© 2003 by European Society of Cardiology
Copyright © 2003, European Society of Cardiology
Cardiovascular effects of sildenafil in experimental ischemia–reperfusion (reply to "Sildenafil-induced cardioprotection in rabbits, by Kukreja, R.C., Ockaili, R., Salloum, F., Xi, L.")
The Heart Institute, Good Samaritan Hospital, Division of Cardiovascular Medicine, University of Southern California, 1225 Wilshire Boulevard, Los Angeles, CA 90017-2395, USA
*Corresponding author. Tel.: +1-213-977-4050; fax: +1-213-977-4107. Email address: rkloner{at}goodsam.org
Received 24 September 2003; accepted 1 October 2003
In their letter to the editor, Kukreja, Ockaili, Salloum and Xi comment on our recently published study in which intravenous application of 1.45 or 0.7 mg/kg sildenafil 30 min prior to coronary artery occlusion did not reduce myocardial infarct size in an experimental rabbit model of 30 min ischemia followed by 180 min of reperfusion [1]. Since our results contrast with a recent investigation by this research group [2] that described pronounced infarct size reduction after pretreatment with sildenafil in a very similar model, the authors are "puzzled by our negative findings". Indeed, we appreciate the editorial decision from Cardiovascular Research to publish our results, as in these days publication bias in favor of positive results occasionally makes it difficult to publish "negative findings". We believe that the constructive debate, initiated by Kukreja et al. in this letter to the editor, may encourage this editorial policy.
When comparing the mentioned investigations by Ockaili et al. [2] with our study, again—and we think that the extensive work from Dr. Kukreja's group on delayed cardioprotective effects of sildenafil [2,3] should be clearly differentiated from acute effects—we must reiterate the very similar study design in Ockaili's and our investigations. The only noticeable differences are the use of atropine when initiating anesthesia, the slightly lower body temperature in Ockaili's study, the use of pentobarbital for maintenance of anesthesia, and the higher infarct size in the control group in our study. Additionally, for preparation of a sildenafil solution, Ockaili et al. crushed Viagra tablets and dissolved them in saline, while we used the pure powder of sildenafil citrate. Whether these subtle differences in the study protocol are responsible for the different findings remains unresolved to date.
Our experimental model of coronary artery occlusion and reperfusion in the rabbit is a valid and established tool for investigating cardioprotective interventions. In our laboratory, infarct size reduction was demonstrated after application of the adenosine receptor agonist R-PIA [4], the Na+/H+-exchange inhibitor cariporide [5,6], regional myocardial hypothermia [7], or ischemic preconditioning [6]. Whether the failure to demonstrate a statistically significant reduction in infarct size after diazoxide [5] (although we did observe a trend of 23% nonsignificant reduction of infarct size with diazoxide) or after sildenafil [1], two interventions that were shown to be cardioprotective in Dr. Kukreja's group [8,2] and appear to involve mitochondrial KATP channels, is just an expression of statistical variation, a characteristic feature in our model, or related to other causes is not known, but potentially of importance when transferring experimental data to clinical applications.
Kukreja et al. mention another study by Das et al. [9] that did show protective effects of sildenafil in the rat. Notably, these investigations, performed in working-heart preparations, demonstrated infarct size reduction with 0.05 mg/kg sildenafil but substantial exacerbation of myocardial necrosis with higher sildenafil concentrations over a very narrow dosing range (0.1–0.5 mg/kg sildenafil). Thus, assuming such a steep dose–effect relationship for the rabbit heart as well, one might speculate that slight inaccuracy in the dosage of sildenafil could lead to marked differences in the results. The accuracy of the concentrations might be a special challenge when crushing the Viagra tablets and dissolving them in saline. On the other hand, even if several clinical applications of sildenafil are now conceivable [10], a dosage window, as it was found for sildenafil in this model of experimental myocardial ischemia–reperfusion, is probably too narrow ever to be tested in patients with acute myocardial infarction.
The conceptual basis, however, that Kukreja et al. present in their letter to the editor is interesting and potentially of therapeutic importance, linking nitric oxide, cyclic guanosine monophosphate (cGMP), mitochondrial KATP channels, and eventually attenuation of myocardial necrosis in ischemia–reperfusion. A key question in this concept is whether ventricular cardiomyocytes express phosphodiesterase 5 (PDE-5), a crucial prerequisite for sildenafil to elevate cytosolic cGMP levels in acute studies. Elevation of cGMP in whole heart tissue preparations after application of sildenafil, as demonstrated in the investigations by Das et al. [9], may be the result of elevated cGMP levels in cardiomyocytes or other cells such as vascular smooth muscle cells. Importantly, the systematic work by Wallis et al. [11] did not provide evidence of PDE-5 expression in ventricular cardiomyocytes. However, this important issue is still controversial [12,13]. The theoretical possibility that expression of PDE-5 in the animals could vary depending on age, genetic predisposition, or environmental factors could provide another explanation for the controversial results. Provided PDE-5 is present in cardiomyocytes, one might also conjecture that the degree of pre-activation of the nitric oxide–guanylate cyclase pathway in cardiomyocytes is the major determinant of the cardiovascular, and maybe cardioprotective, response to sildenafil, as it inhibits the hydrolysis of previously synthesized cGMP. The complex regulation of this pathway, as discussed by Kukreja et al., may not easily be standardized in experimental animal models.
In conclusion, we highly appreciate the interest Kukreja, Ockaili, Salloum, and Xi take in our article, and we hope that the descriptive presentation of our data in the original article and now this controversial, but hopefully constructive, discussion of potential reasons for the discrepancies will help to identify additional factors in the models we use for our biological research.
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