GIP's effect on bone metabolism is reduced by the selective GIP receptor antagonist GIP(3-30)NH₂

Infusion of the incretin hormone glucose-dependent insulinotropic polypeptide (GIP) suppresses the bone resorption marker carboxy-terminal type 1 collagen crosslinks (CTX). Using separate and combined infusions of the selective GIP receptor (GIPR) antagonist, GIP(3-30)NH₂, and GIP, we investigated how GIPR inhibition affects bone turnover markers. Ten healthy men (median age 22.5 years (range 21-25), BMI 21.3kg/m² (19.9-24.7)) participated in a randomized, doubled blinded, placebo-controlled, crossover study with four 1h 12mmol/l-hyperglycemic clamps on four separate study days with concomitant infusions of GIP, GIP+GIP(3-30)NH₂, GIP(3-30)NH₂, and placebo, respectively, separated by a period of at least one week. GIP was infused at 1.5pmol/kg/min and GIP(3-30)NH₂ at 800pmol/kg/min. Plasma glucose was clamped at 12.0±1.2mmol/l and plasma levels of GIP and GIP(3-30)NH₂ amounted to ∼80pmol/l and ∼50nmol/l, respectively. GIP suppressed CTX more than placebo (baseline-subtracted AUC -6,811±1,260 vs. -3,012±3,018ng/l×min, P= 0.002) and resulted in CTX values of 53 ± 6.9% (GIP) versus 81 ± 10% of baseline (placebo), respectively (P = 0.0006), at the end of the hyperglycemic clamp. Co-infusion of GIP and GIP(3-30)NH₂ attenuated the GIP-induced CTX suppression by 51±33% (P = 0.01). The peak value of the bone formation marker N-terminal propeptide of type 1 procollagen (P1NP) peaked at higher levels during GIP (109±6.7% of baseline) than during GIP(3-30)NH₂ infusion (101±8.9%) (P = 0.049) and GIP suppressed PTH levels compared to GIP(3-30)NH₂ alone (P = 0.0158). In conclusion, blockade of the GIPR with GIP(3-30)NH₂ diminished GIP-induced CTX and P1NP responses, showing that these effects are GIPR-mediated and that GIPR antagonism might interfere with bone resorption.