Determination of polymyxin E1 in rat plasma by high-performance liquid chromatography

A precise and accurate HPLC assay for polymyxin E(1) in rat and dog plasma has been validated. Samples and standards are extracted from plasma with a 96-well C(8) extraction disk plate. Sample extracts are derivatized with dansyl chloride, and polymyxin E(1) derivative is quantitated on a C(8) column by HPLC with fluorescence detection. The assay is linear in the range of 0.050-5.00 micro g/ml for polymyxin E(1). The precision and accuracy of polymyxin E(1) plasma assay was well within the recommended limits set in the FDA Guidance for Bioanalytical Method Validation. Polymyxin E(1) stability in rat and dog plasma for 24 h at room temperature and through three freeze-thaw cycles was demonstrated.

Inorganic iron effects on in vitro hypoxic proximal renal tubular cell injury

Iron-dependent free radical reactions and renal ischemia are believed to be critical mediators of myohemoglobinuric acute renal failure. Thus, this study assessed whether catalytic iron exacerbates O2 deprivation-induced proximal tubular injury, thereby providing an insight into this form of renal failure. Isolated rat proximal tubular segments (PTS) were subjected to either hypoxia/reoxygenation (H/R: 27:15 min), "chemical anoxia" (antimycin A; 7.5 microM x 45 min), or continuous oxygenated incubation +/- ferrous (Fe2+) or ferric (Fe3+) iron addition. Cell injury (% lactic dehydrogenase [LDH] release), lipid peroxidation (malondialdehyde, [MDA]), and ATP depletion were assessed. Under oxygenated conditions, Fe2+ and Fe3+ each raised MDA (approximately 7-10x) and decreased ATP (approximately 25%). Fe2+, but not Fe3+, caused LDH release (31 +/- 2%). During hypoxia, Fe2+ and Fe3+ worsened ATP depletion; however, each decreased LDH release (approximately 31 to approximately 22%; P < 0.01). Fe(2+)-mediated protection was negated during reoxygenation because Fe2+ exerted its intrinsic cytotoxic effect (LDH release: Fe2+ alone, 31 +/- 2%; H/R 36 +/- 2%; H/R + Fe2+, 41 +/- 2%). However, Fe(3+)-mediated protection persisted throughout reoxygenation because it induced no direct cytotoxicity (H/R, 39 +/- 2%; H/R + Fe3+, 25 +/- 2%; P < 0.002). Fe3+ also decreased antimycin toxicity (41 +/- 4 vs. 25 +/- 3%; P < 0.001) despite inducing marked lipid peroxidation and without affecting ATP. These results indicate that catalytic iron can mitigate, rather than exacerbate, O2 deprivation/reoxygenation PTS injury.

Direct amphotericin B-mediated tubular toxicity: assessments of selected cytoprotective agents

Amphotericin B (AB) may induce acute renal failure by vasoconstrictive and tubulo-toxic effects. Although mannitol, Ca2+ channel blockers, and lipid-based AB preparations have been suggested to mitigate in vivo AB nephrotoxicity, whether they confer direct tubular cytoprotection has not been defined. Therefore, this study assessed the impact of mannitol, verapamil/extracellular Ca2+, and cholesteryl sulfate (CS) AB binding on AB cytotoxicity, employing an isolated rat proximal tubular segment (PTS) preparation. After 30 to 60 minutes of incubation, 0.2 mg/ml of AB (Fungizone) caused marked toxicity, as assessed by LDH release (29 to 44%) and ATP depletion (greater than 90%). Approximately 40% of the LDH release could be attributed to deoxycholate, the standard AB (Fungizone) solubilizing agent. Both 100 mM mannitol and 100 mM glucose decreased AB-mediated LDH release, despite having a quantitatively trivial impact on ATP concentrations (increments of less than or equal to 1% at normal values). Dimethylthiourea (25 mM; equipotent to 100 mM mannitol/glucose as a hydroxyl radical scavenger) did not decrease LDH release. Neither verapamil addition (100 microM) nor Ca2+ removal from the PTS buffer had a protective effect. CS binding completely eliminated AB's toxicity (no LDH or ATP losses). The effect of AB and CS-AB on concomitant O2 deprivation/reoxygenation (30 min/15 min) PTS injury was also assessed. AB and hypoxia/reoxygenation caused additive, not synergistic, LDH release whereas CS-AB had no adverse effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against increased hydroxyl radical production during oxygen deprivation-reoxygenation proximal tubular injury

