Insulin, Glucagon and Blood Glucose

Why does oral glucose evoke more insulin than intravenous glucose at the same blood glucose? You can separate the glucose stimulus from the route, then ask what the gut adds. Start with the beta cell’s electrical response; finish with a whole-body glucose balance. The basic endocrine loop is in the endocrine system.

A beta cell reads glucose by burning it

An islet is an endocrine cell cluster in the pancreas; its beta cells secrete insulin. Mouse beta cells take up glucose through GLUT2. In the isolated nondiabetic human beta-cell preparations examined by de Vos and colleagues in 1995, GLUT1 predominated over GLUT2. Predominance does not mean exclusivity. Facilitated diffusion belongs to membrane transport.

Glucokinase phosphorylates glucose at the start of beta-cell glucose metabolism. Treat this phosphorylation as the rate-setting step in a working sensing model. Further metabolism raises ATP relative to ADP; the model connects that rise to closure of KATP channels, potassium channels sensitive to intracellular nucleotides. Less outward potassium current favors depolarization, a shift to a less negative membrane potential. Voltage-gated calcium channels open, calcium enters, and exocytosis, granule fusion with the surface membrane, releases insulin. This is the triggering pathway; further metabolic signals, and cAMP from hormones such as the incretins, amplify how much insulin that calcium releases. The voltage principles are in resting membranes and action potentials; calcium-triggered fusion machinery is in synaptic transmission.

In 1984, Frances M. Ashcroft, Donna E. Harrison and Stephen J. H. Ashcroft recorded single channels in isolated rat beta cells. In cell-attached patch clamp, a pipette seals against a membrane patch while the cell stays intact. If extracellular glucose binding alone caused closure, metabolism would not be required. If metabolism connected glucose to the channel, metabolically processed glucose should inhibit its activity.

A potassium-selective channel active at low glucose closed when glucose increased, and closure required glucose metabolism. That supported a metabolic link to depolarization in this rat preparation. It argued against extracellular binding alone as a sufficient explanation. It did not identify ATP as the messenger or establish the entire secretion sequence.

Separately, Cook and Hales’s 1984 paper reported ATP inhibition from the cytoplasmic side of excised beta-cell membrane patches. Together the experiments support the metabolic-channel model. Earlier, Noma’s 1983 report found that intracellular ATP depressed potassium channels in cyanide-treated mammalian heart cells; neither beta-cell paper by itself establishes priority across tissues. Later reconstitution supports a channel containing Kir6.2 and SUR1; sulfonylureas close it, whereas diazoxide opens it, as illustrated by the reconstitution experiment.

Insulin comes in two phases

A hyperglycemic clamp maintains raised plasma glucose while you follow secretion over time. DeFronzo, Tobin and Andres reported an early insulin burst within the first six minutes in their 1979 human experiments. A progressively increasing second phase follows while the glucose stimulus remains. A maintained stimulus therefore does not produce a flat insulin response.

What changes at the membrane? Ohara-Imaizumi and colleagues used total internal reflection fluorescence (TIRF) microscopy, which selectively images fluorescent structures close to the cell surface. In normal primary rat beta cells in their 2004 study, early fusion mostly involved granules already present at the membrane, which they called docked; second-phase fusion mostly involved newcomer granules. This connects timing to granule traffic in rats. Later TIRF work found that newcomers fusing immediately on arrival also contribute to the first phase, so the split is not strict. It does not establish that every human granule follows the same division.

An insulin concentration in peripheral blood also reflects removal. In the standard processing model, insulin and C-peptide, the connecting peptide cleaved from proinsulin, are co-secreted in a 1:1 molar ratio. C-peptide undergoes little hepatic extraction compared with insulin, consistent with human secretion measurements and clearance research. Deconvolution estimates secretion from a concentration time course using a kinetic model of removal. C-peptide deconvolution estimates prehepatic insulin secretion; it is not a direct count of released granules. The portal-liver context is in absorption and the liver.

