The Positive Effects of Growth Hormone-Releasing Peptide-6 on Weight Gain and Fat Mass Accrual Depend on the Insulin/Glucose Status

Ghrelin and GH secretagogues, including GH-releasing peptide (GHRP)-6, stimulate food intake and adiposity. Because insulin modulates the hypothalamic response to GH secretagogues and acts synergistically with ghrelin on lipogenesis in vitro, we analyzed whether insulin plays a role in the metabolic effects of GHRP-6 in vivo. Streptozotocin-induced diabetic rats received saline, GHRP-6, insulin, or insulin plus GHRP-6 once daily for 8 wk. Rats receiving saline suffered hyperglycemia, hyperphagia, polydipsia, and weight loss. Insulin, but not GHRP-6, improved these parameters (P < 0.001 for all), as well as the diabetes-induced increase in hypothalamic mRNA levels of neuropeptide Y and agouti-related peptide and decrease in proopiomelanocortin. Cocaine amphetamine-related transcript mRNA levels were also reduced in diabetic rats, with GHRP-6 inducing a further decrease (P < 0.03) and insulin an increase. Diabetic rats receiving insulin plus GHRP-6 gained more weight and had increased epididymal fat mass and serum leptin levels compared with all other groups (P < 0.001). In epididymal adipose tissue, diabetic rats injected with saline had smaller adipocytes (P < 0.001), decreased fatty acid synthase (FAS; P < 0.001), and glucose transporter-4 (P < 0.001) and increased hormone sensitive lipase (P < 0.001) and proliferator-activated receptor-gamma mRNA levels (P < 0.01). Insulin normalized these parameters to control values. GHRP-6 treatment increased FAS and glucose transporter-4 gene expression and potentiated insulin's effect on epididymal fat mass, adipocyte size (P < 0.001), FAS (P < 0.001), and glucose transporter-4 (P < 0.05). In conclusion, GHRP-6 and insulin exert an additive effect on weight gain and visceral fat mass accrual in diabetic rats, indicating that some of GHRP-6's metabolic effects depend on the insulin/glucose status. (Endocrinology 151: 2008-2018, 2010)