Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine. II. Electrophysiological and fluorimetric studies

Changes in granule mobility and age contribute to changes in insulin secretion after desensitization or rest

INTRODUCTION

Functional impairment of the stimulus secretion coupling in pancreatic beta cells is an essential component of type 2 diabetes. It is known that prolonged stimulation desensitizes the secretion of insulin and thus contributes to beta cell dysfunction. Beta cell rest, in contrast, was shown to enhance the secretory response. Here, the underlying mechanisms were investigated.

RESEARCH DESIGN AND METHODS

To characterize the consequences of desensitization or rest for the number and mobility of submembrane granules, insulin-secreting MIN6 cells were desensitized by 18-hour culture with 500 µM tolbutamide or rested by 18-hour culture with 1 µM clonidine. The granules were labeled by hIns-EGFP or hIns-DsRed E5, imaged by TIRF microscopy of the cell footprint area and analyzed with an observer-independent program. Additionally, the insulin content and secretion were measured.

RESULTS

Concurrent with the insulin content, submembrane granules were only slightly reduced after desensitization but markedly increased after rest. Both types of pretreatment diminished arrivals and departures of granules in the submembrane space and increased the proportion of immobile long-term resident granules, but desensitization lowered and rest increased the number of exocytoses, in parallel with the effect on insulin secretion. Labeling with hIns-DsRed E5 ('timer') showed that desensitization did not affect the proportion of aged granules, whereas rest increased it. Aged granules showed a high mobility and made up only a minority of long-term residents. Long-term resident granules were more numerous after rest and had a lower lateral mobility, suggesting a firmer attachment to the membrane.

CONCLUSION

The number, mobility and age of submembrane granules reflect the preceding functional states of insulin-secreting cells. Representing the pool of releasable granules, their quantity and quality may thus form part of the beta cell memory on renewed stimulation.

The role of membrane excitability in pancreatic β-cell glucotoxicity

Persistent hyperglycemia is causally associated with pancreatic β-cell dysfunction and loss of pancreatic insulin. Glucose normally enhances β-cell excitability through inhibition of K_ATP channels, opening of voltage-dependent calcium channels, increased [Ca²⁺]_i, which triggers insulin secretion. Glucose-dependent excitability is lost in islets from K_ATP-knockout (K_ATP-KO) mice, in which β-cells are permanently hyperexcited, [Ca²⁺]_i, is chronically elevated and insulin is constantly secreted. Mouse models of human neonatal diabetes in which K_ATP gain-of-function mutations are expressed in β-cells (K_ATP-GOF) also lose the link between glucose metabolism and excitation-induced insulin secretion, but in this case K_ATP-GOF β-cells are chronically underexcited, with permanently low [Ca²⁺]_i and lack of glucose-dependent insulin secretion. We used K_ATP-GOF and K_ATP-KO islets to examine the role of altered-excitability in glucotoxicity. Wild-type islets showed rapid loss of insulin content when chronically incubated in high-glucose, an effect that was reversed by subsequently switching to low glucose media. In contrast, hyperexcitable K_ATP-KO islets lost insulin content in both low- and high-glucose, while underexcitable K_ATP-GOF islets maintained insulin content in both conditions. Loss of insulin content in chronic excitability was replicated by pharmacological inhibition of K_ATP by glibenclamide, The effects of hyperexcitable and underexcitable islets on glucotoxicity observed in in vivo animal models are directly opposite to the effects observed in vitro: we clearly demonstrate here that in vitro, hyperexcitability is detrimental to islets whereas underexcitability is protective.

Antagonism of the insulinotropic action of first generation imidazolines by openers of KATP channels

The antagonism between K(ATP) channel-blocking insulinotropic imidazolines - phentolamine, alinidine, idazoxan and efaroxan - and K(ATP) channel openers, diazoxide and nucleoside diphosphates, was studied in mouse pancreatic islets and B-cells. In inside-out patches from B-cells, 500muM MgGDP abolished the inhibitory effect of the imidazolines. 300muM diazoxide further increased channel activity. The depolarizing effect of all imidazolines (100muM) on the B-cell membrane potential was practically completely antagonized by 300muM diazoxide. In contrast, diazoxide was unable to decrease the cytosolic Ca(2+) concentration ([Ca(2+)](i)) which was elevated by phentolamine, whereas the [Ca(2+)](i) increases induced by the other imidazolines were promptly antagonized. The effects on [Ca(2+)](i) were reflected by the secretory activity in that the stimulatory effects of alinidine, idazoxan and efaroxan, but not that of phentolamine were antagonized by diazoxide. Metabolic inhibition of intact B-cells by 250muM NaCN, most likely by a decrease of the ATP/ADP ratio, significantly diminished the K(ATP) channel-blocking effect of a low concentration of alinidine (10muM), whereas efaroxan proved to be susceptible even at a highly effective concentration (100muM). This may explain the oscillatory pattern of the [Ca(2+)](i) increase typically produced by efaroxan in pancreatic B-cells. In conclusion, the inhibitory effect of imidazolines on K(ATP) channels, which is exerted at the pore-forming subunit, Kir6.2, is susceptible to the action of endogenous and exogenous K(ATP) channel openers acting at the regulatory subunit SUR, which confers tissue specificity. With intact cells this antagonism can be obscured, possibly by intracellular accumulation of some imidazolines.

