Canyon, Sarah (2003) A novel cardioprotective therapy : adenosine and lidocaine solution in an in vivo rat model of acute myocardial ischemia-reperfusion. PhD thesis, James Cook University.

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Abstract

Abstract Background: Recently, our laboratory demonstrated that an adenosine and lidocaine (AL) non-depolarizing cardioplegic arrest solution conferred superior protection during arrest and recovery compared with the hyperkalemic depolarizing St. Thomas’ Hospital cardioplegic solution in isolated rat hearts. The aim of this thesis was to extend those findings by applying an adenosine and lidocaine (AL) solution at nonarresting concentrations before and during ischemia in an in vivo rat model of acute myocardial ischemia. No study has investigated the effect of AL combination treatment to reduce ischemic injury. Yet, previous studies in the 1990s have used the sequential and separate administration of lidocaine (2 mg/kg i.v.) and adenosine (150 μg/kg/ml/min i.c) as reperfusion therapy with conflicting results. Methods: In all four studies, ischemia-reperfusion was achieved by placing a reversible tie around the left coronary artery of anaesthetized (sodium pentobarbital, 60 mg/ml/kg i.p.) and ventilated male Sprague-Dawley rats (300 - 400 g). The ischemic period lasted 30 min while reperfusion times were maintained for either 40 min or 120 min. Where applicable, a lead II electrocardiogram, heart rate, and systolic and diastolic pressures were recorded and mean arterial pressure and rate-pressure product calculated. The primary end points included infarct size, episodes and durations of ventricular arrhythmias, pH and changes in the concentration of ATP ([ATP]) and phosphocreatine ([PCr]) during ischemia-reperfusion recorded every 5 min. The level of statistical significance was P < 0.05. Experimental design: The first two studies examined the cardioprotective potential of adenosine and lidocaine using the following treatment strategies: i) three AL solutions with varying concentrations of adenosine (A: 152, 305 and 457 μg/kg/min plus L: 608 μg/kg/min i.v., n = 18) compared to saline controls (0.9% saline, n = 12), adenosine only (adeno-only, 305 μg/kg/min i.v., n = 8) and lidocaine only (lido-only, 608 μg/kg/min i.v., n = 8); all of these treatments were given 5 before ischemia and continued throughout 30 min ischemia but not reperfusion; and ii) the separate and sequential administration of adenosine (150 μg/kg/min i.v.) and lidocaine (2 mg/kg i.v) during reperfusion (n = 7); the sequential administration of AL solution (A: 305 μg/kg/min plus L: 608 μg/kg/min i.v.) of 5 min pretreatment and again 5 min before and during 30 min reperfusion, n = 6 and a 5 min pretreatment of AL solution (A: 305 μg/kg/min plus L: 608 μg/kg/min i.v.) continued throughout ischemia and 30 min of reperfusion (n = 6). In the third study, 31P nuclear magnetic resonance was used to investigate the changes in [ATP], [PCr] and pH during ischemia (30 min) and reperfusion (40 min) with AL solution treatment (n = 6) or in controls (n = 7). In the fourth study, AL solution (A: 305 μg/kg/min plus L: 608 μg/kg/min i.v.) was compared to that of ischemic preconditioning (three 3 min cycles of ischemia-reperfusion) (IPC) (n = 6), the adenosine receptor A1 agonist, 2-chloro-N6-cyclopentyladenosine (CCPA) (5 μg/kg i.v.) plus lidocaine (n = 6), and CCPA alone (n = 7). Results: Seven of the 12 saline-control rats and 4 of the 8 Adeno-only treated rats died during the ischemic period from an episode of ventricular fibrillation. No deaths occurred in the Lido-only treated rats (n = 6) or in any group where AL solution was infused. Ventricular tachycardia (VT) occurred in 100% of saline controls (18 ± 9 episodes), 50% of the adeno-only group (11 ± 7 episodes), and 83% of lido-only treatment (23 ± 11 episodes). VT was also experienced in 60% of low-dose AL treated rats (2 ± 1 episodes) (P < 0.05), 57% of mid-dose AL (2 ± 1 episodes) (P < 0.05), and 67% of high-dose AL treated rats (6 ± 3 episodes). Ventricular fibrillation (VF) occurred in 75% of saline