Effects of anoxia on intracellular Ca2+ and contraction in isolated guinea pig cardiac myocytes

Dexmedetomidine Protects Cardiomyocytes against Hypoxia/Reoxygenation Injury by Suppressing TLR4-MyD88-NF-κB Signaling

OBJECTIVE

We previously reported that dexmedetomidine (DEX) offers cardioprotection against ischemia/reperfusion injury in rats. Here, we evaluated the role of toll-like receptors 4- (TLR4-) myeloid differentiation primary response 88- (MyD88-) nuclear factor-kappa B (NF-κB) signaling in DEX-mediated protection of cardiomyocytes using in vitro models of hypoxia/reoxygenation (H/R).

METHODS

The experiments were carried out in H9C2 cells and in primary neonatal rat cardiomyocytes. Cells pretreated with vehicle or DEX were exposed to hypoxia for 1 h followed by reoxygenation for 12 h. We analyzed cell viability and lactate dehydrogenase (LDH) activity and measured tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-1β mRNA levels, TLR4, MyD88, and nuclear NF-κB p65 protein expression and NF-κB p65 nuclear localization. TLR4 knock-down by TLR4 siRNA transfection and overexpression by TLR4 DNA transfection were used to further confirm our findings.

RESULTS

DEX protected against H/R-induced cell damage and inflammation, as evidenced by increased cell survival rates, decreased LDH activity, and decreased TNF-α, IL-6, and IL-1β mRNA levels, as well as TLR4 and NF-κB protein expression. TLR4 knock-down partially prevented cell damage following H/R injury, while overexpression of TLR4 abolished the DEX-mediated protective effects.

CONCLUSIONS

DEX pretreatment protects rat cardiomyocytes against H/R injury. This effect is partly mediated by TLR4 suppression via TLR4-MyD88-NF-κB signaling.

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.

Effect of metabolic inhibition on couplon behavior in rabbit ventricular myocytes

We investigated the effect of combined inhibition of oxidative and glycolytic metabolism on L-type Ca(2+) channels (LCCs) and Ca(2+) spikes in isolated patch-clamped rabbit ventricular myocytes. Metabolic inhibition (MI) reduced LCC open probability, increased null probability, increased first latency, and decreased open time but left conductance unchanged. These results explain the reduction in macroscopic Ca(2+) current observed during MI. MI also produced a gradual reduction in action potential duration at 90% repolarization (APD(90)), a clear decline in spike probability, and an increase in spike latency and variance. These effects are consistent with the changes we observed in LCC activity. MI had no effect on the amplitude or time to peak of Ca(2+) spikes until APD(90) reached 10% of control, suggesting preserved sarcoplasmic reticulum Ca(2+) stores and ryanodine receptor (RyR) conductance in those couplons that remained functioning. Ca(2+) spikes disappeared completely when APD(90) reached <2% of control, although in two cells, spikes were reactivated in a highly synchronized fashion by very short action potentials. This reactivation is probably due to the increased driving force for Ca(2+) entry through a reduced number of LCCs that remain open during early repolarization. The enlarged single channel flux produced by rapid repolarization is apparently sufficient to trigger RyRs whose Ca(2+) sensitivity is likely reduced by MI. We suggest that loss of coupling fidelity during MI is explained by loss of LCC activity (possibly mediated by Ca(2+)-calmodulin kinase II activity). In addition, the results are consistent with loss of RyR activity, which can be mitigated under conditions likely to enlarge the trigger.

