GIP and GLP-1: Stepsiblings Rather Than Monozygotic Twins Within the Incretin Family

Abstract

The importance of the incretin effect for the postprandial augmentation of insulin released has long been recognized (1). However, there has been a long-standing controversy as to the relative importance of the various gastrointestinal hormones in mediating this effect. While earlier studies suggested a contribution of gastrin, secretin, and cholecystokinin (2), it later became obvious that in humans, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are the predominant incretin hormones (1). Which of the two incretins is the most relevant one is still a matter of debate.Gasbjerg et al.(3) performed studies in healthy human subjects receiving oral glucose loads of 75 g in the fasting state on four different days. The known incretin hormones, GIP and GLP-1, which are secreted in response to glucose administration/absorption, were antagonized with the established peptide receptor antagonist exendin(9-39), blocking the stimulation of GLP-1 receptors, or with the novel GIP receptor antagonist, GIP(3-30)amide, a naturally occurring fragment of intact GIP (full sequence 1-42), which has recently been characterized as a specific GIP receptor antagonist for use in human studies (4). A similar GIP receptor antagonist, GIP(7-30)amide, had previously been used in animal studies but had not been validated for human experiments (5). These incretin receptor antagonists were compared with placebo and used alone and in combination. The result was a slight rise in postload glucose concentrations and a minor reduction in insulin secretory responses, when the GLP-1 receptor was blocked, and a substantial rise in glycemic excursions after the glucose load and a more pronounced reduction in insulin secretory responses, when the recently validated GIP antagonist GIP(3-30)amide was administered at an approximately 1,000-fold excess (;60 nmol/L) over endogenous peak GIP concentrations (;60 pmol/L). The combination of exendin(9-39) and GIP(3-30)amide further raised plasma glucose and reduced insulin secretory responses over and above the degree observed with single receptor blockers. The difference between experiments with exendin(9-39) and GIP(3-30)amide used as single receptor blockers was significant with respect to plasma glucose rises, insulin and C-peptide responses, insulin secretion rates calculated by deconvolution, and various measures of in vivo b-cell function (3). Therefore, the main conclusion from this important study is that blocking GIP receptors has more impact on postglucose insulin secretory responses and on oral glucose tolerance than interfering with GLP-1 action. GIP, thus, is the more effective and more important incretin hormone in healthy subjects receiving an oral glucose challenge of this (75 g) size, which is a standardized challenge used in diagnosing disturbances of glucose tolerance. These findings revise the commonly expressed view that GLP-1 is the most important incretin hormone, a misconception that was mainly based on its therapeutic effectiveness in patients with ...

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Tirzepatide: a glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) dual agonist in development for the treatment of type 2 diabetes

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