The purpose of this study was to assess whether proximal renal tubules generate excess hydroxyl radical (.OH) during hypoxia/reoxygenation or ischemia/reperfusion injury, thereby supporting the hypothesis that reactive oxygen species contribute to the pathogenesis of postischemic acute renal failure. In the first phase of the study, rat isolated proximal tubular segments (PTS) were subjected to hypoxia (95% N2- 5% CO2) for 15, 30, or 45 min, followed by 15 to 30 min of reoxygenation in the presence of sodium salicylate, a stable .OH trap. Cellular injury after hypoxia and reoxygenation was assessed by lactate dehydrogenase release; .OH production was gauged by hydroxylated salicylate by-product generation (2,3-, 2,5-dihydroxybenzoic acids (DHBA); quantified by HPLC/electrochemical detection). Continuously oxygenated PTS served as controls. Despite substantial lactate dehydrogenase release during hypoxia (8 to 46%) and reoxygenation (8 to 11%), DHBA production did not exceed that of the coincubated, continuously oxygenated control PTS. In the second phase of the study, salicylate-treated rats were subjected to 25 or 40 min of renal arterial occlusion +/- 15 min of reperfusion. No increase in renal DHBA concentrations occurred during ischemia or reperfusion, compared with that in sham-operated controls. To validate the salicylate trap method, PTS were incubated with a known .OH-generating system (Fe2+/Fe3+); in addition, rats were treated with antioxidant interventions (oxypurinol plus dimethylthiourea). Fe caused marked DHBA production, and the antioxidants halved in vivo DHBA generation. In conclusion, these results suggest that exaggerated .OH production is not a consequence of O2 deprivation/reoxygenation tubular injury.

Temperature effects on ischemic and hypoxic renal proximal tubular injury

Body temperature (T) profoundly alters the severity of ischemic acute renal failure. Therefore, the present study evaluated T effects on: (a) in vitro proximal tubular cell killing during hypoxia/reoxygenation to assess when it exacerbates injury; (b) renal ATP losses and metabolic rate (O2 consumption) during hemorrhagic shock (55-60 mm Hg); and (c) membrane deacylation (assessed by free fatty acid, FFA, release) to determine if T modifies this pathway of ischemic renal damage. Hypoxic cell kill (45 minutes) was 20 +/- 1% 47 +/- 4%, and 61 +/- 2% at 32 degrees C, 37 degrees C, and 40 degrees C respectively (by lactate dehydrogenase release; p less than 0.001). During reoxygenation (15 minutes), minimal lactate dehydrogenase was released, irrespective of T. ATP decrements during shock were profoundly T dependent (% loss of ATP; 15%, 30%, 58% at 32.5 degrees C, 37 degrees C, and 39.5 degrees C, respectively; p less than 0.001), reflecting T-dependent increments in renal metabolic rate, not decreased O2 delivery (arterial O2 content; renal blood flow). ATP losses during shock correlated with the extent of S3 proximal tubular morphologic damage. O2 deprivation dramatically increased FFA levels both in vivo and in vitro but the increments were only slightly T dependent. In vitro, % lactate dehydrogenase release and FFA levels did not significantly correlate and bovine serum albumin, a FFA binder, conferred no protection.

CONCLUSIONS

(a) T dramatically accentuates hypoxic, not reoxygenation, injury; (b) changes in membrane deacylation do not appear to underlie this effect; and (c) T has a profound impact on renal ATP losses during shock, thereby affecting the severity of ischemic renal damage.

Regional responses within the kidney to ischemia: assessment of adenine nucleotide and catabolite profiles

Renal cortex (C) has predominantly aerobic metabolism, whereas inner medulla (IM) has both aerobic and anaerobic capacities. This study was undertaken (1) to assess how well rat IM anaerobic metabolism maintains this region's ATP content during ischemia; and (2) to determine whether regional variations in adenylate pool/catabolite responses to ischemia exist, obscuring interpretation of cellular energetics in rat studies of acute renal failure (ARF). Adenine nucleotides/catabolites were measured in rat C, IM and outer medulla (OM) after 15 and 45 min of ischemia. After 15 min, all regions showed profound ATP depletion, although the IM maintained slightly higher (by 0.23 mumol/g) absolute ATP levels than C/OM tissues (normal ATP value = 8.7 mumol/g). By 45 min, significant differences in regional ATP levels did not exist. Striking regional catabolite differences were apparent at both 15 and 45 min. Most prominent were: (1) intrarenal purine base/inosine gradients, levels falling approx. 22-50% from C to IM; and (2) preferential OM AMP/IMP/adenosine accumulation. To assess whether more homogeneous results might be found in rabbit kidney, possibly making this animal preferable to rats for studies of renal ischemia, rabbit C, OM and IM adenylate pools were analyzed after 15 min of ischemia. C vs. IM ATP differences were greater (approx. 1.3 mumol/g) and large catabolite concentration differences were still apparent.