Desensitization of insulin secretion by depolarizing insulin secretagogues

Prolonged stimulation of insulin secretion by depolarization and Ca2+ influx regularly leads to a reversible state of decreased secretory responsiveness to nutrient and nonnutrient stimuli. This state is termed "desensitization." The onset of desensitization may occur within 1 h of exposure to depolarizing stimuli. Desensitization by exposure to sulfonylureas, imidazolines, or quinine produces a marked cross-desensitization against other ATP-sensitive K+ channel (KATP channel)-blocking secretagogues. However, desensitized beta-cells do not necessarily show changes in KATP channel activity or Ca2+ handling. Care has to be taken to distinguish desensitization-induced changes in signaling from effects due to the persisting presence of secretagogues. The desensitization by depolarizing secretagogues is mostly accompanied by a reduced content of immunoreactive insulin and a marked reduction of secretory granules in the beta-cells. In vitro recovery from a desensitization by the imidazoline efaroxan was nearly complete after 4 h. At this time point the depletion of the granule content was partially reversed. Apparently, recovery from desensitization affects the whole lifespan of a granule from biogenesis to exocytosis. There is, however, no direct relation between the beta-cell granule content and the secretory responsiveness. Even though a prolonged exposure of isolated islets to depolarizing secretagogues is often associated with the occurrence of ultrastructural damage to beta-cells, we could not find a cogent link between depolarization and Ca2+ influx and apoptotic or necrotic beta-cell death.

β-Cell toxicity of ATP-sensitive K+ channel-blocking insulin secretagogues

A prolonged exposure of isolated pancreatic islets to insulin secretagogues, the imidazolines phentolamine, alinidine and idazoxan (100microM each), the sulfonylurea tolbutamide (500microM), or the alkaloid quinine (100microM) resulted in morphological damage of 4-18% of beta-cells compared to less than 2% in controls. Thus, the question arose whether K(ATP) channel-blocking insulin secretagogues are beta-cell toxic as has already been suggested for sulfonylureas. The concentration- and time-dependency of the secretagogue-associated toxicity was documented by viability assays in insulin-secreting HIT T15 cells. Treatment for 24h with idazoxan reduced MTT conversion by 50% at 100microM and by 98% at 1000microM. Phentolamine and quinine reduced viability comparably at 1000microM, but were less toxic at 100microM. On the other hand, the imidazoline alinidine and the sulfonylurea tolbutamide were only moderately toxic (less than 40% viability loss at 1000microM). The imidazoline efaroxan appeared even to be non-toxic. Apoptotic DNA fragmentation and DEVD-caspase activation was observed at 100microM of idazoxan and phentolamine, whereas at 1000microM signs of necrosis predominated. Alinidine, tolbutamide and quinine treatment did not increase markers of apoptotic cell death. Blocking Ca(2+) influx by D600 did not diminish secretagogue-associated toxicity. Electron microscopy confirmed the validity of these observations for beta-cells in intact mouse islets. In summary, beta-cell toxicity of the tested insulin secretagogues varied widely and did not depend on a prolonged Ca(2+) influx via L-type Ca(2+) channels. Thus, secretagogue-mediated closure of K(ATP) channels is apparently not per se beta-cell toxic.

Desensitization of insulin secretion

Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.

Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine. I. Secretory and morphological studies

The desensitization of pancreatic B-cells against stimulation by insulin secretagogues that inhibit ATP-dependent K(+) channels (K(ATP) channels) was investigated by measuring insulin secretion of perifused pancreatic islets. Additionally, the islet insulin content and the number of secretory granules per B-cell were determined. Prior to the measurement of secretion, islets were cultured for 18 h in the presence or absence of the test agents in a cell-culture medium containing 5 mM glucose. The effects of three imidazolines, phentolamine, alinidine, and idazoxan (100 microM each) were compared with those of the well-characterized sulfonylurea, tolbutamide (500 microM), and those of the ion channel-blocking alkaloid, quinine (100 microM). Insulin secretion was strongly reduced upon re-exposure to phentolamine, alinidine, tolbutamide, and quinine, whereas idazoxan, which stimulated secretion only weakly, had no significant effect. The imidazoline secretagogues phentolamine and alinidine induced a cross-desensitization against the stimulatory effect of tolbutamide and quinine. A long-term depolarization with 40 mM KCl was also able to induce a significant reduction of the secretory response to all of the above secretagogues. The insulin content of cultured islets was moderately, but significantly reduced by alinidine, whereas the reduction by phentolamine, tolbutamide, and quinine was not significant. In contrast to these observations, the ultrastructural examination revealed that tolbutamide-treated B-cells had a high degree of degranulation, whereas the other test agents and 40 mM KCl produced only a partial degranulation, except for phentolamine, which produced no significant degranulation at all. These results suggest that the desensitization of insulin secretion is a common property of all agents that stimulate insulin secretion by depolarisation of the plasma membrane. Depending on the specific secretagogue, additional mechanisms, proximal and distal to Ca(2+) influx, appear to contribute to the desensitization (see Rustenbeck et al., pages 1695-1703, this issue).