STONE Heart: Mechanisms of Injury

As of yet, only a few strategies to prevent myocardial reperfusion injury have been tested clinically. In the first minutes of reperfusion, the myocardium can be damaged by contracture development, causing mechanical stiffness, tissue necrosis, and the “stone heart” phenomenon. Reperfusion-induced contracture can have two different causes, namely, Ca²⁺overload–induced contracture or rigor-type contracture. Ca²⁺ contracture results from rapid re-energization of contractile cells with a persistent Ca²⁺ overload. Strategies to prevent this type of injury are directed at cytosolic Ca²⁺ control or myofibrillar Ca²⁺ sensitivity. Rigor-contracture occurs when re-energization proceeds very slowly. It does not depend on Ca²⁺ overload. It may be prevented by strategies improving early mitochondrial reactivation

Contracture (ie, a sustained shortening and stiffening of myocardium) can have several causes. In ischemic myocardium contracture develops by means of a rigor-type mechanism. Studies on skinned cardiac cells or muscle fibers have shown that a force-generating cross bridge cycling is initiated when cytosolic adenosine triphosphate (ATP) is reduced to a low (<100 μmol/L) but nonzero level [1, 2]. In ischemia, this window of low cytosolic ATP concentrations is open only during a brief period, because cellular ATP reserves are quickly exhausted. The myofibrillar shortening then stays fixed, as all cross bridges between actin and myosin remain in an attached state. The contracture developed by this ischemic mechanism does not actually cause major structural damage but leads to cytoskeletal defects. These defects render cardiomyocytes more fragile and thus susceptible to mechanical damage [3]. When energy depletion is rapidly relieved, ischemic rigor contracture is usually reversible.

After prolonged ischemia myocardial cells may develop severe contracture, which can lead to cytoskeletal defects that consequently increase the fragility of cardiomyocytes upon reperfusion. As a consequence, end-diastolic ventricular pressure increases and ventricular compliance decreases. Substantial contracture is accompanied by a specific form of tissue necrosis, the so-called contraction band necrosis [4]. The histologic picture is characterized by coexistence of supercontracted sarcomeres, overextension of spaces in between, and sarcolemmal disruptions, all in the same cells. This picture results from strong and inhomogeneous mechanical forces. In a number of studies we have shown that pathogenesis of reperfusion-induced contracture can be analyzed on the cellular level. This analysis revealed two independent causes of reperfusion-induced contracture: (1) Ca²⁺overload–induced contracture, and (2) rigor contracture. Ca²⁺ overload–induced contracture is elicited in a cardiomyocyte, if it develops Ca²⁺ overload during ischemia and is then rapidly reenergized. High cytosolic Ca²⁺plus energy leads to uncontrolled activation of the contractile machinery. Rigor-contracture may be activated during reoxygenation, if reenergization of the ischemic cardiomyocytes occurs at a very low rate. It may, therefore, be observed after prolonged or severe ischemia. Rigor-contracture is not essentially dependent on Ca²⁺ overload. These two causal mechanisms for reperfusion-induced contracture are described separately below.

Ischemic cells become energy depleted and subsequently develop a Ca²⁺ overload of the cytosol due to a reverse-mode operation of the sarcolemmal Na⁺/Ca²⁺ exchanger (Fig 1). If the ability of mitochondria to resume ATP synthesis is not critically impaired during the ischemic period, reoxygenation leads to a rapid recovery of energy production. Resynthesis of ATP can enable cardiomyocytes to recover from the loss of cytosolic cation balance, but it also reactivates the contractile machinery that had been fixed in rigor contracture after ischemic loss of ATP. The latter effect is normally faster then the former, which leads to an uncontrolled Ca²⁺-dependent contraction. When analyzed in detail, it was found that cyclic uptake and release of Ca²⁺ by the sarcoplasmic reticulum (SR) in the reoxygenated cardiomyocytes triggers a Ca²⁺ overload–induced contracture [5, 6] (Fig 2). These oscillatory Ca²⁺ shifts lead to high cytosolic peak Ca²⁺ concentrations (Fig 3). The frequency of these Ca²⁺peaks is influenced by an ongoing Ca²⁺ influx across the sarcolemma during the early phase of reoxygenation [6]. During this period the transsarcolemmal Na⁺ gradient is still reduced and the Na⁺/Ca²⁺exchanger still operates in reverse mode. Experimentally, various protocols have been shown to interfere with Ca²⁺ overload–induced contracture: First, contracture can be prevented by an initial, time-limited inhibition of the contractile machinery. For this purpose, the chemical phosphatase 2,3 butane dione monoxime has been used [7, 8]. Part of the protective effects of cGMP-mediated effectors (NO, atrial natriuretic peptides) or cytosolic acidosis can also be attributed to contractile inhibition, as these agents reduce Ca²⁺ sensitivity of myofibrils. Second, contracture can be reduced by reducing SR-dependent Ca²⁺ oscillations. This can either be achieved by agents interfering with SR–Ca²⁺ sequestration or by inhibition of the Ca²⁺ influx into the cells still occurring during the early phase of reoxygenation. Ca²⁺ cycling across the SR can be inhibited by specific agents interfering with SR Ca²⁺ ATPase or SR Ca²⁺ release [5] or with less specific means such as the anesthetic halothane or intracellular acidosis [9, 10].

As long as mitochondrial energy production recovers rapidly upon reperfusion/reoxygenation, reoxygenated cardiomyocytes are in acute jeopardy by Ca²⁺ overload–induced contracture. After prolonged ischemia the ability of mitochondria to rapidly restore a normal cellular state of energy is reduced. However, during the early phase of reoxygenation cardiomyocytes may then contain very low (even though rising) concentrations of ATP which provoke rigor contracture (see above) (Fig 4). In comparison to ischemia, upon reoxygenation cardiomyocytes may spend much more time at the window of low cytosolic ATP suitable to induce rigor-type contracture. Therefore, cell shortening can be much more pronounced than observed in ischemic rigor contracture. In fact, the rigor mechanism may become the major contributor to reoxygenation-induced contracture (Piper HM, unpublished data). In the event that rigor contracture prevails in acute reperfusion injury, therapeutic actions aiming at cytosolic Ca²⁺ overload are not effective, inasmuch as rigor contracture is essentially Ca²⁺ independent (Fig 5). It can be shown experimentally that one can reduce rigor contracture by improving the conditions for energy recovery. A first approach is application of mitochondrial energy substrates, eg, succinate, with the aim of accelerating oxidative energy production. Second, one may speculate about means to protect mitochondria and resume respiratory activity in the early phase of reperfusion from compulsory calcium uptake.