrose can inhibit a small fraction of the CK release from septic hearts during the calcium paradox as compared to the large CK loss associated with sham sepsis. We have concluded that induction of sepsis made the heart more susceptible to a calcium paradox-mediated myocardial injury.

Hexarelin, a growth hormone secretagogue, protects the isolated rat heart from ventricular dysfunction produced by exposure to calcium-free medium

The effect of hexarelin, a potent synthetic growth hormone (GH)-secretagogue, and of human GH were studied on the mechanical and metabolic changes elicited by the calcium-paradox phenomenon in isolated rat hearts submitted to 5 min Ca(2+)-depletion followed by reperfusion with reintegrated Ca(2+)medium. Hexarelin, (80 microg kg(-1)s.c.) administered to normal male young rats for 3 and 7-day, time-dependently antagonized the sudden increase in resting tension of the isolated perfused hearts upon Ca(2+)-repletion. The beneficial effect of hexarelin was particularly evident in the 7-day treatment. In this instance, ventricular contraction peaked at 30 +/- 2 mmHg (controls, 76 +/- 7 mmHg) and the recovery of left ventricular developed pressure (LVDP) was two times higher (P<0.001) than that recorded in controls (LVDP, 29 +/- 2 mmHg). Moreover, the release of creatine kinase into the heart effluent during Ca(2+)-repletion was reduced by 40% (P<0.001) as compared to controls. The protecting activity of hexarelin against the damage induced by calcium-paradox in the heart was apparently divorced from any stimulation of the GH/insulin-like growth factor (IGF) axis, since plasma and heart concentrations of IGF-1 were similar to those measured in control rats. In contrast to hexarelin, administration of GH (400 microg kg(-1) s.c.) for 7 days did not affect the mechanical and metabolic manifestations of calcium-paradox in the perfused rat hearts. Hexarelin (8 microg ml(-1)) perfused for 60 min through the hearts in recirculating conditions did not modify heart contractility and failed to prevent ventricular hypercontractility developed on Ca(2+)-readmission. In conclusion, the mode of action of hexarelin in protecting the rat heart from calcium-paradox events is presently unknown; it would seem, however, that only prolonged exposure to hexarelin makes myocardial cells competent to maintain cytoplasmatic electrolyte balance and to control of Ca(2+)gain, two functions that are impaired during the 'calcium-paradox' phenomenon.

The hamster heart: a paradox in itself

Perfusion of all mammalian heart muscle except hamster with Ca(2+)-free Tyrode and thereafter reperfusion with normal Tyrode causes irreversible damage, the calcium paradox. Our study aims at deciphering the role of creatine kinase, high energy phosphates and Ca(2+)influx in the genesis of myocardial injury in the rat and comparing it with the hamster. Isolated hearts from hamster and rats were perfused in the Langendorff mode at 37 degrees C for 30 min with normal Tyrode, for 15 min with Ca(2+)-free Tyrode and thereafter for 30 min of reperfusion with normal Tyrode. The 'high energy phosphate compound' levels were monitored by(31)P-NMR, creatine kinase (CK) release was measured in the perfusate.(45)Ca influx was estimated in the papillary muscle. We observed that in the rat heart: (a) high energy phosphate levels declined significantly within 1 min of Ca(2+)reperfusion; (b) a massive release of CK occurred upon Ca(2+)reperfusion; (c) there was a significant increase of Ca(2+)influx. In the hamster heart, there was preservation of high energy phosphates, CK release was prevented completely and no rise in(45)Ca influx was observed upon Ca(2+)reperfusion. These results suggest that the hamster heart has a remarkable capacity for Ca(2+)homeostasis which protects the heart from Ca(2+)overload.

An increase in intracellular [Na+] during Ca2+ depletion is not related to Ca2+ paradox damage in rat hearts

Ca2+ paradox damage has been suggested to be determined by Na+ entry during Ca2+ depletion and exchange of Na+ for Ca2+ during Ca2+ repletion. With the use of 23Na nuclear magnetic resonance, we previously observed a Ca2+ paradox without a prior Na+ increase. We have now demonstrated a Na+ increase during Ca2+ and Mg2+ depletion without the occurrence of the Ca2+ paradox during Ca2+ repletion. Isolated rat hearts were perfused for 20 min with a Ca(2+)-free or a Ca(2+)- and Mg(2+)-free (Ca2+/Mg(2+)-free) solution under hypothermic conditions (20 and 25 degrees C). Intracellular Na+ concentration ([Na+]i) increased from 11.9 +/- 1.2 to 26.9 +/- 5.8 mM (P < 0.001) during Ca2+/Mg(2+)-free perfusion at 20 degrees C, whereas no significant change in [Na+]i occurred during 20 min of Ca(2+)-free perfusion at 20 degrees C. In addition, we confirmed that [Na+]i did not change significantly during 20 min of normothermic Ca(2+)-free perfusion. Creatine kinase release during normothermic Ca2+ repletion in the 20 degrees C groups was approximately 10% and in the 25 degrees C groups 75% of the release in the normothermia group. Recovery of rate-pressure product was approximately 50% in the 20 degrees C groups versus 0% in the normothermia group. In conclusion, hypothermic Ca2+/Mg(2+)-free perfusion results in a significant increase of [Na+]i, which does not contribute to the extent of the Ca2+ paradox on normothermic Ca2+ repletion.