Insulin says store, glucagon says release, and the liver hears both

Use this working organ-level model for fed and fasting adult humans. Its supporting literature includes human and rodent experiments; the tissue distinctions matter more than a single circulating insulin value. Lipolysis is breakdown of stored triglyceride into fatty acids and glycerol.

Tissue	Insulin arm of the working model	Glucagon evidence and limit
Liver	Promotes glycogen synthesis and suppresses glucose output; GLUT2 glucose transport is insulin-independent.	Mammalian liver studies support increased glycogen breakdown and glucose synthesis.
Skeletal muscle	Recruits GLUT4 to the surface and increases glucose uptake and glycogen synthesis.	A 2019 review found no support for direct glucagon-receptor signaling in human muscle.
White adipose tissue	Recruits GLUT4 for glucose uptake and suppresses lipolysis.	Physiological-range glucagon did not increase lipolysis in the tested human abdominal subcutaneous depot.

Human muscle biopsies support insulin-dependent GLUT4 redistribution; human liver experiments support insulin-stimulated glycogen synthesis. Hepatic transport and net hepatic glucose output are different variables. The intracellular insulin cascade belongs to PI3K–Akt–insulin signaling. In the standard model, muscle glycogen supplies muscle metabolism rather than exported free glucose because muscle lacks glucose-6-phosphatase.

Greater insulin influence after feeding favors storage and uptake; lower insulin influence during fasting permits greater hepatic output and adipose lipolysis in this model. The fuel relay is in integration of metabolism.

Glucagon acts at hepatic receptors through Gs signaling and cAMP–PKA, increasing glycogenolysis, glycogen breakdown, and gluconeogenesis, new glucose synthesis. Pathway chemistry belongs to gluconeogenesis. In Gravholt and colleagues’ 2001 experiment, microdialysis in seven healthy young men detected no increased abdominal subcutaneous lipolysis with physiological-range glucagon under somatostatin and hormone replacement. Keep that negative result within its tested depot and conditions.

Alpha cells secrete glucagon through calcium-dependent exocytosis. Low glucose may stimulate release through intrinsic electrical sensing and autonomic input. Amino acids can stimulate glucagon in the liver–alpha-cell feedback axis, which links amino-acid availability with hepatic metabolism. Paracrine signals act locally between neighboring cells: insulin from beta cells and somatostatin from delta cells inhibit alpha-cell release in the human islet experiments.

Omar-Hmeadi and colleagues in 2020 compared dispersed human alpha cells with intact donor islets. Dispersed cells showed a V-shaped glucose–exocytosis curve; raised glucose inhibited exocytosis in intact nondiabetic islets. In type 2 diabetes donor islets, 10 mM glucose no longer inhibited exocytosis, although 7 mM still did; inhibition at raised glucose was also lost after blocking the inhibitory effects of paracrine insulin or somatostatin. Reduced surface somatostatin-receptor-2 expression suggested one resistance mechanism. These comparisons support local inhibition without proving an exclusive explanation in vivo. Capozzi and colleagues’ 2019 experiments support cooperation: disrupting proglucagon-peptide signaling reduced stimulated insulin secretion in mouse and human islets, with lower beta-cell cAMP demonstrated in mice.

The gut sends word ahead: incretins

An incretin is a gut hormone that amplifies glucose-stimulated insulin secretion. The incretin effect is the extra insulin response after oral glucose relative to intravenous glucose reproducing the same plasma glucose trajectory, an isoglycemic comparison. Equal administered glucose would not guarantee equal glucose stimuli.

Comparisons of oral and intravenous glucose date to the 1960s; Nauck, Stöckmann, Ebert and Creutzfeldt made the matched-glucose comparison in people with and without type 2 diabetes in 1986. In eight metabolically healthy controls, incretin factors accounted for 72.8 ± 6.9% of the total insulin response; in the 14-member type 2 diabetes cohort, the value was 36.0 ± 8.8%. These are cohort estimates with the paper’s reported error terms, not universal fractions. They establish reduced oral-route amplification in that cohort; they do not locate the change in hormone supply or beta-cell responsiveness.