CONCLUSIONS

(1) anaerobic mechanisms support IM ATP levels during ischemia but, in terms of normal concentrations, the impact is small, particularly in the rat; and (2) marked regional differences in adenylate catabolite levels exist within ischemic kidneys. These need to be recognized when analyzing adenylate pool responses in ischemic ARF.

Degree and time sequence of hypothermic protection against experimental ischemic acute renal failure

The purpose of this study was to assess the degree, time sequence, and biochemical correlates of hypothermic protection against ischemic acute renal failure. Rats subjected to 40 minutes of bilateral renal artery occlusion (RAO) were made mildly hypothermic (32 degrees-33 degrees C, by cold saline peritoneal lavage) during the following time periods: 1) RAO only, 2) reperfusion only (beginning at 0, 15, 30, or 60 minutes after RAO and maintained for 45 minutes), or 3) during and after (0-45 minutes) RAO. Continuously normothermic (37 degrees C) RAO rats served as controls. The control rats developed severe acute renal failure (blood urea nitrogen [BUN], 95 +/- 4 mg/dl; creatinine, 2.2 +/- 0.1 mg/dl; and extensive tubular necrosis at 24 hours). Hypothermia confined to RAO was highly protective (BUN, 33 +/- 5 mg/dl; creatinine, 0.62 +/- 0.07 mg/dl; and minimal necrosis). Hypothermia partially preserved ischemic renal adenylate high-energy phosphate (ATP and ADP), increased AMP and inosine monophosphate concentrations, and lessened hypoxanthine/xanthine buildup (assessed at end of RAO). Hypothermia confined to the reflow period (beginning at 0, 15, and 30 minutes) was only mildly protective (e.g., BUN, 58-63 mg/dl); the degree of protection did not differ according to the time of hypothermic onset. Lowering reflow temperature to 26 degrees C had no added benefit. Hypothermia that started at 60 minutes after RAO conferred no protection. Combining ischemic and postischemic hypothermia abolished all renal failure (assessed at 24 hours). This study offers the following conclusions: Mild hypothermia can totally prevent experimental ischemic acute renal failure. Hypothermia is highly effective during ischemia, and it is mildly protective during early reflow; these benefits are additive. During early reflow, hypothermic protection is not critically time dependent. By 60 minutes of reflow, no effect is elicited; this absence of effect possibly signals completion of the reperfusion injury process. Hypothermia's protective effects may be mediated, in part, by improvements in renal adenine nucleotide content and, possibly, by decreasing postischemic oxidant stress.

Delayed cerebroventricular metabolism of [125I]angiotensins in the spontaneously hypertensive rat

This study was designed to evaluate the hypothesis that impaired brain angiotensin signal termination contributes to the sustained blood pressure elevations noted in the genetically hypertensive rat model of human essential hypertension. A technique that combined the intracerebroventricular injection of [125I]angiotensins, followed by focused microwave fixation to stop all peptidase activity and subsequent HPLC analyses, was used for determining half-lives of [125I]angiotensin II and [125I]angiotensin III in the ventricular space. The results indicate that the spontaneously hypertensive rat evidenced significantly longer half-lives for intracerebroventricularly injected [125I]angiotensin II over those measured for the Wistar-Kyoto and Sprague-Dawley normotensive rat strains: 45.0, 27.2, and 25.0 s, respectively. This was also true for intracerebroventricularly administered [125I]angiotensin III: 19.5, 11.4, and 9.0 s, respectively. These results support the notion that a dysfunction in central aminopeptidase activity in the spontaneously hypertensive rat may result in prolonged half-lives of endogenously synthesized angiotensins II and III, which are known to serve as ligands at central angiotensin receptors responsible for the control of cardiovascular function. The extended half-lives of these ligands may contribute to the sustained elevations in blood pressure observed in this animal model.