Lidocaine does not prevent the calcium paradox in rat hearts: a laser microprobe mass analysis (LAMMA) study

The calcium paradox stands for the cell damage that occurs when isolated hearts are perfused with a Ca(2+)-free solution followed by perfusion with a Ca(2+)-containing solution. Although it is generally accepted that a massive Ca2+ influx during the Ca(2+)-repletion phase is responsible for the cell damage, there is no consensus about what makes the heart susceptible to the calcium paradox during the Ca(2+)-depletion phase. It has been suggested that the extent of the calcium paradox is primarily determined by accumulation of Na+ during Ca2+ depletion and a subsequent accumulation of Ca2+ via reverse Na(+)-Ca2+ exchange during Ca2+ repletion. According to another theory, weakening of intercalated disc junctions during Ca2+ depletion and contracture-mediated disruption of the cell membrane during Ca2+ repletion are responsible for the calcium paradox. In the present study we further investigated the possible role of Na+ in the development of the calcium paradox. During Ca2+ depletion, lidocaine was used to inhibit Na+ entry through the Na+ channels. Isolated rat hearts were perfused with Krebs Henseleit buffer (KH) containing 1.4 mM Ca2+ for 15 min, followed by 10 min of Ca(2+)-free perfusion and 10 min of reperfusion with Ca2+. In the treated group 0.1 mM lidocaine was present throughout the experiment. At the end of each experiment, Ca2+ cytochemistry was performed and the intracellular Ca2+ content was analyzed by laser microprobe mass analysis (LAMMA). The results show that during Ca2+ depletion, the intracellular Ca2+ content did not change significantly. Ca2+ repletion, however, gave rise to a full calcium paradox irrespective of the presence of lidocaine: massive cell damage and Ca2+ accumulation in the mitochondria. The results provide further evidence that intracellular Na+ accumulation during Ca2+ depletion is not involved in the occurrence of the calcium paradox.

Intracellular free Ca2+, Na+, and H+ concentrations in the isolated perfused rat heart during the Ca2+ paradox

Enzyme release from the normothermic perfused rat heart was determined to evaluate myocardial cell damage during the perfusion sequence of the Ca2+ Ca2+ paradox. In addition sarcosolic free Ca2+ and monovalent cation concentrations were measured using ion-selective microelectrodes (ISMEs). A reduction of the extracellular Ca2+ concentration, [Ca2+]e, from 1.0 mmol/l to 0.3 or 0.1 mmol/l, respectively, during the re-perfusion period markedly decreased the rate of enzyme release. Slow Ca2+ channel blockers were less (verapamil) or not at all (nifedipine) effective in providing protection. The sarcosolic free Ca2+ and Na+ concentrations, [Ca2+]i and [Na+]i, were significantly (P < 0.01) elevated during Ca(2+)-free perfusion, [H+]i was not significantly changed, while the membrane potential became continuously more positive. Addition of verapamil to the perfusion medium increased [Na+]i, but did not further increase [Ca2+]i. The critical [Ca2+]i for cell damage was between 12 and 18 mumol/l. It could be demonstrated, that Ca2+ entry during re-perfusion via Na/Ca exchange is thermodynamically unlikely to elevate [Ca2+]i to critical values. It is concluded therefore that the predisposition of the rat heart for the Ca2+ paradox is brought about by membrane leaks, which form during Ca(2+)-free perfusion and that Ca2+ influx into the sarcosol proceeds mainly through these leaks.

Decrease in the transmembrane sodium activity gradient in ferret papillary muscle as a prerequisite to the calcium paradox

Sodium-dependent calcium exchange may be an important mediator of calcium reperfusion damage during the calcium paradox phenomenon. We measured intracellular sodium activity with ion-selective electrodes during a 15-min period of calcium reperfusion in isolated ferret papillary muscles. During the calcium-free period, alpha Nai increased from 9.0 +/- 0.9 to 18.9 +/- 4.3 mM. With reinstitution of calcium there was a significant contracture. The amount of contracture after calcium reinstitution was related to sodium loading during the calcium-free period. We were unable to block sodium entry during the calcium-free period with either nitrendipine, tetrodotoxin, or low concentrations of amiloride. 10(-3) M amiloride or lithium for sodium substitution in the calcium-free period, however, obliterated the increase in alpha Nai activity and the subsequent paradox. These data suggest that sodium loading is a necessary prerequisite for the calcium paradox and that one mechanism of sodium entry is through Na+/Ca2+ exchange. Under these conditions, no increase in the rest force is seen without previous sodium gains, suggesting that sodium-dependent calcium exchange is an important trigger for the calcium reflow, the calcium paradox.