The Role of Ca²⁺-Related Events in Glucose-Stimulated Desensitization of Insulin Secretion*

Abstract

The spontaneous decline of insulin secretion (third phase) that occurs under a variety of secretory conditions is well documented and suggests a general impairment or desensitization of the secretory process. We have examined several aspects of Ca2+ flux as well as regulators of Ca-linked second messenger events in freshly isolated rat islets chronically stimulated with glucose over 24 h, a period that encompasses initial (hour 1), peak (hour 3), and subsequent impaired or desensitized (hour 20-22) secretion. In islets incubated for these periods in HB104 medium with 22 mM glucose, 45Ca2+ uptake did not vary (12.6 +/- 1.6 vs. 10.2 +/- 1.7 vs. 13.2 +/- 3.4 pmol Ca2+/islet.10 min at 1, 3, and 22 h, respectively). Chronic incubation in 2 mM glucose reduced total Ca2+ uptake at each of the time periods, but, again, uptake did not change with desensitization (9.8 +/- 1.4 vs. 6.6 +/- 2.1 vs. 7.8 +/- 2.3 pmol Ca2+/islet.10 min). In 11 mM glucose, the Ca channel antagonist verapamil (1-10 microM) reduced insulin secretion by 55-80% in a dose-dependent manner over 1-3 h; islets continuously exposed to verapamil escaped inhibition by 20 h even at the highest concentration. However, in islets first exposed to 10 microM verapamil only during 20-22 h, hourly insulin secretion was suppressed 25%, 45%, and 33% at 20, 21, and 22 h, respectively, indicating that glucose-desensitized islets were still sensitive to further inhibition of Ca channels. Staurosporine (1 microM), an inhibitor of protein kinase-C activity, progressively inhibited glucose-stimulated insulin secretion from 48% at 1 h to more than 80% by 3 h; again, this inhibitory effect was lost by 20 h of chronic staurosporine. When staurosporine was first administered at 20 h, insulin secretion was modestly suppressed and returned to control values in the next hour. With continuous glucose, the islet response to positive stimulation of endogenous C-kinase activity by carbachol was maintained. The Ca/calmodulin inhibitor trifluoroperazine also inhibited insulin secretion by 75-80% during 1-3 h and continued to exert inhibitory effects through 23 h of continuous administration. We conclude that even though insulin secretion has desensitized to glucose, 1) Ca2+ entry is unchanged and is still regulated by glucose, 2) voltage-dependent Ca channels are still sensitive to blockade by acute verapamil, but can desensitize to chronic verapamil; 3) stimulus-enhanced C-kinase activity may be especially labile during glucose-induced desensitization, while 4) possible Ca/calmodulin potentiation of secretion persists through the three secretory phases.(ABSTRACT TRUNCATED AT 400 WORDS)

Desensitization of the insulin‐secreting beta cell

“…Yet these agents are fully active if added acutely at a time when their chronic effect would have waned. This is illustrated in Figure 6 [45] where the inhibitory effect of verapamil (presumably on the voltagedependent Ca channel) is lost with continuous, but not acute, administration. Similarly, the inhibitory effect of somatostatin diminishes during chronic exposure while inhibition by an acute challenge is retained [37].…”

“…In the presence of added carbachol, the decline of second-phase secretion may be associated with a depletion of endogenous enzyme [59-611. In third phase, insulin secretion loses responsiveness to stimulation of PKC by phorbol ester and to inhibition by staurosporine [45,621. Thus the ability to activate PKC is suppressed in glucosedesensitized islets.…”

“…Similarly, the inhibitory effect of somatostatin diminishes during chronic exposure while inhibition by an acute challenge is retained [37]. This phenomenon is not universally observed since inhibition to trifluoroperazine is retained even after 20 h of continuous administration [45] has the ability to desensitize to both positive and negative regulators of cell function at the level of stimulus-secretion coupled activity. It is probable that a and 6, as well as other non-islet secretory cells, similarly maintain modulated secretory rates.…”

Physiologic Implications of Phosphoinositides and Phospholipase C in the Regulation of Insulin Secretion

“…This could not be attributed to any significant reduction in insulin content of the pancreas, suggesting a lesion not in biosynthesis but in signal transduction. Additionally, Grodsky and Bolaffi probed the evolution of the secretory effect in more detail and the term "third phase," which is after the biphasic pattern, was applied to the reduced secretion that accompanied sustained glycemic stimulation (33,34,(61)(62)(63). Their results suggest that signal generation plays an important role in ␤-cell desensitization.…”

Alterations in Calcium Channel Currents Underlie Defective Insulin Secretion in a Transgenic Mouse

“…Given that insulin secretion requires elevation of [Ca 2ϩ ] i (23,31,32), it is likely that this defect in Ca 2ϩ signaling accounts for the mild hyperglycemic phenotype demonstrated by the CaM-8 mice at the age we have examined (6 -8 days old). Prolonged elevation of blood glucose levels in these young mice acts as a catalyst to further desensitize the beta-cells to external stimuli, so that the diabetic condition will become more severe as the animals get older (33).…”