LAUSR

Abstract Ghrelin is a 28-peptide hormone that was originally discovered as a ligand for the growth hormone secretagogue receptor (GHSR) and noted for its roles in food intake and feeding behavior. Beyond this, ghrelin may also regulate glucose tolerance and insulin secretion. In times of starvation, ghrelin defends against hypoglycemia. Contrastingly, ghrelin removal increases insulin secretion and improves glucose tolerance in metabolically-stressed mice. Though ghrelin is predominantly produced by stomach X/A cells, it is also made in the pancreas. Within the pancreatic islet, ghrelin is produced by ε-cells and GHSR is predominantly expressed in somatostatin-secreting δ-cells. This positioning suggests that, locally, islet ghrelin could reach interstitial levels that are higher than detected in circulation and act in a paracrine manner to inhibit insulin secretion. Additionally, GHSR has high constitutive activity and may act in the absence of ghrelin. We hypothesized that a ghrelin-GHSR paracrine axis within the islet modulates insulin secretion and tested this using parallel in vivo and ex vivo approaches in genetic rodent models lacking ghrelin (Ghrl-/-) or GHSR (GHSR-/-). Glycemic responses in GHSR-/- and GHSR+/+male mice did not differ after i.p. glucose tolerance tests (IPGTT) after a 6- or 16-hr fast, oral mixed meal tolerance test (MMTT), or insulin sensitivity during insulin tolerance test (ITT). Female GHSR-/- mice had impaired glucose tolerance after 6-hr fast IPGTT compared to GHSR+/+ (glucose AUC, p=0.01) but did not differ from controls during 16-hr fasted IPGTT, MMTT, or ITT. These data suggest constitutive GHSR activity does not mediate in vivo glucose tolerance. Ghrl-/- and Ghrl+/+ male mice did not differ in glycemia in response to IPGTTs, MMTT, or ITT. Female Ghrl-/- mice showed a slight improvement compared to Ghrl+/+ during MMTT (glucose AUC, p=0.04) but did not differ in other tests. Thus, endogenous ghrelin does not substantially alter glucose tolerance. Given that many factors can affect in vivo glucose tolerance, we perifused isolated islets. Here, exogenous ghrelin robustly decreased glucose-stimulated insulin secretion (GSIS, p<0.001 at 16.7 mM glucose) in a GHSR-dependent manner. Insulin secretion from Ghrl-/- or GHSR-/- islets did not differ from their wild type controls during a glucose ramp or in response to alanine or GLP-1 stimulation. Ex vivo, islet ghrelin and constitutive GHSR activity do not appear to mediate insulin secretion in the paradigms we tested. Collectively, our data provide strong evidence against a paracrine ghrelin-GHSR axis in mediating insulin secretion in lean, chow-fed adult mice. Islet ghrelin and GHSR may impact insulin secretion at earlier stages of development, during times of extreme nutritional stress (e.g., starvation or overnutrition), or in the transition from fasting to fed state.