Protection against cellular damage in the perfused rat heart by lowered pH

Neurologic Preservation by Na+-H+ Exchange Inhibition Prior to 90 Minutes of Hypothermic Circulatory Arrest

BACKGROUND

The effects of pretreatment with cariporide (HOE 642 Aventis Pharma, Strasbourg-Cedex, France), a Na+-H+ exchanger (NHE) blocker, were studied in a cerebral ischemia-reperfusion model of hypothermic arrest.

METHODS

Fifteen Yorkshire-Duroc pigs (37.1 +/- 4.2 kg) underwent femoral-jugular bypass and 90 minutes of deep hypothermic circulatory arrest at 19 degrees C. Ten animals were untreated, whereas 5 received 5 mg/kg of intravenous cariporide before cooling. After rewarming and off cardiopulmonary bypass, the pigs were weaned from anesthesia and followed for 24 hours. A standardized neurologic scoring system assessed brain functional recovery. Biochemical markers were used to analyze cellular injury. Control studies without circulatory arrest were done in 2 animals that underwent similar cooling and rewarming.

RESULTS

Neurologic recovery was rapid and complete in the nonischemic controls and in all pretreated animals. Conversely, at 24 hours, all untreated pigs exhibited a cloudy or stuporous level of consciousness, abnormal positioning, and with only one exception, could not sit or stand. The gradation of neurologic score (evaluating central nervous system, motor and sensory functions, respiration condition, level of consciousness, and behavior) was 0 +/- 0 (0 = normal, 500 = brain death) in the treated group, compared with 124 +/- 59 in the untreated animals. Biochemical analysis showed every variable of whole-body injury (including conjugated dienes (p < 0.05), serum aspartate amino transferase (p < 0.01), creatine kinase p < 0.001) and endothelin-1 (p < 0.001) to be higher in the untreated group.

CONCLUSIONS

NHE function alters experimental brain ischemia-reperfusion damage. These observations imply that NHE inhibition therapy before ischemia may improve neurologic protection in adult and infant patients undergoing cerebral ischemia during procedures that use hypothermic circulatory arrest.

Specific electromechanical responses of cardiomyocytes to individual and combined components of ischemia

The main factors of myocardial ischemia are hypoxia, substrate deprivation, acidosis, and high extracellular potassium concentration ([K+]e), but the influence of each of these factors has not yet been evaluated in a cardiomyocyte (CM) culture system. Electromechanical responses to the individual and combined components of ischemia were studied in CM cultured from newborn rat ventricles. Action potentials (APs) were recorded using glass microelectrodes and contractions were monitored photometrically. Glucose-free hypoxia initially reduced AP duration, amplitude, and rate and altered excitation-contraction coupling, but AP upstroke velocity (Vmax) remained unaffected. Early afterdepolarizations appeared, leading to bursts of high-rate triggered impulses before the complete arrest of electromechanical activity after 120 min. Acidosis reduced Vmax whereas AP amplitude and rate were moderately decreased. Combining acidosis and substrate-free hypoxia also decreased Vmax but attenuated the effects of substrate-free hypoxia on APs and delayed the cessation of the electrical activity (180 min). Raising [K+]e reduced the maximal diastolic potential and Vmax. Total ischemia (substrate deletion, hypoxia, acidosis, and high [K+]e) decreased AP amplitude and Vmax without changing AP duration. Moreover, delayed afterdepolarizations appeared, initiating triggered activity. Ultimately, 120 min of total ischemia blocked APs and contractions. To conclude, glucose-free hypoxia caused severe functional defects, acidosis delayed the changes induced by substrate-free hypoxia, and total ischemia induced specific dysfunctions differing from those caused by the former conditions. Heart-cell cultures thus represent a valuable tool to scrutinize the individual and combined components of ischemia on CMs.

