Jochen Rose

Calcium responsiveness in regional myocardial short-term hibernation and stunning in the in situ porcine heart : Inotropic responses to postextrasystolic potentiation and intracoronary calcium

BACKGROUND We tested the hypothesis that decreased calcium responsiveness is responsible for the reduction in contractile function in regional hibernating and stunned myocardium in situ. METHODS AND RESULTS In 19 anesthetized swine, the left anterior descending coronary artery flow was reduced to decrease anterior myocardial work index (sonomicrometry) by approximately 60%. During 90 minutes of hypoperfusion, creatine phosphate recovered (as determined by biopsy specimens and bioluminescence) and no necrosis developed (as determined by staining with triphenyl tetrazolium chloride). In 10 swine, changes in the intracellular calcium concentration were induced by systematic variation of the postextrasystolic time interval at a constant prematurity. In 9 additional swine, a graded IC calcium infusion was performed. Under control conditions, anterior myocardial work increased with a fully compensated postextrasystolic time interval from 380+/-93 (mean+/-SD) to 523+/-98 mm Hg . mm. IC calcium infusion increased anterior myocardial work under control conditions from 356+/-85 to a maximum of 428+/-93 mm Hg . mm. Although the maximal responses were decreased during postextrasystolic potentiation (222+/-68 versus 523+/-98 mm Hg . mm) and calcium infusion (176+/-32 versus 428+/-93 mm Hg . mm) after 90 minutes of ischemia, the relationships between increases in anterior myocardial work and, respectively, postextrasystolic time interval and IC calcium were not different. The same was true after 30 minutes of reperfusion. CONCLUSIONS Both regional hibernating myocardium and stunned myocardium in situ are characterized by a decrease in overall myocardial calcium responsiveness; however, there appears to be no significant myocardial desensitization to calcium.

Involvement of endogenous adenosine in ischaemic preconditioning in swine

Adenosine release and the subsequent activation of adenosine receptors are involved in ischaemic preconditioning in dogs and rabbits. In the present study, we investigated whether adenosine also mediates ischaemic preconditioning in swine. Swine were used since, due to the lack of an innate collateral circulation, infarct development in this species most closely resembles that observed in humans. In 36 enflurane-anaesthetized swine the impact of increased adenosine breakdown with exogenous porcine adenosine deaminase (5 IU/ml blood/min) on global and regional myocardial function (sonomicrometry), subendocardial blood flow (ENDO, microspheres) and infarct size (IS, triphenyl tetrazolium chloride staining following 90 min ischaemia and 120 min reperfusion) were analysed. Low-flow ischaemia for 90 min at an ENDO of 0.09±0.04 (mean±SD) ml/min/g caused an IS of 13.2±9.7% (n=8) of the area at risk. Ischaemic preconditioning by a cycle of 10 min low-flow ischaemia followed by 15 min reperfusion prior to the 90-min ischaemic period (ENDO=0.06±0.03 ml/min/g) reduced IS to 2.6±3.0% (n=11, P<0.05). The interstitial adenosine concentration (microdialysis) increased from 1.60±0.87 nmol/ml to above 10 μM during ischaemia; with intracoronary adenosine deaminase, the interstitial adenosine concentration fell from 1.65±0.23 to 0.12±0.07 nmol/ml and did not increase during ischaemia. Adenosine deaminase per se did not alter IS after 90 min ischaemia (n=7, ENDO=0.08±0.04 ml/min/g, IS=12.1±6.9%) but abolished the beneficial effect of ischaemic preconditioning (n=10, ENDO=0.06±0.03 ml/min/g, IS=8.8±5.8%). For any given ENDO, IS was significantly reduced in the ischaemic preconditioned group compared with the other three groups. Global and regional myocardial function were comparable among all groups of swine. We conclude that endogenous adenosine mediates ischaemic preconditioning also in swine.

Inorganic phosphate content and free energy change of ATP hydrolysis in regional short-term hibernating myocardium

OBJECTIVE Short-term myocardial hibernation is characterized by an adaptation of contractile function to the reduced blood flow, the recovery of creatine phosphate content and lactate balance back towards normal, whereas ATP content remains reduced at a constant level. We examined the hypothesis that, despite the absence of ATP recovery, the short-term hibernating myocardium regains an energetic balance. METHODS An enzymatic method was modified for the measurement of inorganic phosphate (Pi) in transmural myocardial drill biopsies (about 5 mg). In 12 anaesthetized swine, moderate ischemia was induced by reduction of coronary inflow into the cannulated left anterior descending coronary artery to decrease regional myocardial function (sonomicrometry) by 50%. RESULTS The development of short-term hibernation was verified by the recovery of creatine phosphate content, the persistence of inotropic reserve in response to dobutamine and the absence of necrosis (triphenyl tetrazolium chloride). At 5-min ischemia, Pi was increased from 3.6 +/- 0.3 (SD) to 8.1 +/- 1.1 mumol/gwet wt (p < 0.05). The free energy of ATP hydrolysis (delta GATP) was decreased from -57.8 +/- 0.8 to -52.2 +/- 1.4 kJ/mol (p < 0.05). The relationships between function and Pi (r = -0.81) and delta GATP (r = -0.83), respectively, during control and at 5-min ischemia became invalid at 90-min ischemia, as myocardial blood flow and function remained reduced at a constant level, but Pi decreased back to 4.9 +/- 0.9 mumol/g (p < 0.05 vs. control and 5-min ischemia), and delta GATP fully recovered back to -57.2 +/- 1.3 kJ/mol (p < 0.05 vs. 5-min ischemia). CONCLUSIONS In short-term hibernating myocardium, myocardial inorganic phosphate content recovers partially and the free energy change of ATP hydrolysis returns to control values. Contractile function remains reduced by mechanisms other than an energetic deficit.

