The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are degraded by dipeptidyl peptidase IV (DPP IV), thereby losing insulinotropic activity. DPP IV inhibition reduces exogenous GLP-1 degradation, but the extent of endogenous incretin protection has not been fully assessed, largely because suitable assays which distinguish between intact and degraded peptides have been unavailable. Using newly developed assays for intact GLP-1 and GIP, the effect of DPP IV inhibition on incretin hormone metabolism was examined. Conscious dogs were given NVP-DPP728, a specific DPP IV inhibitor, at a dose that inhibited over 90% of plasma DPP IV for the first 90 min following treatment. Total and intact incretin concentrations increased (P<0.0001) following a mixed meal, but on control days (vehicle infusion), intact peptide concentrations were lower (P<0.01) than total peptide concentrations (22.6 +/- 1.2% intact GIP; 10.1 +/- 0.4% intact GLP-1). Following inhibitor treatment, the proportion of intact peptide increased (92.5 +/- 4.3% intact GIP, P<0.0001; 99.0 +/- 22.6% intact GLP-1, P<0.02). Active (intact) incretins increased after NVP-DPP728 (from 4797 +/- 364 to 10 649 +/- 106 pM x min for GIP, P<0.03; from 646 +/- 134 to 2822 +/- 528 pM x m in for GLP-1, P<0.05). In contrast, total incretins fell (from 21 632 +/- 654 to 12 084 +/- 1723 pM x min for GIP, P<0.002; from 5145 +/- 677 to 3060 +/- 601 pM x min for GLP-1, P<0.05). Plasma glucose, insulin and glucagon concentrations were unaltered by the inhibitor. We have concluded that DPP IV inhibition with NVP-DPP728 prevents N-terminal degradation of endogenous incretins in vivo, resulting in increased plasma concentrations of intact, biologically active GIP and GLP-1. Total incretin secretion was reduced by DPP IV inhibition, suggesting the possibility of a feedback mechanism.

The circadian rhythms of most night shift workers do not adapt fully to the imposed behavioural schedule, and this factor is considered to be responsible for many of the reported health problems. One way in which such disturbances might be mediated is through inappropriate hormonal and metabolic responses to meals, on the night shift. Twelve healthy subjects (four males and eight females) were studied on three occasions at the same clock time (1330 h), but at different body clock times, after consuming test meals, first in their normal environment, secondly after a forced 9 h phase advance (body clock time approximately 2230 h) and then again 2 days later in the normal environment. They were given a low-fat pre-meal at 0800 h, then a test meal at 1330 h with blood sampling for the following 9 h. Parameters measured included plasma glucose, non-esterified fatty acids (NEFAs), triacylglycerol (TAG), insulin, C-peptide, proinsulin and glucose-dependent insulinotropic polypeptide, and urinary 6-sulphatoxymelatonin. In contrast with a previous study with a high-fat pre-meal, postprandial glucose and insulin responses were not affected by the phase shift. However, basal plasma NEFAs were lower immediately after the phase shift (P < 0.05). Incremental (difference from basal) TAG responses were significantly higher (P < 0.05) immediately after the phase shift compared with before. Two-day post-phase shift responses showed partial reversion to baseline values. This study suggests that it takes at least 2 days to adapt to eating meals on a simulated night shift, and that the nutritional content of the pre-meals consumed can have a marked effect on postprandial responses during a simulated phase shift. Such findings may provide a partial explanation for the increased occurrence of cardiovascular disease reported in shift workers.