Protection against cellular damage in the rat heart by iodoacetate

In this comparative study, rat hearts were perfused at 37 degrees C with three clearly defined protocols: the Ca2+ paradox, the O2 paradox, and with 20 mM caffeine. Each protocol involved an initial priming (Ca2+o depletion or anoxia; stage 1) and subsequent full activation (Ca2+o repletion, caffeine or reoxygenation; stage 2) of the damage system of the sarcolemma. Iodoacetate (1 mM) provided complete protection in the O2 paradox and over 80% protection in the Ca2+ paradox and caffeine protocols against creatine kinase release when present throughout the experiment (P < 0.001). Almost identical protection was found when iodoacetate was present only in stage 2 (P < 0.001). However, it was concluded that iodoacetate had limited protective effects when present only in stage 1 in any of the three protocols and that its action is to inhibit the activity of the transsarcolemma damage system in stage 2 when it has been activated in stage 1. It is suggested that iodoacetate interacts with thiol groups on the damage system of the sarcolemma.

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.

Protection against cellular damage in the rat heart by hyperosmotic solutions

In this comparative study, rat hearts were perfused at 37 degrees C with three clearly defined protocols: the Ca2+ paradox, the O(2) paradox, and with 20 mM caffeine. Each protocol involved an initial priming (Ca(2+)(0) depletion or anoxia; stage 1) and subsequent full activation (Ca(2+)(0) repletion, caffeine or reoxygenation; stage 2) of the damage system of the sarcolemma. Creatine kinase release in stage 2 was completely inhibited (P < 0.001) in all three protocols when 420 mOsm was added to the perfusion medium throughout the experiments, or only during stage 1, or only during stage 2. Increasing the perfusion pressure in the Ca2+ paradox significantly (P < 0.001) exacerbated creatine kinase release, although this was still completely inhibited at 28 degrees C. Amiloride (1 mM) inhibited creatine kinase release completely at 40 cm of water pressure but only some 50% at 80 cm of water pressure. It is suggested that the transmembrane damage system needs to be uncoupled or deactivated by modifying its relationship with the cytosol or with the underlying cytoskeleton by hyperosmotic cell shrinkage for only one of the stages in all three protocols to block the damage pathway. Increased perfusion pressure has the opposite effect and exacerbates damage.

Monovalent cation and L-type Ca2+ channels participate in calcium paradox-like phenomenon in rabbit aortic smooth muscle cells

1. The effects of removal of extracellular divalent cations (experimental calcium paradox conditions) were studied on the whole-cell current in freshly isolated smooth muscle cells (SMCs), and on contraction in rabbit aortic rings. 2. Aortic rings treated for 30-60 min with extracellular Ca2+- and Mg2+-free solution contracted following readmission of extracellular Ca2+, even in the presence of nifedipine. 3. In isolated SMCs, the removal of extracellular Ca2+ and Mg2+ induced a non-inactivating whole-cell inward current and membrane depolarization. This current was a monovalent cation (MC) current which reversed at around 0 mV and conducted K+ >= Cs+ > Na+ > Li+. Extracellular divalent cations (Ca2+, Mg2+, Ba2+, Mn2+ and Ni2+) inhibited MC current. 4. Using noise analysis of the whole-cell MC current, the single MC channel conductance was estimated to be < 450 fS. 5. MC current was insensitive to nifedipine, TEA, 4-aminopyridine, SK&F 96365 and S-nitroso-N-acetyl-penicillamine (SNAP), but was decreased by amiloride and low pH. 6. When EGTA was present in Ca2+- and Mg2+-free solution, a significant nifedipine-sensitive Na+ current through L-type Ca2+ channels developed in addition to MC current. 7. It is concluded that upon the removal of extracellular Ca2+ and Mg2+ from resting SMCs, an inward MC current develops allowing Na+ influx and causing SMC depolarization which could be the important steps leading to vessel contraction upon Ca2+ readmission. Addition of EGTA to Ca2+- and Mg2+-free solution greatly potentiates Na+ influx and vessel contraction by allowing additional Na+ influx through L-type Ca2+ channels which are activated presumably by MC current-induced depolarization.