Acute changes of bone turnover and PTH induced by insulin and glucose: euglycemic and hypoglycemic hyperinsulinemic clamp studies

Effects of glucose-dependent insulinotropic peptide on osteoclast function

Acute nutrient ingestion leads to a rapid inhibition of bone resorption while effects on makers of bone formation are less marked or absent, suggesting that there is a transient shift toward skeletal accretion in the immediate postprandial period. The cellular bases for these effects are not clear. Glucose-dependent insulinotropic peptide (GIP), a known modulator of glucose-induced insulin secretion, is secreted from intestinal endocrine cells in response to nutrient ingestion. In addition to the effect of GIP on pancreatic beta-cells, GIP receptors are expressed by osteoclastic cells [corrected] in bone, suggesting a role for this incretin hormone in bone formation. To determine whether GIP also plays a role in the anti-resorptive effect of nutrient ingestion, osteoclasts were analyzed for the presence of GIP receptors by PCR, immunohistochemical and immunocytochemical analyses of bone tissue, and freshly isolated mature osteoclasts and osteoclast-like cells cultured in vitro. Osteoclast function was assessed by fetal long bone resorption assay and by use of the Osteologic disc assay. Our results demonstrate that GIP receptor transcripts and protein are present in osteoclasts. In addition, with the use of an in vitro organ culture system and mature osteoclasts, GIP was found to inhibit bone resorption in the organ culture system and the resorptive activity of mature osteoclasts. These data are consistent with the hypothesis that GIP inhibits bone breakdown through a direct effect on osteoclast-resorptive activity and suggest one mechanism for the postprandial reduction in markers of bone breakdown.

Role of gastrointestinal hormones in postprandial reduction of bone resorption

Collagen type I fragments, reflecting bone resorption, and release of gut hormones were investigated after a meal. Investigations led to a dose escalation study with glucagon like peptide-2 (GLP-2) in postmenopausal women. We found a dose-dependent effect of GLP-2 on the reduction of bone resorption.

INTRODUCTION

The C-terminal telopeptide region of type I collagen as measured in serum (s-CTX) can be used to assess bone resorption. This marker of bone resorption has a significant circadian variation that is influenced by food intake. However, the mediator of this variation has not been identified.

MATERIALS AND METHODS

We studied the release of the gut hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2; a representative of the intestinal proglucagon-derived peptides) after ingestion of glucose, fat, protein, and fructose, as well as their effects after parenteral administration in relation to bone turnover processes in healthy volunteers. Furthermore, we studied the effect on bone turnover of a single subcutaneous injection of GLP-2 in four different dosages (100, 200, 400, or 800 microg GLP-2) or placebo in 60 postmenopausal women (mean age, 61 +/- 5 years).

RESULTS

All macronutrients significantly (p < 0.05) reduced bone resorption as assessed by s-CTX (39-52% from baseline), and only the glucagon-like peptides were secreted in parallel. Parenteral administration of GIP and GLP-1 did not result in a reduction of the s-CTX level, whereas GLP-2 caused a statistically significant and dose-dependent reduction in the s-CTX level from baseline compared with placebo (p < 0.05). Urine DPD/creatinine, a marker of bone resorption, was significantly reduced by 25% from baseline in the 800-microg GLP-2 group (p < 0.01). An area under the curve (AUC(0-8h)) analysis for s-CTX after GLP-2 injection confirmed the dose-dependent decrease (ANOVA, p = 0.05). The s-osteocalcin level was unaffected by the GLP-2 treatment.

CONCLUSION

These studies exclude both GIP and GLP-1 as key mediators for the immediate reduction in bone resorption seen after a meal. The dose-dependent reduction of bone resorption markers found after subcutaneous injection of GLP-2 warrants further investigation into the mechanism and importance of GLP-2 for the bone turnover processes.

Octreotide abolishes the acute decrease in bone turnover in response to oral glucose

Feeding or oral intake of glucose results in an acute suppression of bone turnover. This does not appear to be mediated by insulin. Several gastrointestinal hormones modulate bone turnover in vitro and may mediate this response. We examined whether inhibiting the production of gastrointestinal hormones using octreotide could block glucose-mediated suppression of bone turnover. Fifteen subjects were each studied on four occasions in a randomized, single-blind, crossover study after receiving 1) oral placebo, iv saline; 2) oral glucose, iv saline; 3) oral glucose, iv octreotide; or 4) iv octreotide alone. We measured serum C-terminal telopeptide of type I collagen, urinary N-terminal telopeptide of type I collagen, osteocalcin, procollagen type I N-terminal propeptide, PTH, insulin, ionized calcium, and glucose over 4 h. All bone turnover markers decreased significantly after oral glucose (P < 0.001). At 120 min serum C-terminal telopeptide decreased by 45 +/- 2%, urinary N-terminal telopeptide by 31 +/- 7%, osteocalcin by 16 +/- 1%, and procollagen type I N-terminal propeptide by 8 +/- 1%. There was no significant decrease in bone turnover in response to oral glucose during octreotide infusion. Octreotide alone resulted in a significant increase in all bone turnover markers (P < 0.05) and PTH (P < 0.01). We conclude that octreotide completely abolishes the bone turnover response to glucose intake and increases PTH secretion. The apparent bone turnover response to feeding is probably mediated by an octreotide-inhibitable endocrine factor.