Effects of [5-(2-methoxy-5-fluorophenyl)furan-2-ylcarbonyl]guanidine (KR-32560), a novel sodium/hydrogen exchanger-1 inhibitor, on myocardial infarct size and ventricular arrhythmias in a rat model of ischemia/reperfusion heart injury

Determinants of Cation Permeation and Drug Sensitivity in Predicted Transmembrane Helix 9 and Adjoining Exofacial Re-entrant Loop 5 of Na+/H+ Exchanger NHE1

Mammalian Na(+)/H(+) exchangers (NHEs) regulate numerous physiological processes and are involved in the pathogenesis of several diseases, including tissue ischemia and reperfusion injuries, cardiac hypertrophy and failure, and cancer progression. Hence, NHEs are being targeted for pharmaceutical-based clinical therapies, but pertinent information regarding the structural elements involved in cation translocation and drug binding remains incomplete. Molecular manipulations of the prototypical NHE1 isoform have implicated several predicted membrane-spanning (M) helices, most notably M4, M9, and M11, as important determinants of cation permeation and drug sensitivity. Here, we have used substituted-cysteine accessibility mutagenesis and thiol-modifying methanethiosulfonate (MTS) reagents to further probe the involvement of evolutionarily conserved sites within M9 (residues 342-363) and the adjacent exofacial re-entrant loop 5 between M9 and M10 (EL5; residues 364-415) of a cysteine-less variant of rat NHE1 on its kinetic and pharmacological properties. MTS treatment significantly reduced the activity of mutants containing substitutions within M9 (H353C, S355C, and G356C) and EL5 (G403C and S405C). In the absence of MTS, mutants S355C, G403C, and S405C showed modest to significant decreases in their apparent affinities for Na(+) o and/or H(+) i. In addition, mutations Y370C and E395C within EL5, whereas failing to confer sensitivity to MTS, nevertheless, reduced the affinity for Na(+) o, but not for H(+) i. The Y370C mutant also exhibited higher affinity for ethylisopropylamiloride, a competitive antagonist of Na(+) o transport. Collectively, these results further implicate helix M9 and EL5 of NHE1 as important elements involved in cation transport and inhibitor sensitivity, which may inform rational drug design.

Aquaporin-1 expression and angiogenesis in rabbit chronic myocardial ischemia is decreased by acetazolamide

Aquaporin-1 (AQP1) is a water channel protein expressed in endothelial and epithelial cells of many tissues, including the vasculature, where it serves to increase water permeability of the cell membrane. Prior studies have also reported that AQP1 plays a central role in tumor angiogenesis by promoting endothelial cell migration. To investigate whether AQP1 might also influence vascular angiogenesis in ischemic myocardium, the expression level of AQP1 for 21 days post myocardial infarction in rabbit hearts was observed. Aquaporin-1 mRNA and protein levels in day 3, and peaked on day 7 post surgery. This correlated well with the pattern of neovascularization and increased water content of infarct border tissue, and suggested that AQP1 may be involved in myocardial angiogenesis in response to ischemia injury. These AQP1-induced changes were tempered by acetazolamide, a carbonic anhydrase inhibitor, which acted by downregulating AQP1 expression. Acetazolamide treatment did not significantly affect the expression of vascular endothelial growth factor in the tissues studied. Our findings indicate a novel role for AQP1 in postnatal angiogenesis, which has implications in diverse pathophysiological conditions including wound healing, tumor metastasis, and organ regeneration.

KR-31761, a novel K+(ATP)-channel opener, exerts cardioprotective effects by opening both mitochondrial K+(ATP) and Sarcolemmal K+(ATP) channels in rat models of ischemia/reperfusion-induced heart injury

The cardioprotective effects of KR-31761, a newly synthesized K+(ATP) opener, were evaluated in rat models of ischemia/reperfusion (I/R) heart injury. In isolated rat hearts subjected to 30-min global ischemia/30-min reperfusion, KR-31761 perfused prior to ischemia significantly increased both the left ventricular developed pressure (% of predrug LVDP: 17.8, 45.1, 54.2, and 62.6 for the control, 1 microM, 3 microM, and 10 microM, respectively) and double product (DP: heart rate x LVDP; % of predrug DP: 17.5, 44.9, 56.2, and 64.5 for the control, 1 microM, 3 microM, and 10 microM, respectively) at 30-min reperfusion while decreasing the left ventricular end-diastolic pressure (LVEDP). KR-31761 (10 microM) significantly increased the time to contracture during the ischemic period, whereas it concentration-dependently decreased the lactate dehydrogenase release during reperfusion. All these parameters were significantly reversed by 5-hydroxydecanoate (5-HD, 100 microM) and glyburide (1 microM), selective and nonselective blockers of the mitochondrial K+(ATP) (mitoK+(ATP)) channel and K+(ATP) channel, respectively. In anesthetized rats subjected to 30-min occlusion of left anterior descending coronary artery/2.5-h reperfusion, KR-31761 administered 15 min before the onset of ischemia significantly decreased the infarct size (72.2%, 55.1%, and 47.1% for the control, 0.3 mg/kg, i.v., and 1.0 mg/kg, i.v., respectively); and these effects were completely and almost completely abolished by 5-HD (10 mg/kg, i.v.) and HMR-1098, a selective blocker of sarcolemmal K+(ATP) (sarcK+(ATP)) channel (6 mg/kg, i.v.) administered 5 min prior to KR-31761 (72.3% and 67.9%, respectively). KR-31761 only slightly relaxed methoxamine-precontracted rat aorta (IC50: > 30.0 microM). These results suggest that KR-31761 exerts potent cardioprotective effects through the opening of both mitoK+(ATP) and sarcK+(ATP) channels in rat hearts with a minimal vasorelaxant effect.

Pravastatin Inhibits Arrhythmias Induced by Coronary Artery Ischemia in Anesthetized Rats

We have reported that chronically administered pravastatin prevented coronary artery reperfusion-induced lethal ventricular fibrillation (VF) in anesthetized rats without lowering the serum cholesterol level. The present study was undertaken to evaluate whether pravastatin prevents ischemia-induced lethal VF, simultaneously examining myeloperoxidase (MPO) activity in ischemic myocardial tissues. Anesthetized rats were subjected to 30-min ischemia and 60-min reperfusion after chronic administration of pravastatin (0.02, 0.2, and 2 mg/kg), fluvastatin (2 and 4 mg/kg), or vehicle for 22 days, orally, once daily. ECG and blood pressure were continually recorded, and MPO was measured by a spectrophotometer. Pravastatin and fluvastatin significantly (P<0.05) decreased MPO activities, but only pravastatin decreased the incidence of ischemia-induced lethal VF. Both statins had no significant effects on body weight, blood pressure, heart rate, and QT interval as we reported earlier. Our results prove further that pravastatin has benefits to decrease cardiovascular mortality beyond its cholesterol-lowering effect. Pravastatin is more potent than fluvastatin in prevention of arrhythmias. A decrease in the neutrophil infiltration may be partly involved in the inhibitory effect of pravastatin on the ischemia-induced VF.