Pörksen, Niels

Although it is well known that somatostatin inhibits net insulin secretion, it is unknown whether this is achieved by regulation of the basal or pulsatile components of insulin secretion and, if the latter, whether this is through modulation of pulse mass or frequency. We addressed these questions with a canine model. Portal vein blood was sampled at 1-min intervals in five dogs for 60 min before (basal) and 90 min after ingestion of 30 g glucose on two different occasions, during a saline (SAL) or a somatostatin (SMS, 175 ng/min) infusion. Plasma glucose concentrations were similar during SAL and SMS. SMS had no effect on pulse frequency before (8.4 +/- 0.7 vs. 9.2 +/- 1.0 pulses/h, SMS vs. SAL, P = 0.54) or after glucose (13.3 +/- 1.1 vs. 11.6 +/- 0.9 pulses/h, SMS vs. SAL, P = 0.22). In contrast, SMS decreased insulin pulse mass in the postabsorptive (84 +/- 28 vs. 214 +/- 73 pmol/pulse, SMS vs. SAL, P < 0.05) and fed states (676 +/- 143 vs. 913 +/- 183 pmol/pulse, SMS vs. SAL, P < 0.05). In the postabsorptive state, SMS decreased insulin clearance by approximately 50% (0.32 +/- 0.04 vs. 0.60 +/- 0.09 l/min, P < 0.05), but after glucose ingestion, insulin clearance was comparable during SMS or SAL (0.72 +/- 0.04 vs. 0.80 +/- 0.08 l/min, P = 0.4). SMS appeared to alter insulin clearance through modulation of insulin pulse amplitude, because in the postabsorptive state clearance was closely correlated to the pulse amplitude (r = + 0.87, P < 0.0001). In conclusion, somatostatin regulates the rate of insulin secretion by selective inhibition of pulsatile insulin secretion. Regulation of secretory burst mass (and amplitude) may secondarily influence transhepatic and thus total body clearance of endogenously secreted insulin and thereby serve as a novel mechanism to dictate the systemic insulin concentration.

The purpose of the present study was to determine the contributions of discrete insulin secretory bursts vs. basal insulin release to total insulin secretion in vivo. Quantification of the partitioning of pulsatile and basal insulin secretion is complicated by physiological delivery of these pulses into the portal vein and the absence of validated methods of measuring the rates of pulsatile and basal insulin secretion in vivo. We therefore 1) developed a canine model with chronically implanted portal vein catheters, 2) validated an established deconvolution technique as well as a novel direct catheterization technique (Clustcath) for measurement of pulsatile and nonpulsatile insulin secretion rates in this model, and 3) applied these methods to study insulin secretion in the overnight-fasted dog in vivo to determine the contribution of pulsatile vs. basal insulin secretion to total rates of endogenous insulin secretion. Rates of total, pulsatile, and nonpulsatile endogenous insulin secretion measured by Cluscath closely parallel those measured by deconvolution analysis (54 +/- 15 vs. 51 +/- 11, 38 +/- 12 vs. 36 +/- 11, and 16 +/- 4 vs. 14 +/- 4 pmol/min, respectively). Clustcath and deconvolution indicated that the majority of insulin was secreted as pulses (70 +/- 6 and 66 +/- 7%, respectively). These data infer that any process that selectively decreases the pulsatile component of insulin secretion (e.g., diabetes mellitus) will likely have a major impact on total insulin secretion.

Impaired insulin secretion in type 2 diabetes is characterized by decreased first-phase insulin secretion, an increased proinsulin-to-insulin molar ratio in plasma, abnormal pulsatile insulin release, and heightened disorderliness of insulin concentration profiles. In the present study, we tested the hypothesis that these abnormalities are at least partly reversed by a period of overnight suspension of β-cell secretory activity achieved by somatostatin infusion. Eleven patients with type 2 diabetes were studied twice after a randomly ordered overnight infusion of either somatostatin or saline with the plasma glucose concentration clamped at ∼8 mmol/l. Controls were studied twice after overnight saline infusions and then at a plasma glucose concentration of either 4 or 8 mmol/l. We report that in patients with type 2 diabetes, 1) as in nondiabetic humans, insulin is secreted in discrete insulin secretory bursts; 2) the frequency of pulsatile insulin secretion is normal; 3) the insulin pulse mass is diminished, leading to decreased insulin secretion, but this defect can be overcome acutely by β-cell rest with somatostatin; 4) the reported loss of orderliness of insulin secretion, attenuated first-phase insulin secretion, and elevated proinsulin-to-insulin molar ratio also respond favorably to overnight inhibition by somatostatin. The results of these clinical experiments suggest the conclusion that multiple parameters of abnormal insulin secretion in patients with type 2 diabetes mechanistically reflect cellular depletion of immediately secretable insulin that can be overcome by β-cell rest.