Cardioprotection by ACE inhibitors in myocardial ischaemia/reperfusion : The importance of bradykinin

SummaryMyocardial ischaemia, when severe and sustained for more than 40 minutes, results in irreversible damage, i.e. myocardial infarction. However, with early reperfusion, damage is reversible. Complete recovery of contractile function requires some time, despite fully or almost fully restored blood flow. This phenomenon has been termed myocardial stunning. There is experimental evidence showing that angiotensin converting enzyme (ACE) inhibitors limit the development of infarct size, reduce the incidence of ischaemic and reperfusion arrhythmias, and enhance the recovery of contractile function of stunned myocardium. These cardioprotective effects of ACE inhibitors are mediated by an attenuated degradation of bradykinin.

Attenuation of regional myocardial stunning by felodipine

Dear Sir, Following short periods of ischemia leaving only completely reversible damage, contractile function remains depressed and recovers spontaneously, but slowly, although blood flow has been restored by reperfusion; this phenomenon has been termed myocardial stunning [1,2]. The mechanisms underlying the prolonged depression of contractile function in reperfused myocardium are still under debate but include damage by free radicals and alterations in excitationcontraction coupling secondary to an increase in free cytosolic calcium [3]. Treatment with calcium antagonists prior to the ischemic insult attenuates myocardial stunning [4]. Whether or not treatment with calcium antagonists after established reperfusion also attenuates stunning remains somewhat controversal [5-9]. We have recently demonstrated that treatment with nisoldipine prior to the ischemic insult attenuates regional myocardial stunning in anesthetized dogs and that this effect could not be attributed to more favorable systemic hemodynamics or improved blood flow during ischemia or reperfusion [8]. In the present study using the same model, the effect of pretreatment with the dihydropyridine calcium antagonist felodipine on the functional recovery of reperfused myocardium was investigated. The dogs were handled according to the guidelines of the American Physiological Society, and the protocol was approved by the bioethical committee of the district of Dfisseldorf, FRG. The experimental model has been described in detail previously [8]. In brief, dogs were anesthetized with an initial bolus of sodium thiamylal, and anesthesia was maintained by ventilation with enflurane and nitrous oxide. Body temperature was maintained between 37°C and 38°C. Left ventricular and aortic pressures were measured, and heart rate and left ventricular dP/dt were derived from the left ventricular pressure signal. A balloon catheter was placed in the descending aorta to compensate for potential decreases in mean aortic pressure. Ultrasonic crystals were implanted in the anterior left ventricular wall (control area) and in the posterolateral wall perfused by the left circumflex coronary artery (LCx) for measurement of regional myocardial wall thickness using sonomicrometry. Regional myocardial function was determined as systolic wall thickening (systolic wall excursion in percent of end-diastolic wall thickness). End-diastolic wall thickness was normalized to an end-diastolic wall thickness of 10 mm under control conditions to account for interindividual differences. Regional myocardial blood flow was determined with colored microspheres. After control measurements of systemic hemodynamics, regional myocardial blood flow, and contractile function, eight dogs received felodipine intravenously (10 ~g/kg dissolved in 7 ml aqua bidest., 1.5 ml ethanol, and 1.5 ml polyethylene glycol 400, infused over 1 minute). The drug-induced decrease in mean aortic pressure was compensated for by inflation of the intraaortic balloon, and measurements were repeated. The LCx was then occluded for 15 minutes. At 10 minutes of LCx occlusion, and at 1, 2, 3, and 4 hours after release of the LCx occlusion, measurements were repeated. Data on dogs receiving felodipine were compared with those of a historic placebo group of dogs (n = 11), which was also subjected to 15 minutes of LCx occlusion and 4 hours of reperfusion [10]. Data are reported as mean values ± standard deviation (SD). Changes in systemic hemodynamics, regional myocardial blood flow, and dimensions were estimated by a two-way analysis of variance for repeated measurements, accounting for the different points of time throughout the experiments and the two different groups of dogs. When a significant overall effect was detected, Tukeys t-tests were performed to compare