78 S. HU et al. desensitized secretion to GLY, REP and GLI. The insulinotropic action of REP, GLY and GLI was desensitized by lasting SU exposure, suggestive of common site of action for these drugs. On the other hand, the inability of both TOL and GLY to affect NAT-induced insulin secretion suggested a distinct mode of action of NAT. Provided that the mechanism of desensitization was indeed asso- ciated to the refractoriness of SUR1/KATP chan- nels in ]3-cells, a possible explanation for the effectiveness of NAT vs. ineffectiveness of GLY, REP and GLI in stimulation of insulin following prolonged pretreatment of SUs would be the existence of distinct binding sites on SUR1 for NAT vs. SUs and REP. It is possible that the site on SUR1 for SUs has become hypo-responsive to GLY, GLI and REP due to its desensitization induced by chronic treatment of GLY or TOL, while the site on SUR1 for NAT remained fully available to be activated with or without pretreatment of SUs. Alternatively, the present data would be reconciled by proposing a second mechanism of the insulinotropic action of NAT that is independent of KATP channels. of some that Being cautious not to over-interpret the in vitro findings in the present study, we only speculate aspects signaling pathways mediating NAT’s effect are markedly dissimilar from those involving in the effects of SUs and REP. Although the present study did not deal with time-dependent secretory pattern but cumulative stimulation of insulin secre- tion from islets during certain period of time, earlier works by others and us reveale essential differences of NAT action from those of GLY, GLI and REP with respect to (1) the kinetics of the interaction of NAT with SUR1 receptor/KATp channels; [17] (2) the kinetics of in vitrofin vivo insulinotropic action;[18-22] (3) the glucose-dependence; [23,24] and (4) the sensitivity to metabolic inhibitors. [25] These findings collectively indicated the uniqueness of NAT action and possibly a distinct receptor/ effector system(s). By virtue of these properties, NAT is able to ameliorate postprandial hyper- INTERNATIONAL JOURNAL OF EXPERIMENTAL DIABETES RESEARCH glycemia by augmenting early insulin release, while SUs and REP may preferentially decrease postabsorptive hyperglycemia due to their slower onset and sustained duration of action. In conclusion, the maintenance of insulino- tropic efficacy of NAT in islets desensitized to SUs adds to the list of properties of this agent that distinguish it from other SUR ligands, despite the presumed common basic mechanism of action of said agents. References [1] Aguilar-Bryan, L. and Bryan, J., Molecular biology of adenosine triphosphate-sensitive potassium channels, Endocr. Rev., 20, 101-135. [2] Inagaki, N., Gonoi, T., Clement, J. P., Namba, N., Inazawa, J., Gonzalez, G., Aguilar-Bryan, L., Seino, S. and Bryan, J. (1995). Reconstitution of IKATP: An inward rectifier subunit plus the sulfonylurea receptor, Science, 270, 1166-1170. [3] Ashcroft, F. M. and Rorsman, P. (1989). Electrophys- iology of the pancreatic ]B-cell, Prog. Biophys. Mol. Biol., 54, 87-143. [4] Dunbar, J. C. and Foa, P. P. (1974). An inhibitory effect of tolbutamide and glibenclamide on the pancreatic islets of normal animals, Diabetologia, 10, 27-32. [5] Filipponi, P., Marcelli, M., Nicoletti, I., Pacifici, R., .Santeusanio, E and Brunetti, P. (1983). Suppressive effect of long-term sulfonylurea treatment on A, B, and D-cells of normal rat pancreas, Endocrinology, 113, 1972-1979. [6] Karam, J. H., Sanz, E., Salomon, E. and Nolte, M. S. (1986). Selective unresponsiveness of pancreas B-cells to acute sulphonylurea stimulation during sulphonylurea therapy in NIDDM, Diabetes, 35, 1314-1320. [7] Grunberger, G. (1993). Continuous versus intermittent sulphonylurea therapy in non-insulin-dependent diabetes mellitus, Drug Safety, 9, 249-253. [8] Bolaffi, J. L., Heldt, A., Lewis, L. D. and Grodsky, G. M. (1986). The third phase of in vitro insulin secretion: evidence for glucose insensitivity, Diabetes, 35, 370-373. [9] Zawalich, W. S. (1989). Phosphoinoside hydrolysis and insulin secretion in response to glucose are im- paired in isolated rat islets by prolonged exposure to the sulphonylurea tolbutamide, Endocrinology, 125, 281-286. [10] Gullo, D., Rabuazzo, A. M., Vetri, M., Gatta, C., Vinci, C., Buscema, M., Vigneri, R. and Purrello, F. (1991). Chronic exposure to glibenclamide impairs insulin secretion in isolated rat pancreatic islets, J. Endocrinol. Invest., 14, 287-291. [11] Rabuazzo, A. M., Buscema, M., Vinci, C., Caltabiano, V., Vetri, M., Forte, F., Vigneri, R. and Purrello, F. (1992). Glyburide and tolbutamide induce desensiti- zation of insulin release in rat pancreatic islets by different mechanisms, Endocrinology, 131, 1815-1820. [12] Kahn, S. E., Montgomery, B., Howell, W. M., Ligueros- Saylan, M., Hsu, C. H., Devineni, D., Mcleod, J. F. and Horowitz, A. D. (2000). Nateglinide increases first phase