Nonetheless, a glutamic acid to glutamine substitution at position 354 in GIPR��s 6th transmembrane domain results in 4-Chloro-7H-pyrrolo[2,3-d]pyrimidine lowered basal activity. Subjects homozygous for the E354Q polymorphism were found to have reduced fasting and post oral glucose tolerance test serum C-peptide concentration, suggesting that GIPR��s constitutive activity may play a role in glucose homeostasis. More recently, the same GIPR polymorphism has been shown to be associated with reduced bone mineral density and increased fracture risk, suggesting a role for GIPR��s basal activity in osteoblast function. The traditional role for GRKs and arrestins is to mediate the homologous desensitisation and internalisation of GPCRs, as well as activation of tyrosine kinase signalling pathways. Arr2 has been shown to mediate GLP-1 signalling in cultured pancreatic b-cells. Knockdown of Arr2 by RNAi reduced GLP-1-stimulated cAMP levels and impaired GLP-1-stimulated insulin secretion. Interestingly, Arr2 knockdown did not affect GLP-1R desensitisation or internalisation. Arr3-knockout mice displayed impaired glucose tolerance and insulin secretion; however, GLP-1 amplification of insulin secretion was not affected. In contrast, very little is known regarding the interaction between GIPR and either GRKs or arrestins. We hypothesised that given GIPR��s high level of basal activity observed in the luciferase assay, GIPR may interact with arrestin in a ETC-1002 ligand-independent manner. Several methods were employed to compare the ability of GLP1R and GIPR to interact with Arr3. Initially, we used a commercially available enzyme fragment complementation assay to investigate arrestin recruitment to GLP1-R and GIPR. GLP-1 and GIP stimulated Arr3 recruitment to their respective receptors with comparable potency. However, the signal window for GIPR was substantially smaller than for GLP-1R, and as a result, maximum arrestin binding was also significantly lower. Due to the nature of this assay, we were unable to compare or manipulate variables such as the relative expression of arrestin and/or receptors; therefore, alternative methods were employed.