Chronic intermittent hypoxia alters Ca2+ handling in rat cardiomyocytes by augmented Na+/Ca2+ exchange and ryanodine receptor activities in ischemia-reperfusion

This study examined Ca(2+) handling mechanisms involved in cardioprotection induced by chronic intermittent hypoxia (CIH) against ischemia-reperfusion (I/R) injury. Adult male Sprague-Dawley rats were exposed to 10% inspired O(2) continuously for 6 h daily from 3, 7, and 14 days. In isolated perfused hearts subjected to I/R, CIH-induced cardioprotection was most significant in the 7-day group with less infarct size and lactate dehydrogenase release, compared with the normoxic group. The I/R-induced alterations in diastolic Ca(2+) level, amplitude, time-to-peak, and the decay time of both electrically and caffeine-induced Ca(2+) transients measured by spectrofluorometry in isolated ventricular myocytes of the 7-day CIH group were less than that of the normoxic group, suggesting an involvement of altered Ca(2+) handling of the sarcoplasmic reticulum (SR) and sarcolemma. We further determined the protein expression and activity of (45)Ca(2+) flux of SR-Ca(2+)-ATPase, ryanodine receptor (RyR) and sarcolemmal Na(+)/Ca(2+) exchange (NCX) in ventricular myocytes from the CIH and normoxic groups before and during I/R. There were no changes in expression levels of the Ca(2+)-handling proteins but significant increases in the RyR and NCX activities were remarkable during I/R in the CIH but not the normoxic group. The augmented RyR and NCX activities were abolished, respectively, by PKA inhibitor (0.5 microM KT5720 or 0.5 microM PKI(14-22)) and PKC inhibitor (5 microM chelerythrine chloride or 0.2 microM calphostin C) but not by Ca(2+)/calmodulin-dependent protein kinase II inhibitor KN-93 (1 microM). Thus, CIH confers cardioprotection against I/R injury in rat cardiomyocytes by altered Ca(2+) handling with augmented RyR and NCX activities via protein kinase activation.

Negative inotropic drugs alter indexes of cytosolic [Ca2+]-left ventricular pressure relationships after ischemia

Negative inotropic agents may differentially modulate indexes of cytosolic [Ca(2+)]-left ventricular (LV) pressure (LVP) relationships when given before and after ischemia. We measured and calculated [Ca(2+)], LVP, velocity ratios [[(d[Ca(2+)]/dt(max))/(dLVP/dt(max)); VR(max)] and [(d[Ca(2+)]/dt(min))/(dLVP/dt(min)); VR(min)]], and area ratio (AR; area [Ca(2+)]/area LVP per beat) before and after global ischemia in guinea pig isolated hearts. Ca(2+) transients were recorded by indo 1-AM fluorescence via a fiberoptic probe placed at the LV free wall. [Ca(2+)]-LVP loops were acquired by plotting LVP as a function of [Ca(2+)] at multiple time points during the cardiac cycle. Hearts were perfused with bimakalim, 2,3-butanedione monoxime (BDM), nifedipine, or lidocaine before and after 30 min of ischemia. Before ischemia, each drug depressed LVP, but only nifedipine decreased both LVP and [Ca(2+)] with a downward and leftward shift of the [Ca(2+)]-LVP loop. After ischemia, each drug depressed LVP and [Ca(2+)] with a downward and leftward shift of the [Ca(2+)]-LVP loop. Each drug except BDM decreased d[Ca(2+)]/dt(max); nifedipine decreased d[Ca(2+)]/dt(min), whereas lidocaine increased it, and bimakalim and BDM had no effect on d[Ca(2+)]/dt(min). Each drug except bimakalim increased VR(max) and VR(min) before ischemia; after ischemia, only BDM and nifedipine increased VR(max) and VR(min). Before and after ischemia, BDM and nifedipine increased AR, whereas lidocaine and bimakalim had no effect. At 30 min of reperfusion, control hearts exhibited marked Ca(2+) overload and depressed LVP. In each drug-pretreated group Ca(2+) overload was reduced on reperfusion, but only the group pretreated with nifedipine exhibited both higher LVP and lower [Ca(2+)]. These results show that negative inotropic drugs are less capable of reducing [Ca(2+)] after ischemia so that there is a relatively larger Ca(2+) expenditure for contraction/relaxation after ischemia than before ischemia. Moreover, the differential effects of pretreatment with negative inotropic drugs on [Ca(2+)]-LVP relationships after ischemia suggest that these drugs, especially nifedipine, can elicit cardiac preconditioning.