Little is known about the optimal experimental conditions for assessing pulsatile insulin secretion in vivo. To address this, we employed a recently validated canine model (n = 12) to determine the consequences of 1) sampling from the systemic circulation (SC) vs. the portal vein (PV), 2) sampling intensity and duration, and 3) deconvolution vs. cluster analysis on assessing pulsatile insulin secretion. PV vs. SC sampling resulted in a approximately 40% higher pulse frequency by deconvolution (9.0 +/- 0.5 vs. 6.6 +/- 0.9 pulses/h, P < 0.02) and cluster analysis (7.5 +/- 0.3 vs. 5.6 +/- 0.6 pulses/h, P < 0.01) due to a higher signal-to-noise ratio (19 +/- 4.8 PV vs. 12 +/- 1.8 SC). PV sampling also disclosed a higher calculated contribution of the pulsatile vs. nonpulsatile mode of delivery to total insulin secretion (57 +/- 4 vs. 28 +/- 5%, P < 0.001). Analysis of the relevance of sampling intensity revealed that 1-min data yielded a markedly higher estimate of pulse frequency with PV sampling than 2-min data (9.0 +/- 0.5 vs. 5.4 +/- 0.5, P < 0.02, deconvolution; 7.5 +/- 0.3 vs. 4.3 +/- 0.6 pulses/h, P < 0.001, cluster). Optimal sampling duration was shown to be 40 min or more. We conclude that the resolving power of the analytical tool, the anatomic site of blood withdrawal, the frequency of blood sampling, and the duration of the total observation interval all significantly influence estimated insulin secretory pulse frequency and the fraction of insulin secreted in pulses. With the assumption that PV 1-min insulin data constitute the "gold standard," our in vivo inferences of 7.5-9.0 insulin pulses/h closely recapitulate in vitro islet secretory activity.

Although sulfonylureas enhance insulin secretion, it is unknown whether these hypoglycemic chemicals stimulate insulin secretion through the augmentation of the pulsatile or basal modes of insulin release. Enhanced pulsatile insulin could occur in turn through amplification of the burst mass or an increase in burst frequency. To address the mechanism of sulfonylurea action, we employed a recently validated canine model with a portal vein sampling catheter and flow probe to measure pulsatile insulin secretion in vivo directly in response to tolbutamide infusion or ingestion. After a 16-h fast, seven dogs were studied in the postabsorptive basal state and during a tolbutamide (0.2 mg/min) infusion when their plasma glucose concentrations were clamped at euglycemia. Insulin concentrations in the carotid artery (basal vs. tolbutamide, 85 ± 12 vs. 325 ± 66 pmol/l; P &lt; 0.01) and portal vein (basal vs. tolbutamide, 345 ± 55 vs. 1,288 ± 230 pmol/l; P &lt; 0.01) increased during tolbutamide infusion, but the portal vein plasma flow did not change. Increased plasma insulin concentrations were achieved by a fourfold increase in the total insulin secretion rate (2.3 ± 0.2 to 9.4 ± 1.9 pmol · kg−1 · min−1; basal vs. tolbutamide, P &lt; 0.01). The augmented total insulin secretion was achieved mechanistically via a marked and selective increase in the insulin secretory burst mass (basal vs. tolbutamide, 266 ± 64 vs. 817 ± 144 pmol/pulse; P &lt; 0.01), with no change in portal-vein insulin pulse frequency (basal vs. tolbutamide, 10.1 ± 0.6 vs. 11.1 ± 0.8 pulses/h; P = 0.3). Oral (250 mg) tolbutamide also magnified the endogenous insulin secretion rate by the preferential amplification of the secretory pulse mass (basal vs. tolbutamide, 167 ± 37 vs. 362 ± 50 pmol/pulse; P &lt; 0.01). Neither the infusion nor the ingestion of tolbutamide changed the calculated clearance rates of endogenously secreted insulin. We conclude that sulfonylurea (tolbutamide) induced insulin secretion in vivo is achieved by the highly selective amplification of insulin secretory burst mass with no change in basal insulin release or the frequency of the β-cell-network pacemaker.

In the present studies, we used a recently validated canine model to determine 1) if glucose ingestion stimulates insulin secretion by amplifying the pulsatile component of insulin release, and if so, 2) whether this effect is achieved preferentially through burst mass or frequency modulation, and 3) if the mechanism of incretin effect of insulin secretion is mediated via the pulsatile mode of secretion. We report that 30 g of glucose ingestion stimulates an ∼ 550% increase in the overall rate of insulin secretion (1.8 ± 0.2 to 11.6 ± 1.5 pmol.kg−1 · min−1), which is achieved via an ∼ 400% increase in the mass of insulin secreted per burst (202 ± 38 to 1,003 ± 147 pmol/pulse, P &lt; 0.001) and a ∼ 40% increase in burst frequency (8.7 ± 0.5 to 12.3 ± 0.6 pulse/h, P &lt; 0.001). Of the insulin secreted after glucose ingestion, 68% (±4) was released in discrete secretory bursts. Further analyses showed that the incretin effect of ingested (GPO) versus infused glucose (GIV) is achieved through regulation of pulsatile insulin secretion. Glucose ingestion led to an ∼ 70% greater rate of insulin secretion than intravenous glucose delivery (10.0 ± 1.6 vs. 5.9 ± 0.9 pmol.kg−1 · min−1, P &lt; 0.005, GPO vs. GIV). This incretin effect was achieved by the specific mechanism of an ∼ 70% greater pulse mass (930 ± 196 vs. 558 ± 97 pmol/pulse, P &lt; 0.02, GPO vs. GIV) but with a comparable pulse frequency (13.1 ± 0.9 vs. 12.0 ± 0.5 pulses/h, P = 0.14, n = 9 dogs, GPO vs. GIV). We conclude that in vivo glucose regulates overall insulin secretion almost exclusively by amplification of the pulsatile mode of insulin secretion, and that the incretin effect is achieved by preferential enhancement of insulin secretory burst mass.