Response to Letter Regarding Article, “High-Dose Folic Acid Pretreatment Blunts Cardiac Dysfunction During Ischemia Coupled to Maintenance of High-Energy Phosphates and Reduces Postreperfusion Injury”
In his letter, Greyson questions whether enhanced myocardial blood flow rather than altered high-energy phosphate (HEP) metabolism reduced infarct size and better preserved ATP and ADP levels in hearts pretreated with folic acid (FA). He argues that by measuring relative rather than absolute flow reduction between remote and ischemic territories, we missed higher total flows to the heart in FA-pretreated animals despite similar relative flow reduction. However, this question presents a chicken-egg problem: Does FA primarily enhance flow to improve function and ischemic outcome, or is it the other way around? Although our flow analysis has limitations, and further studies with more comprehensive flow analysis would be useful, we believe our findings1 favor the latter hypothesis.
First, in isolated hearts in which coronary perfusion was constant, postischemic myocardial preservation was similar to that for the in vivo studies. Although HEP analysis was not performed in these hearts, the data indicate that relevant alternative mechanisms beyond coronary flow must exist. Second, systolic pressure during ischemia was 80 versus 100 mm Hg for FA-treated versus control rats, respectively. Both values fall in an autoregulating range for rats,2 so this difference would not necessarily result in a meaningful disparity of absolute flow and improved function. Although left ventricular end-diastolic pressure rose somewhat more in controls, potentially further reducing the transcoronary pressure gradient, this occurred in the first 10 minutes after occlusion, when function (dP/dtmax, stroke work, relaxation, etc) was similarly maintained in both groups.
Third, systolic pressure declined 10 to 20 minutes after coronary occlusion, yet a similar relative flow reduction was observed at both 5 and 30 minutes. If function in FA-treated hearts represented a primary effect on perfusion regulation and/or collaterals, this mechanism would appear to be activated well into the ischemic period, which is somewhat unusual. Fourth, although FA indeed is a vasodilator in isolated rat hearts3 when added acutely to the perfusate, this effect is modest (≈25%). More importantly, animals were pretreated orally (ie, global administration) in our study, with no direct FA infusion during ischemia.
Lastly, the flow hypothesis ignores striking alterations in HEP catabolism observed after FA pretreatment alone, as well as the directionally opposite changes in AMP catabolites (eg, a marked decline in uric acid and hypoxanthine in the FA-treated group but a marked rise in controls) during ischemia. It is hard to explain this solely by regional perfusion changes.
On the basis of this reasoning, we stand by our conclusion that improved coronary flow did not likely explain our findings of a protective effect of FA on ischemia-reperfusion injury. More likely, the changes in HEP catabolism in the basal state (before ischemia) and their preservation during ischemia reveal a novel target and effect of FA on the heart.
Moens AL, Champion HC, Claeys MJ, Tavazzi B, Kaminski PM, Wolin MS, Borgonjon DJ, Nassauw LV, Haile A, Zviman M, Bedja D, Wuyts FL, Elsaesser RS, Cos P, Gabrielson KL, Lazzarino G, Paolocci N, Timmermans JP, Vrints CJ, Kass DA. High-dose folic acid pretreatment blunts cardiac dysfunction during ischemia coupled to maintenance of high-energy phosphates and reduces postreperfusion injury. Circulation. 2008; 117: 1810–1819.
Sato F, Isoyama S, Takishima T. Normalization of impaired coronary circulation in hypertrophied rat hearts. Hypertension. 1990; 16: 26–34.