How Inotropic Drugs Alter Dynamic and Static Indices of Cyclic Myoplasmic [Ca2+] to Contractility Relationships in Intact Hearts

The authors examined effects of positive (dopamine and digoxin) and negative (nifedipine and lidocaine) inotropic interventions on the instantaneous cyclic relationship between myoplasmic [Ca2+] and simultaneously developed left ventricular pressure (LVP) in intact guinea pig hearts. Novel indices were developed to quantify this relationship based on (1) transient [Ca2+] and LVP signal morphology, ie, maxima and minima, peak derivatives, beat areas, durations, and ratios of indices of LVP to [Ca2+]; (2) temporal delay; and (3) LVP versus [Ca2+] loop morphology, ie, orientation, size, hysteresis, position, shape, and duration. These analyses were used to assess the cost of phasic [Ca2+] for contraction and relaxation over one beat after inotropic intervention. It was found that dopamine and digoxin increased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike dopamine, digoxin did not decrease relaxation response time. Nifedipine and lidocaine decreased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike lidocaine, nifedipine decreased net available Ca2+ and Ca2+ influx. Positive inotropic agents increased [Ca2+]-LVP loop area and hysteresis and resulted in a more vertically oriented loop. Nifedipine and lidocaine decreased these loop indices and lidocaine exhibited greater loop hysteresis than did nifedipine. These novel indices provide a quantitative assessment of myoplasmic [Ca2+] handling for cardiac contractile function.

Early effects of metabolic inhibition on intracellular Ca2+ in toad pacemaker cells: involvement of Ca2+ stores

The early effects of metabolic inhibition on intracellular Ca(2+) concentration ([Ca(2+)](i)), Ca(2+) current, and sarcoplasmic reticulum (SR) Ca(2+) content were studied in single pacemaker cells from the sinus venosus of the cane toad. The amplitude of the spontaneous elevations of systolic [Ca(2+)](i) (Ca(2+) transients) was reduced after 5-min exposure to 2 mM NaCN from 338 +/- 30 to 189 +/- 37 nM (P < 0.005, n = 9), and the spontaneous firing rate was reduced from 27 +/- 2 to 12 +/- 4 beats/min (P < 0.002, n = 9). It has been proposed that CN(-) acts by inhibition of cytochrome P-450, resulting in a reduction of cAMP and Ca(2+) current. To test this proposal, we used clotrimazole, a cytochrome P-450 inhibitor, which also decreased the Ca(2+) transients and firing rate. CN(-) caused an insignificant fall of Ca(2+) current (23 +/- 11%) but a substantial reduction of SR Ca(2+) content (by 65 +/- 5%), whereas clotrimazole produced a larger reduction of Ca(2+) current and did not affect the SR Ca(2+) content. Thus the main effect of CN(-) does not seem to be through inhibition of cytochrome P-450. In conclusion, CN(-) appears to reduce Ca(2+) release from the SR mainly by reducing SR Ca(2+) content. A likely cause of the decreased SR content is reduced Ca(2+) uptake by the SR pump.

Effects of pharmacological preconditioning with U50488H on calcium homeostasis in rat ventricular myocytes subjected to metabolic inhibition and anoxia