ABSTRACT

The acute effects of different macronutrients on the secretion of glucagon-like peptide-1(7–36)amide (GLP-1(7–36)amide) and glucose-dependent insulinotropic polypeptide (GIP) were compared in healthy human subjects. Circulating levels of the two hormones were measured over a 24-h period during which subjects consumed a mixed diet. In the first study, eight subjects consumed three equicaloric (375 kcal) test meals of carbohydrate, fat and protein. Small increases in plasma GLP-1(7–36) amide were found after all meals. Levels reached a maximum 30 min after the carbohydrate and 150 min after the fat load. Ingestion of both carbohydrate and fat induced substantial rises in GIP secretion, but the protein meal had no effect. In a second study, eight subjects consumed 75 g glucose or the equivalent portion of complex carbohydrate as boiled brown rice or barley. Plasma GIP, insulin and glucose levels increased after all three meals, the largest increase being observed following glucose and the smallest following the barley meal. Plasma GLP-1(7–36)amide levels rose only following the glucose meal. In the 24-h study, plasma GLP-1(7–36)amide and GIP concentrations were increased following every meal and remained elevated throughout the day, only falling to fasting levels at night. The increases in circulating GLP-1(7–36)amide and GIP levels following carbohydrate or a mixed meal are consistent with their role as incretins. The more sustained rises observed in the daytime during the 24-h study are consistent with an anabolic role in lipid metabolism.

Journal of Endocrinology (1993) 138, 159–166

This study was undertaken to determine whether the internal clock contributes to the hormone and metabolic responses following food, in an experiment designed to dissociate internal clock effects from other factors. Nine female subjects participated. They lived indoors for 31 days with normal time cues, including the natural light: darkness cycle. For 7 days they retired to bed from 0000 h to 0800 h. They then underwent a 26-h 'constant routine' (CR) starting at 0800 h, being seated awake in dim light with hourly 88 Kcal drinks. They then lived on an imposed 27-h day (18 h of wakefulness, 9 h allowed for sleep), for a total of 27 days. A second 26-h CR, starting at 2200 h, was completed. During each CR salivary melatonin and plasma glucose, triacylglycerol (TAG), non-essential fatty acids (NEFA), insulin, gastric inhibitory peptide (GIP) and glucagon-like peptide-1 (GLP-1) were measured hourly. Melatonin and body temperature data indicated no shift in the endogenous clock during the 27-h imposed schedule. Postprandial NEFA, GIP and GLP-1 showed no consistent effects. Glucose, TAG and insulin increased during the night in the first CR. There was a significant effect of both the endogenous clock and sleep for glucose and TAG, but not for insulin. These findings may be relevant to the known increased risk of cardiovascular disease amongst shift workers.

Abstract

This study was designed to investigate postprandial responses to a mixed meal in simulated shift work conditions. Nine normal healthy subjects (six males and three females) were studied on two occasions at the same clock time (1330 h) after consuming test meals, first in their normal environment and secondly after a 9 h phase advance (body clock time 2230 h). Plasma glucose, insulin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), triacylglycerol (TAG) and non-esterified fatty acids (NEFAs) were determined at intervals for 6 h after each test meal. Postprandial plasma glucose, insulin, GIP and GLP-1 profiles were evaluated by calculating areas under the curve (AUC) for the first 2 h and the last 4 h of the sampling together with total AUC. Significantly higher postprandial glucose responses (total AUC) were observed after the phase shift than before (AUC 0–360 min, 2·01 (1·51–2·19) vs 1·79 (1·56–2·04) mmol/l.min; P<0·02; mean (range)). No significant difference was observed when the first 2 h of each response was compared, but significantly higher glucose levels were observed in the last 4 h of the study after the phase shift than before (AUC 120–360 min, 1·32 (1·08–1·42) vs 1·16 (1·00–1·28) mmol/l.min; P<0·05). Similar results were obtained for insulin (AUC 0—360 min, 81·72 (30·75– 124·97) vs 58·98 (28·03–92·57) pmol/l.min; P<0·01; AUC 120–360 min, 40·73 (16·20–65·25) vs 25·71 (14·25–37·33) pmol/l.min; P<0·02). No differences were observed in postprandial plasma GIP and GLP-1 responses before and after the phase shift. Postprandial circulating lipid levels were affected by phase shifting. Peak plasma TAG levels occurred 5 h postprandially before the phase shift. Postprandial rises in plasma TAG were significantly delayed after the phase shift and TAG levels continued to rise throughout the study. Plasma postprandial NEFA levels fell during the first 3 h both before and after the phase shift. Their rate of return to basal levels was significantly delayed after the phase shift compared with before. This study demonstrates that a simulated phase shift can significantly alter pancreatic B-cell responses and postprandial glucose and lipid metabolism.

Journal of Endocrinology (1996) 151, 259–267