controls (4 ± 3 episodes), 100% of adeno-only (3 ± 2 episodes), and in 33% lido-only treated rats (2 ± 1 episodes). Low-dose AL and mid-dose AL completely prevented VF from occurring during ischemia. The mean infarct size of mid dose-AL (38 ± 6%) was significantly reduced from saline controls (61 ± 5%), adeno-only (56 ± 4%), and lido-only (66 ± 8%) (P < 0.05) but not from low-dose AL (45± 9%) and high-dose AL animals (45 ± 6%). The separate and sequential administration of lidocaine and adenosine resulted in 2 out of 7 deaths and ischemia-induced VT (6 ± 3 episodes, 4 ± 2 sec) was not prevented while VF (1 ± 0 episodes, 1 ± 0 sec) was reduced. Infarct size (52 ± 5%) was not significantly different from saline-controls (61 ± 5%). When AL was given at pretreatment, stopped for ischemia and resumed 5 min before reperfusion, infarct size reduction (67 ± 8%) and protection from ventricular arrhythmias (VT: 39 ± 23 episodes, 84 ± 49 sec; VF: 2 ± 1 episodes, 21 ± 8 sec) were lost; though, there were no deaths in this group. AL solution given continuously from pretreatment through ischemia and reperfusion provided similar protection to AL infusion during pretreatment and ischemia (41 ± 10% vs. 38 ± 6%, 2 ± 1 VT and 0 VF). During ischemia, control [ATP] fell to 61% of baseline at 15 min and recovered 68% - 88% of baseline during reperfusion. AL treatment maintained [ATP] in a steady state throughout ischemia and reperfusion with changes ranging of 95 ± 7 % to 117 ± 10% of baseline. Control [PCr] was significantly reduced compared to AL treated hearts during ischemia at 10 min (62 ± 7 vs. 89 ± 9%), 15 min (45 ± 4% vs 81 ± 7%), 20 min (44 ± 9% vs. 92 ± 9%) and 30 min (45 ± 8% vs. 77 ± 7%) and during reperfusion at 10 min (44 ± 19% vs. 92 ± 9%) and 15 min (50 ± 8% vs. 90 ± 7%) (P < 0.05). The pH of AL and control hearts were similar throughout ischemia ranging from pH 7.6 to 6.4 in control and pH 7.5 to 6.8 in AL hearts. Controls maintained a mean pH below baseline for the first 20 min of reperfusion (pH 7.1) while AL hearts pH recovered to baseline within the first 5 min of reperfusion (pH 7.4 ± 0.1). Pretreating the heart before and during ischemia with AL or with CCPA plus lidocaine resulted in no deaths, and no lethal arrhythmias. Infarct size reduction in CCPA plus lidocaine treated rats (12 ± 4 %) was similar to ischemic preconditioning (11± 3%), whereas in AL- and CCPA- treated rats, the infarct size was 38 ± 6% and 42 ± 7% respectively. Conclusions: i) The intravenous infusion of AL solution before or during 30 min was more cardioprotective than adenosine alone, lidocaine alone, or the separate and sequential infusion of adenosine and lidocaine; ii) the AL combination led to no death, virtually no episodes of VF, few episodes of VT, and a significantly reduced infarct size; iii) AL cardioprotection appears to be associated with preservation of high energy phosphates and a better balance between supply and demand during ischemic conditions; iv) low pH was not an indicator of myocardial damage in AL treated rats, and v) when adenosine was substituted with an adenosine A1 receptor agonist, CCPA, plus lidocaine cardioprotection was significantly enhanced and similar to IPC. In summary, targeting adenosine receptors, especially the A1 adenosine receptor, with lidocaine Na+ fast channel modulation may offer a new combination therapy to delay myocardial damage during ischemia and prevent ischemia-induced arrhythmias.

Item ID:	1642
Item Type:	Thesis (PhD)
Keywords:	heart, ischemia, heart protection, organ preservation, preconditioning, heart surgery, cardioprotective therapy, adenosine, lidocaine, acute myocardial ischemia-reperfusion injury, ventricular arrythmias, cardioplegic arrest solutions, rat models
Date Deposited:	03 Sep 2007
FoR Codes:	11 MEDICAL AND HEALTH SCIENCES > 1102 Cardiovascular Medicine and Haematology > 110201 Cardiology (incl Cardiovascular Diseases) @ 100%
SEO Codes:	92 HEALTH > 9201 Clinical Health (Organs, Diseases and Abnormal Conditions) > 920103 Cardiovascular System and Diseases @ 100%
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