GIP, glucose-dependent insulinotropic polypeptide, comes from intestinal K cells; GLP-1, glucagon-like peptide 1, comes from L cells, with nutrient stimuli covered in gut hormones. In the working signaling model, both receptors recruit Gs and cAMP to amplify glucose-stimulated secretion rather than provide sufficient triggers alone.

Gasbjerg and colleagues tested separate contributions in 18 healthy men in 2019. Each man completed randomized, double-blind crossover conditions with GIP-receptor blockade, GLP-1-receptor blockade, both, or saline during oral glucose testing. Receptor blockade reduces signaling through the targeted receptor. If one hormone accounted for nearly all amplification, its blockade alone should approach the dual-blockade effect.

Combined blockade reduced the C-peptide-to-glucose-ratio secretion index by 37 ± 16%; GIP-receptor blockade alone reduced it by 30 ± 17%, both mean ± SD relative to saline. GLP-1-receptor blockade alone reduced it by 8.6 ± 16%; the single effects (30 and 8.6) roughly add up to the combined effect (37), which is what the authors mean by additive. The authors concluded additive contributions, with the larger contribution from GIP in this preparation. Combined and GLP-1-receptor blockade also raised glucagon and sped gastric emptying, so GLP-1’s role reaches beyond insulin. The factorial comparison asks about each receptor while varying the other; crossover comparisons reduce differences between individuals, and blinding reduces expectation-related bias. A ratio index is not a secretion flux. Incomplete blockade and different glucose trajectories limit inference, and these shares cannot be assigned unchanged to women or mixed meals. Dividing 30 by 37 does not establish a universal GIP percentage.

Measuring secretion and sensitivity: the OGTT and the clamp

An oral glucose tolerance test (OGTT) follows glucose and, when measured, hormones after oral glucose. The curve integrates absorption, secretion and insulin action; peripheral insulin also reflects hepatic extraction. A larger glucose excursion alone cannot distinguish reduced secretion from reduced action.

A hyperinsulinemic-euglycemic clamp maintains elevated insulin while feedback-controlled glucose delivery holds plasma glucose near its basal value. Compare achieved plateaus, not merely intended conditions. Endogenous glucose production (EGP) is glucose entering blood from internal sources; Rd is whole-body glucose disposal. A tracer, distinguishably labeled glucose, can estimate production separately. Rizza, Mandarino and Gerich’s 1981 healthy-human study found half-maximal production suppression at 29 ± 2 microU per mL insulin, complete suppression near 60 microU per mL, and maximal utilization of 10–11 mg per kg per min. These observed responses are preparation-specific.

Worked problem: account for every source

These made-up analyzed clamp data describe two healthy adults at matched steady insulin and glucose, with negligible urinary glucose loss. External glucose appearance, J, is 5.71 mg per kg per min in A and 2.86 mg per kg per min in B. These are mass-normalized research fluxes.

At steady state: Rd = J + EGP, with every term in mg per kg per min.

First assume EGP is zero. Then A’s Rd is 5.71 mg per kg per min and B’s is 2.86 mg per kg per min. Calculate B/A: 2.86 / 5.71 = 0.501, approximately half. B shows lower insulin-stimulated disposal under these matched conditions; the result does not identify an intracellular defect.

Now a tracer detects A’s EGP at 0.50 mg per kg per min. Add the sources: Rd = 5.71 + 0.50 = 6.21 mg per kg per min. Check by subtraction: 6.21 minus 5.71 = 0.50 mg per kg per min. Reading J alone underestimates disposal by that amount. At fixed Rd, continuing EGP reduces the external input required to hold glucose steady.