1. The effects of pharmacological preconditioning with U50488H (U(50)), a selective kappa-opioid receptor agonist, on Ca(2+) homeostasis in rat ventricular myocytes subjected for 9 min to metabolic inhibition (MI) and anoxia (A), consequences of ischaemia, were studied and compared with those of preconditioning with brief periods of MI/A. 2. Precondition with 30 micro M of U(50) for three cycles of 1 min each cycle separated by 3 min of recovery (UP) significantly increased the percentage of non-blue cells following MI/A. The effect of UP is the same as that of preconditioning with an inhibitor of glycolysis and an oxygen scavenger for three 1-min cycles separated by three-minute recovery (MI/AP). The results indicate that like MI/AP, UP also confers cardioprotection. 3. MI/A increased intracellular Ca(2+) ([Ca(2+)](i)) and reduced the amplitude of caffeine-induced [Ca(2+)](i) transients, an indication of Ca(2+) content in the sarcoplasmic reticulum (SR). MI/A also reduced the electrically-induced [Ca(2+)](i) transient, that indicates Ca(2+)-release during excitation-contraction coupling, and Ca(2+) sparks in unstimulated myocytes, that indicates spontaneous Ca(2+)-release from SR. It also prolonged the decline of the electrically-induced [Ca(2+)](i) transient and slowed down the recovery of the electrically-induced [Ca(2+)](i) transient after administration of caffeine. In addition, MI/A prolonged the decline of caffeine induced [Ca(2+)](i) transient, an indication of Na(+)-Ca(2+) exchange activity, and UP prevented it. So UP, that confers cardioprotection, prevented the changes induced by MI/A. With the exception of Ca(2+)-spark, which was not studied, the effects of MI/AP are the same as those of UP. 4. It is concluded that pharmacological preconditioning with U(50), that confers immediate cardioprotection, prevents changes of Ca(2+) homeostasis altered by MI/A in the rat heart. This may be responsible, at least partly, for the cardioprotective action. 5. The study also provided evidence that MI/A causes mobilization of Ca(2+) from SR to cytoplasm causing Ca(2+)-overload which may be due to reduced Ca(2+)-uptake by SR. MI/A also reduces spontaneous and electrically induced Ca(2+) release from SR.

Effects of sustained low-flow ischemia and reperfusion on Ca2+ transients and contractility in perfused rat hearts

We investigated changes in cytoplasmic Ca2+ concentration ([Ca2+]i) and in left ventricular contractility during sustained ischemia and reperfusion in isolated beating rat hearts. Hearts from male Sprague-Dawley rats were perfused retrogradely and were loaded with 4 microM fura-2. Low-flow global ischemia was induced by reducing perfusion flow to 10% and by electric pacing. The hearts were exposed to ischemia for 10 min or 30 min and then reperfused. [Ca2+]i was measured by monitoring the ratio of 500 nm fluorescence excited at 340 and 380 nm while simultaneously measuring left ventricular pressure (LVP). To determine diastolic [Ca2+]i, background autofluorescence was subtracted. LVP rapidly decreased from 82.3+/-8.2 to 17.1+/-2.9 mmHg , whereas the amplitude of the Ca2+ transient did not change significantly during the first 1 min of ischemia. After 10 min of ischemia, the amplitude decreased to 60.8+/-10.6% (p < 0.05) and diastolic [Ca2+]i increased by 26.3+/-2.9% (p < 0.001) compared with the pre-ischemic value (n = 8). When the hearts were reperfused after 10 min of ischemia, the amplitude of the Ca2+ transient and LVP recovered to 79.0+/-7.2% and 73.2+/-7.5 mmHg, respectively. Whereas diastolic [Ca2+]i decreased to the preischemic value. In the hearts exposed to 30 min of ischemia (n = 10), diastolic [Ca2+]i increased even further by 32.7+/-5.3% at the end of ischemia and continued increasing during the 10 min of reperfusion by 42.6+/-15.6%. Six of 10 hearts developed ventricular fibrillation (VF) and intracellular Ca2+ overload after reperfusion. Recovery of LVP after reperfusion was significantly smaller in the hearts exposed to 30 min of ischemia than in the hearts exposed to 10 min of ischemia (58.9+/-11.7 vs. 97.2+/-3.0% of pre-ischemic value, p < 0.05). Diastolic [Ca2+]i also increased under hypoxic conditions (N2 bubbling) in this model. These results suggest that increases in diastolic [Ca2+]i might play an important role in myocardial contractile dysfunction and viability in ischemia-reperfusion injury.