Evidence suggests that 1) ischemia-reperfusion injury is due largely to cytosolic Ca 2ϩ accumulation resulting from functional coupling of Na ϩ /Ca 2ϩ exchange (NCE) with stimulated Na ϩ /H ϩ exchange (NHE1) and 2) 17␤-estradiol (E2) stimulates release of NO, which inhibits NHE1. Thus we tested the hypothesis that acute E2 limits myocardial Na ϩ and therefore Ca 2ϩ accumulation, thereby limiting ischemia-reperfusion injury. NMR was used to measure cytosolic pH (pHi), Na ϩ (Na i ϩ ), and calcium concentration ([Ca 2ϩ ]i) in Krebs-Henseleit (KH)-perfused hearts from ovariectomized rats (OVX). Left ventricular developed pressure (LVDP) and lactate dehydrogenase (LDH) release were also measured. Control ischemia-reperfusion was 20 min of baseline perfusion, 40 min of global ischemia, and 40 min of reperfusion. The E2 protocol was identical, except that 1 nM E2 was included in the perfusate before ischemia and during reperfusion. E2 significantly limited the changes in pH i, Na i ϩ , and [Ca 2ϩ ]i during ischemia (P Ͻ 0.05). In control OVX vs. OVXϩE2, pH i fell from 6.93 Ϯ 0.03 to 5.98 Ϯ 0.04 vs. 6.96 Ϯ 0.04 to 6.68 Ϯ 0.07; Na i ϩ rose from 25 Ϯ 6 to 109 Ϯ 14 meq/kg dry wt vs. 25 Ϯ 1 to 76 Ϯ 3; [Ca 2ϩ ]i changed from 365 Ϯ 69 to 1,248 Ϯ 180 nM vs. 293 Ϯ 66 to 202 Ϯ 64 nM. E2 also improved recovery of LVDP and diminished release of LDH during reperfusion. Effects of E2 were diminished by 1 M N -nitro-L-arginine methyl ester. Thus the data are consistent with the hypothesis. However, E2 limitation of increases in [Ca 2ϩ ]i is greater than can be accounted for by the thermodynamic effect of reduced Na i ϩ accumulation on NCE. myocardial ischemia; Na ϩ /H ϩ exchange; Na ϩ /Ca 2ϩ exchange; nuclear magnetic resonance; ischemic biology; ion channels/membrane transport; transplantation IT IS CLEAR THAT OUR UNDERSTANDING of the effects of endogenous and exogenous estrogen on the cardiovascular system, and in particular on its role in modifying susceptibility to ischemic injury, is deficient. It is more important than ever that fundamental research be conducted to understand the basis for the effects of estrogen (50). Because results of various studies are inconsistent and/or difficult to interpret, we have taken a reductionist approach to testing the acute effects of exogenous 17␤-estradiol (E2) on ischemic myocardium within the context of the well-accepted paradigm that ischemic injury is largely the result of the following chain of events. Anaerobic metabolism decreases cytosolic pH (pH i ), which stimulates pHregulatory Na ϩ /H ϩ exchange (NHE1) to increase Na ϩ uptake and thereby increase intracellular Na ϩ (Na i ϩ ). Increases in (3,7,31,37,46,49). In the heart, there are questions about whether the bulk of Na ϩ and Ca 2ϩ entry occurs during ischemia or reperfusion, but there is a growing consensus that much of the Ca 2ϩ entry is Na ϩ dependent (36). Numerous studies have shown that E2 stimulates NOS activity and/or the release of nitric oxide (NO) in the heart (20,39,40) and that female hormonelinked changes in Na ϩ...

Paramagnetic Mn2+ has emerged in the search for non-invasive magnetic resonance imaging (MRI) techniques to monitor Ca2+ in diagnostic and prognostic cardiovascular disease tests because it both alters MRI contrast and behaves as a Ca2+ 'surrogate' in vivo. However, the reliance on macroscopically averaged measurements to infer microscopic processes constitutes a major limitation of MRI. This investigation circumvents this limitation and contributes an MRI-based myocardial Ca2+-transporter assay, which probes the Na+/Ca2+-exchanger involvement in Mn2+ (and presumably Ca2+) transport by virtue of its response to pharmacological inhibition. In the model employed herein, ex vivo arrested rat hearts underwent normoxia and then hypoxia while a constant (hyperkalemic) perfusion minimized flow (and uncontrolled Ca2+-channel) contributions to Mn2+-enhanced MRI measurements. The results (i) demonstrate that Mn2+ (and presumably Ca2+) accumulates via Na+/Ca2+-exchanger-mediated transport during hyperkalemic hypoxia and further, (ii) implicate hypo-perfusion (rather than the diminished participation of an isolated sarcolemmal Ca2+-transporter) as the mechanism that underlies the reported reductions of Mn2+ accumulation (relative to healthy myocardium) subsequent to myocardial insults in MRI studies. Although myriad studies have employed Mn2+-enhanced MRI in myocardial investigations, this appears to be the first attempt to assay the Na+/Ca2+-exchanger with MRI under highly circumscribed conditions. MRI-based Ca2+)transporter assays, such as the Na+/Ca2+-exchanger assay utilized here, will inevitably impact disciplines in the medical sciences and beyond.