Whole-body glucose disposal Rd 6.21 mg per kg per min Residual endogenous production EGP 0.50 mg per kg per min Body mass 70 kg

External glucose J = Rd − EGP

Pump rate = J × mass ÷ 200 mg per mL

Disposal Rd = J + EGP (check)

The same pump rate in two people

At 50 kg

At 100 kg

In this sketch: steady state at matched insulin and glucose in a healthy adult, with negligible urinary glucose loss, as in the lesson’s made-up worked problem; the starting values Rd 6.21 and EGP 0.50 mg per kg per min are that problem’s person A. Made up for the sketch: the slider ranges (the top of the Rd range, 11, follows the 10–11 mg per kg per min maximal utilization that Rizza, Mandarino and Gerich found in healthy humans in 1981, a preparation-specific result), body masses of 50 to 100 kg, and the infusate fixed at 20 percent glucose, 200 mg per mL. The pump rate in mL per min is a unit conversion made up for the sketch; the lesson gives only mg per kg per min. Arrow widths are roughly proportional to flux; there is no time course and no insulin dose-response curve.

At steady state Rd = J + EGP, so the glucose given from outside equals disposal only if endogenous production is suppressed and urinary losses are negligible, and the lesson’s fluxes are mass-normalized, in mg per kg per min.

An area under the curve (AUC) integrates concentration over time. For a separate made-up isoglycemic comparison, incremental insulin areas above baseline are 10,000 and 3,000 pmol per L x min after oral and intravenous glucose. Assuming equal insulin clearance:

Incretin effect = (oral AUC minus intravenous AUC) / oral AUC x 100%.

Subtract to get 7,000 pmol per L x min; divide by 10,000 pmol per L x min to get 0.70; multiply by 100% to get 70%. That is near the 1986 control estimate, but insulin-based and C-peptide-based estimates can differ.

For a mechanism perturbation, reduce beta-cell insulin secretion: muscle and fat uptake should fall and hepatic output should rise, favoring higher plasma glucose. Reduce tissue insulin action instead, holding insulin constant, and the same directions can follow. Type 1 diabetes usually involves profound secretion loss; type 2 combines inadequate secretion with frequently reduced insulin action. Neither label identifies one universal signaling lesion.

Design problem

Nauck’s comparison leaves hormone supply and responsiveness unresolved. Design experiments using plasma from volunteers with and without type 2 diabetes and isolated human donor islets to distinguish lower gut-hormone availability from lower beta-cell responsiveness. Specify controls and the outcomes that would support each mechanism.

One way to do it

Measure circulating GIP and GLP-1 during oral glucose experiments in volunteers with and without type 2 diabetes. For a controlled supply test, compare paired plasma samples with selective hormone depletion and sham depletion, verifying glucose and other measured constituents remain matched. Lower circulating concentrations alone would not prove lower secretion, because clearance also matters.

For responsiveness, expose paired donor-islet aliquots to identical glucose and matched GIP or GLP-1 concentrations, with hormone-free controls and randomized condition order. Measure insulin release and normalize to insulin content and viable beta-cell mass. Compare untreated aliquots with aliquots having experimentally reduced receptor signaling. Less amplification at equal hormone availability supports reduced responsiveness; if depleting the hormone from a sample removes amplification and add-back restores it, the hormone in that sample carries the effect; lower hormone concentrations in samples from people with type 2 diabetes would then point to a supply effect. Both can occur.

As a historical lead, Nauck and colleagues’ 1993 comparison compared infused GIP and GLP-1 under the same clamped glucose in nine people with mild type 2 diabetes and nine matched controls: the maximum effect of GIP was 54% lower in the patients, whereas GLP-1 kept much of its effect (71% of the control increment in C-peptide, not a significant difference). GLP-1, but not GIP, also lowered glucagon in both groups. Receptor blockade during healthy-volunteer oral testing can additionally test endogenous contributions. Islet isolation removes neural and vascular context; depletion may remove other factors. Prior glucose exposure can also change responsiveness: in a 2009 study of eight obese people with type 2 diabetes, four weeks of near-normalized glucose raised the later-phase insulin response to infused GIP and to infused GLP-1 several-fold (Højberg and colleagues, Diabetologia 52:199–207), so donor history and culture conditions also require control.