Hysteresis in force probe measurements: a dynamical systems perspective

BACKGROUND: This study was designed to investigate the effects of a modified University of Wisconsin (UW) solution supplemented with one of four buffering agents (histidine, bicine N,N-bis(2-hydroxyethyl)glycine], tricine N-tris(hydroxymethyl)methylglycine], and Tris) on liver metabolism during cold ischemic storage. METHODS: Rat livers were flushed and stored for a maximum period of 24 hr at 4 degrees C, and tissue energetics, substrate, and anaerobic end-products were assessed; the group exhibiting the best results during storage was recovered in a 60-min period of warm reperfusion. Relative buffering capacities of the experimental solutions (measured over physiological pH range, in mM H+/L) were: UW, 4.1; histidine+UW, 9.8; Tris+UW, 19.0; bicine+UW, 22.5; tricine+UW, 26.8. RESULTS: In the UW group, ATP levels dropped rapidly over the first 4 hr; 1.0 micromol/g (40% of initial) remained after 4 hr of storage. By 2 hr, ATP levels in bicine- and tricine-treated groups were 0.5 and 1.1 micromol/g greater than in the UW-stored livers and by 10 hr, ATP in bicine-treated livers was twofold that of the control (UW) group. Total adenylate levels also reflected a superior elevation of cellular energetics; even after 24 hr, quantities were 1.4 and 2.0 micromol/g higher than the UW group in bicine- and histidine-supplemented organs. The increase in energetics occurred as a result of increased flux through the major anaerobic energy-producing pathway, glycolysis. The glycolytic rate was significantly greater at storage times > 10 hr with solutions supplemented with bicine, histidine, and tricine. Final values for net lactate accumulation over the entire 24-hr storage period were: UW, 10.1 micromol/g; histidine, 14.3 micromol/g; bicine, 15.2 micromol/g; tricine, 13.8 micromol/g. Activities of glycogen phosphorylase revealed that the activity of this enzyme dropped by 50% within 2 hr of storage in UW. However, histidine and bicine supplementation resulted in a substantial elevation of phosphorylase "a" over 4 hr and 10 hr, respectively. The best buffer of the four examined in this study was bicine; energetics, glycolytic flux, and patterns of adenylate regeneration upon reperfusion were markedly superior to modified UW solution. CONCLUSION: The results of this study suggest that supplementing the "gold standard" UW solution with an additional buffering agent (in order of efficacy: bicine>tricine>histidine) may improve the metabolic status of livers during clinical organ retrieval/storage.