EFFECTIVENESS OF NATEGLINIDE ON IN WTRO 77 350 300 []- 100 uM TOL overnight []+ 100 uM TOL overnight Control uM NAT 100 nM GLY 50 nM REP 100 nM GLI FIGURE 4 Insulin release during hour static incubation in response to acute challenge by antidiabetic agents from rat islets following overnight treatment without (empty bars) and with (filled bars) 1001xM TOL at G8. Data were pooled from 16 independent samples/condition and expressed as % of appropriate controls TOL). *p < 0.05 and **p < 0.005 when compare between data with and without pretreatment. Basal insulin secretion during static incubation without and with TOL pretreatment was, respectively, 210 22 and 314 16 xU/islet/lh. treatment of islets with or without 100txM TOL are illustrated in Figure 4. NAT was able to stimulate insulin by 240 23% in control and 160 35 after TOL pretreatment. This change was not statistically significant. On the other hand, the stimulation factors in control or TOL- pretreated islet groups were, respectively, 257+27% or 134+23% with GLY, 298 +27% or 137 + 13% with REP, and 260+38% or 155 +20% with GLI. The results reinforced the argument that islets desensitized to SUs retain responsive- ness to NAT but not GLY, REP and GLI. The changes of insulin secretion in both GLY and TOL pretreated groups are summarized in the Table I. DISCUSSION The present study demonstrated that prolonged exposure of pancreatic islets to SUs, GLY or -TOL, impaired their ability to secrete insulin in response to subsequent secretagogue stimulation a phenomenon known as islet desensitization. Similar results have been previously reported for both drugs. [8-10,16] As KATp channel blockade plays key role in the mechanism of action of SUs and all test drugs in this study, one possible mechanism for SU desensitization is that chronic exposure to SUs would result in lasting binding of SU to SUR1 and closing of KATP channels, and in turn, a lasting membrane depolarization. The constant depolarization would lead the ]3-cells to a refractory state, in which they respond less effectively to a further stimulation of closing KATP channels by test secretagogues. Evidence for this mechanism of SU desensitization associated with chronic exposure to GLY has been previously reported. [111 If this were indeed the case, SU desensitization is likely to be a temporary and reversible condition of refractoriness of the ]3-cells to secretagogue stimulation, since KATP channel activity would eventually recover upon with- drawal of SUs. The key finding in the study was that NAT was the only drug of four test agents which maintained its insulinotropic efficacy in islets desensitized to SUs. Given that islets were still able to vigorously respond to NAT after 18h pretreatment of SUs, the exhaustion of a finite insulin store and/or islet secretory machinery did not appear to be the underlying defect in TABLE Insulin secretion during lh static incubation following overnight treatment of SUs (% of control) -10txM GLY overnight + 10txM GLY overnight -100p,M TOL overnight + 100xM TOL overnight 51xMNAT 126 20 155 14 240 23 160 35 100nM GLY 173 21 112 15 257 27 134 23** 50nM REP 191 18 112 11" 298 27 137 13"* 100nM GLI 251 33 138 __+ 17" 260 38 155 20* All data in SU-pretreated and non-pretreated groups are expressed, respectively, as percent of insulin secretion in appropriate control groups without acute stimulation by secretagogues. *p < 0.05 and **p < 0.005 on data with SU treatment compared to those without. INTERNATIONAL JOURNAL OF EXPERIMENTAL DIABETES RESEARCH