Impact of incretin hormones on beta-cell function in subjects with normal or impaired glucose tolerance.
The mechanisms by which the enteroinsular axis influences beta-cell function have not been investigated in detail. We performed oral and isoglycemic intravenous (IV) glucose administration in subjects with normal (NGT; n = 11) or impaired glucose tolerance (IGT; n = 10), using C-peptide deconvolution to calculate insulin secretion rates and mathematical modeling to quantitate beta-cell function. The incretin effect was taken to be the ratio of oral to IV responses. In NGT, incretin-mediated insulin release [oral glucose tolerance test (OGTT)/IV ratio = 1.59 +/- 0.18, P = 0.004] amounted to 18 +/- 2 nmol/m(2) (32 +/- 4% of oral response), and its time course matched that of total insulin secretion. The beta-cell glucose sensitivity (OGTT/IV ratio = 1.52 +/- 0.26, P = 0.02), rate sensitivity (response to glucose rate of change, OGTT/IV ratio = 2.22 +/- 0.37, P = 0.06), and glucose-independent potentiation were markedly higher with oral than IV glucose. In IGT, beta-cell glucose sensitivity (75 +/- 14 vs. 156 +/- 28 pmol.min(-1).m(-2).mM(-1) of NGT, P = 0.01) and potentiation were impaired on the OGTT. The incretin effect was not significantly different from NGT in terms of plasma glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide responses, total insulin secretion, and enhancement of beta-cell glucose sensitivity (OGTT/IV ratio = 1.73 +/- 0.24, P = NS vs. NGT). However, the time courses of incretin-mediated insulin secretion and potentiation were altered, with a predominance of glucose-induced vs. incretin-mediated stimulation. We conclude that, under physiological circumstances, incretin-mediated stimulation of insulin secretion results from an enhancement of all dynamic aspects of beta-cell function, particularly beta-cell glucose sensitivity. In IGT, beta-cell function is inherently impaired, whereas the incretin effect is only partially affected.
Muscelli
Elza
E
Dept. of Internal Medicine, University of Pisa, 56122 Pisa, Italy.
Mari
Andrea
A
Natali
Andrea
A
Astiarraga
Brenno D
BD
Camastra
Stefania
S
Frascerra
Silvia
S
Holst
Jens J
JJ
Ferrannini
Ele
E
D000328:Adult; D001786:Blood Glucose / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005260:Female; D005749:Gastric Inhibitory Polypeptide / Q000378:metabolism; D052216:Glucagon-Like Peptide 1 / Q000378:metabolism; D018149:Glucose Intolerance / Q000378:metabolism*; D005951:Glucose Tolerance Test; D006728:Hormones / Q000494:pharmacology*; D006801:Humans; D007328:Insulin / Q000097:blood / Q000378:metabolism; D050417:Insulin-Secreting Cells / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D011863:Radioimmunoassay
D001786:Blood Glucose; D005230:Fatty Acids, Nonesterified; D006728:Hormones; D007328:Insulin; D005749:Gastric Inhibitory Polypeptide; D052216:Glucagon-Like Peptide 1