Publications

The lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphosphatidic acid (LPA) reduce mortality in hypoxic cardiac myocytes. S1P is also cardioprotective in both mouse and rat models of cardiac ischemia/reperfusion (I/R) injury. Although these results are consistent with prior work in other cell types, it is not known what signaling events are critical to cardioprotection, particularly with respect to ceramide and the preservation of mitochondrial function, which is essential for cardiac cell survival. Neither receptor regulation nor signaling has been studied during I/R in the heart with or without the application of S1P or LPA. The role of sphingosine kinase in I/R and in ischemic preconditioning (IPC) has not been defined, nor has the fate or function of S1P generated by this enzyme, particularly during preconditioning or I/R, been elucidated. Whether S1P infused systemically in animal models of myocardial infarction in which survival is an end-point will be hemodynamically tolerated has not been determined. If not, the substitution of agents such as the monosialoganglioside GM-1, which activates sphingosine kinase, or the development of alternative ligands for S1P receptors will be necessary.

Hepatic clearance of erythromycin (Ery) is significantly reduced in patients with end stage renal disease. Since Ery is primarily eliminated via excretion of unchanged drug in the bile, we suspect that this change could be due to the effect of uremic toxins on hepatic uptake and/or efflux transporters. Using rat hepatocytes and microsomes as model proof of concept systems, we examined six uremic toxins, 3-carboxy-4-methyl-5-propyl-2-furan-propanoic acid (CMPF), indoxyl sulfate (IS), hippuric acid (HA), indole acetic acid (IA), guanidinosuccinic acid (GSA), and indoxyl-beta-D-glucuronide (IG), for their effects on Ery uptake and metabolism. Ery and the metabolite N-demethyl-Ery were measured by liquid chromatography/tandem mass spectrometry. The uptake of Ery by rat hepatocytes was markedly inhibited by rifampin and digoxin, but not by quinidine, suggesting that Oatp2 plays a major role in the uptake of Ery. At 50 microM, CMPF significantly (p < 0.05) reduced hepatocyte accumulation of Ery and N-demethyl-Ery. At higher concentrations (>200 microM), CMPF appears to also inhibit the enzymatic metabolism of Ery. In contrast, IS did not significantly inhibit the hepatocyte uptake of Ery, even at the highest concentration (800 microM) tested, but reduced metabolite generation (p < 0.001). The other uremic toxins, HA, IA, IG, and GSA, did not affect either hepatic uptake or microsomal metabolism of Ery. CMPF, IS, and HA were shown not to inhibit differential P-glycoprotein transport of Ery in cellular systems. Our results suggest that CMPF can directly inhibit the uptake of Ery by inhibiting Oatp2, whereas IS is more likely to inhibit the enzymatic metabolism of Ery.