Exploring Naltrindole's hypercontracture attenuating effects during myocardial ischemia via a novel mechanism that produces cardioprotective effects in myocardial ischemia/reperfusion injury

Introduction: Previously, naltrindole (NTI) produced robust cardioprotective effects (e.g.∼80% reduction of infarct size) in both ex-vivo (2.5-5[micro]M) and in-vivo (3.75-7.5 mg/kg) rat myocardial ischemia-reperfusion (MIR) injury models. Additionally, we recently showed that NTI 100-200[micro]M attenuated phorbol 12-myristate 13-acetate (PMA) induced polymorphonuclear leukocytes (PMNs) superoxide (SO) release by a novel mechanism of action (MOA) devoid of opioid receptors. Hypothesis: We hypothesize NTI's novel MOA is a reduction of intracellular calcium (Ca2+). Testing this hypothesis, we conducted isolated perfused MIR assays using KB-R7943(KB). KB is known to reduce intracellular Ca2+ via inhibition of reverse mode Na+/Ca2+ exchanger and, therefore, should attenuate myocardial ischemic hypercontracture, infarct size, and left ventricular end diastolic pressure (LVEDP). We predict that both NTI and KB will improve LVEDP and decrease infarct size compared to vehicle (dH2O) control and naloxone (NX) used as a negative control. Methods: Hearts were isolated from anesthetized male Sprague Dawley (SD) rats (∼300g). We then infused NTI 1.25-5[micro]M, KB 10-20[micro]M, or NX 10[micro]M for 5 min into perfused hearts, just prior to global I(30min)/R(45min). Cardiac left ventricular function was measured via a pressure transducer and infarct size using 1% triphenyltetrazolium chloride staining in the MIR assay. Data were analyzed using ANOVA and Fishers post-hoc analysis, with p<0.05 considered statistically significant. Results: MIR assay: NTI 5[micro]M, 2.5[micro]M, 1.25[micro]M and KB 20 [micro]M reduced infarct size (4.8+/-3%, n=5, p<0.05), (6.8+/-0.8%, n=6, p<0.05), (12.2+/-3%, n=5), and (2.9+/-2%, n=5, p<0.05) respectively compared to vehicle (14.1+/-2%, n=9) and NX 10[micro]M controls (18+/-3%, n=3). NTI 5 [micro]M and KB 20 [micro]M had significantly improved LVEDP to (32.4+/-11%, n=5, p<0.05) and (22.8+/-4%, n=5, p<0.05) respectively compared to vehicle control (72.4+/-6%, n=9) and NX 10[micro]M controls (54.2+/-8%, n=3) Conclusion: Both NTI (2.5[micro]M, 5[micro]M) and KB (20[micro]M) reduced infarct size and improved LVEDP. The novel MOA of NTI may be mediated via a reduction in intracellular Ca2+ during myocardial ischemia. The infarct reducing effect of NTI can benefit patients undergoing elective PCI, CABG, and transplant procedures.