Since the first demonstration of ischemic preconditioning in 1986 by Murray et al., IPC has been considered to be one of the strongest forms of endogenous cardioprotection. IPC has two different phases; an early phase that develops within a few minutes and lasts up 2-4 hours and a late phase that develops 6-12 hours and lasts up to 4 days. Complex intracellular signaling pathways mediate IPC, and ROS generated in the mitochondria act as triggers or mediators. Although several effector candidates have been evaluated, the evidence that MPTPs or mitochondrial K_ATP channels are the effectors is most convincing (Fig. 2). Although simplification of the cascades into a trigger, mediator, and effector can help to clarify the mechanism of IPC, it should be kept in mind that there is crosstalk between the underlying signaling components and that the underlying mechanism is more complex.

The effect of ageing on the ability of preconditioning to protect hearts from ischemia-reperfusion injury has been studied intensively (Table 2). The majority of studies demonstrated loss of cardioprotection by IPC with ageing. However, IPC was preserved in 2- 4-year-old rabbits [38] and 5.7- to 8-year-old sheep [39]. Based on the maximal life span of these species (Table 3), however, it is unlikely that these animals are suitable models for investigating ageing in humans.

When preconditioning stimuli were increased, cardioprotection was achieved in middle-aged rats. However, cardioprotection induced by IPC was lost in the aged rats, irrespective of the strength of the stimuli (Fig. 3). Indirect data using agonists for receptors or channel openers involved in cardioprotection showed that IPC signaling cascades are impaired in the aged myocardium [37].

IPC has been shown to occur naturally in the form of prodromal angina prior to acute myocardial infarction, reducing the infarct size in patients [41,42]. In clinical studies, the measurement of infarct size is not available as an end-point, and surrogate end-points are used. For examples, ST changes on EKG, biologic markers such as creatine kinase (CK), CK-MB, and troponin-T, and clinical outcomes such as recovery of cardiac functions, length of hospital stay, mortality, and morbidity are frequently used. Clinical studies of IPC during revascularization and prodromal angina within a 48 h prior infarction have consistently demonstrated that IPC is not cardioprotective in elderly patients in terms of recovery of cardiac function, biologic markers, or clinical outcomes [43-45]. However, in vitro studies have showed less consistent results. After IPC in elderly patients, biological markers such as CK were reduced, but contractile function did not improve. Apart from age, comorbidities and medications such as angiotensin-converting enzymes [46,47], COX-2 inhibitors, or sulfonylureas [48] complicate analysis of the IPC effect in the elderly.

Anesthetic preconditioning (APC) by isoflurane was found to be effective in middle-aged (10-12 months), but not in aged rat hearts (20-24 months) [49,50]. Similarly, in isolated guinea-pig hearts, APC by sevoflurane was attenuated with ageing. Helium preconditioning-induced cardioprotection was not preserved in aged rat hearts [51], and enkephalin preconditioning did not protect aged mouse hearts (24-26 months) [52]. Interestingly, chronic morphine treatment was shown to be cardioprotective via protein kinase A in the aged heart in contrast to loss of acute morphine treatment in the aged heart [53].

Several components of the aged myocardium, included cardioprotection by APC, may be defective. The increase in ROS due to APC is impaired in the aged myocardium, even though the baseline ROS level is high in the aged myocardium [49]. Coupling between the opioid receptor and p38 MAPK (mitogen-activated protein kinase) was shown to be lost in the aged mouse heart, and phosphorylation of the downstream protein HSP27 (Heat shock protein 27) was reduced [52,53].

Interventions performed at reperfusion to protect the myocardium, such as post-conditioning, may be more clinically relevant because it is difficult to anticipate when an ischemic event will occur. IPC is not cardioprotective in aged mice [23]. However, the cardioprotective effect of IPC depends on the stimuli strength in the middle-aged myocardium [54]. The loss of the IPC effect in the aged myocardium was associated with reduced expression and phosphorylation of STAT3 [54,55]. Furthermore, levels of the phosphatase MKP-1 were increased in the aged myocardium, and inhibition of MKP-1 resulted in the restoration of cardioprotection by ischemic postconditioning [23]. ERK, a target protein of MKP-1, was not activated by ischemic postconditioning in the aged myocardium, but was restored in aged hearts treated by a MKP-1 inhibitor [38]. Therefore, in the aged heart, impaired activation of ERK 1/2 might affect downstream components such as STAT3. I found that isoflurane-induced postconditioning was not cardioprotective in old rats (20-24 months) (data not published). The reperfusion salvage kinase pathway, including Akt, its downstream effector GSK3β, and ERK1/2 was not activated by anesthetic postconditioning, which may be associated with the loss of cardioprotection in the aged rat heart. Therefore, the aged myocardium is not protected by pre- or post-conditioning, which is associated with impaired signal transduction pathways.

Caloric restriction attenuates cellular ageing and promotes longevity [4,56]. Furthermore, caloric restriction renders cardiomyocytes resistant to ischemic injury. In food-restricted old rats, the cardioprotective effect of ischemic precondition was preserved [36]. Recovery of the developed pressure after ischemia-reperfusion and the occurrence of arrhythmia were evaluated in 6- and 24-month-old rats that had been on a caloric restriction diet since weaning [36,57]. The recovery of developed pressure was partially improved and arrhythmia was reduced in the food-restricted old rats. In a study of short-term caloric restriction, 24-month-old rats were fed 90% of their caloric intake ad libitum for 2 weeks followed by 65% of their caloric intake ad libitum for 2 weeks. The left ventricular pressure was then evaluated in isolated perfused hearts. These authors found that short-term caloric restriction improved tolerance to ischemia, but did not restore IPC-induced cardioprotection [57]. Increased tolerance to ischemia by short-term caloric restriction was related to increased phosphorylation of AMP-activated kinase [58]. Increased levels of adiponectin induced by caloric restriction may contribute to the restoration of the IPC effect in aged hearts [58,59].

Exercise contributes to the preservation of endogenous cardioprotection in the aged heart. Elderly rats (24-month-old) were trained using a swimming protocol of 40 min/day, 5 days/week for 6 weeks. Developed pressure in the left ventricle was partially improved by IPC in these trained rats. However, the combination of caloric restriction and exercise restored IPC-based cardioprotection more completely [60]. The mechanisms of exercise-induced cardioprotection or restoration of endogenous cardioprotection are not clear. However, a reduction in mitochondrial ROS formation, increased coronary circulation, increased levels of antioxidants such as manganese superoxide dismutase and catalase, altered expression of mitochondrial and sarcoplasmic K_ATP channels, and altered gene/protein expression/activation of factors such as HSP and IGF-1 may contribute to exercise-induced cardioprotection [62].

The beneficial effects of exercise on IPC-induced cardioprotection have been demonstrated in elderly patients. High levels of physical activity preserve the cardioprotective effect of prodromal angina in elderly patients [62].