Insulin secretion in the conscious mouse is biphasic and pulsatile

Abstract

Islets in most species respond to increased glucose with biphasic insulin secretion, marked by a sharp first-phase peak and a slowly rising second phase. Mouse islets in vitro, however, lack a robust second phase. To date, this observation has not been extended in vivo. We thus compared insulin secretion from conscious mice with isolated mouse islets in vitro. The arterial plasma insulin response to a hyperglycemic clamp was measured in conscious mice 1 wk after surgical implantation of carotid artery and jugular vein catheters. Mice were transfused using clamps with blood from a donor mouse to maintain blood volume, allowing frequent arterial sampling. When plasma glucose in vivo was raised from ϳ5 to ϳ13 mM, insulin rose to a first-phase peak of 403 Ϯ 73% above basal secretion (n ϭ 5), followed by a rising second phase of mean 289 Ϯ 41%. In contrast, perifused mouse islets (ϳ75 islets/trial) responded with a similar first phase of 508 Ϯ 94% (n ϭ 4) but a smaller and virtually flat second phase of 169 Ϯ 9% (n ϭ 4, P Ͻ 0.05). Furthermore, the slope of the second-phase response differed significantly from zero in mice (2.63 Ϯ 0.39%/min, P Ͻ 0.01), in contrast to perifused islets (0.18 Ϯ 0.14%/min, P Ͼ 0.30). Mice also displayed pulsatile patterns in insulin concentration (period: 4.2 Ϯ 0.4 min, n ϭ 8). Conscious mice thus responded to increased glucose with biphasic and pulsatile insulin secretion, as in other species. The robust second phase observed in vivo suggests that the processes needed to generate second-phase insulin secretion may be abrogated by islet isolation.insulin; secretion; first phase; second phase; biphasic; mouse; in vivo; pulses; pulsatile INSULIN SECRETION OCCURS IN A BIPHASIC MANNER in response to rapid increases in glucose concentration (2,8,20,45). Firstphase secretion consists of a sharp peak that occurs within 2-5 min of a glucose challenge (29). It is generally accepted that the first-phase peak results from the glucose stimulus increasing ␤-cell ATP/ADP to close ATP-dependent K ϩ (K ATP ) channels, and in turn, depolarizing the ␤-cell membrane potential. This results in the opening of voltage-gated calcium channels, promoting the influx of calcium and secretion of an immediately releasable pool of granules (15,37,46). Secondphase secretion consists of a sustained elevation of insulin secretion, which rises over a period of several minutes (2,8,20,45) and is typically accompanied by insulin pulses occurring at 5-to 15-min intervals (34).This description of biphasic insulin secretion is typical of nearly all species, whether measured from in vivo plasma concentrations (6, 7), perfused pancreas (8, 12), or perifused islets in vitro (21). A notable exception is the isolated mouse islet, which displays a small and relatively static second phase after a fairly typical first-phase response. This type of flat second phase has been observed using a variety of mouse preparations, including the perfused pancreas and cultured islets, and is seen in several different mouse strains (4,13,23,26,42,5...

Dynamin 2 regulates biphasic insulin secretion and plasma glucose homeostasis

“…The first phase peaked at approximately 5 minutes after stimulation and deceased within the following 5~10 minutes; the second phase was relatively slow and long lasting. Such biphasic GSIS in control islets closely mimicked insulin release in vivo, as demonstrated in awake mice under hyperglycemic clamp (6). However, GSIS in KO mice was markedly decreased in the second phase ( Figure 3A).…”

“…EM was carried out as described previously (37), with minor modifications. Purified pancreatic islets were fixed with 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer for 2 hours and post-fixed in 1% OsO 4 , 1.5% K 4 Fe(CN) 6 , and 0.1 M sodium cacodylate for 1 hour. Islets were en bloc stained, dehydrated, embedded, and cut into ultrathin sections (70-90 nm), followed by imaging and analysis (see the Supplemental Methods for details).…”

“…Different control groups had a comparable body weights and responded similarly to glucose stimulation. They maintained a plateau level of glucose, without further significant increase at 30 minutes after the first injection ( Figure 2M), indicating a strong capacity of glucose handling in vivo to stabilize healthy levels of plasma glucose, brane capacitance studies in β cells demonstrate heterogeneous endocytosis kinetics and different endocytosis modulation by Ca 2+ (25,26), IP 6 (27), G proteins (28), and actin (26). However, the molecular nature of endocytosis machinery in β cells is poorly understood, and how endocytosis regulates β cell function and glucose homeostasis in vivo is unknown.…”

“…In response to continuous exposure to high concentrations of glucose, insulin release proceeds in two distinct temporal phases in both humans (4) and rodents (5,6), including a fast, transient first phase and a slow, sustained second phase. In patients with T2D, the second phase is strongly reduced and the first phase is nearly abolished (7).…”

Of rodents and men: Species-specific glucose regulation and type 2 diabetes research

“…extracellular AtP provides an important autocrine regulatory mechanism that enhances the sensitivity and responsiveness of the human β-cell to glucose (Jacques-Silva et al, sociation (Nunemaker et al, 2006). This would complicate interspecies extrapolation by raising the possibility that mice may contain a labile signaling factor absent in humans and rats.…”