Objective: Ghrelin administration can induce fat weight gain and hyperglycemia (potentially through ghrelin-induced hepatic glucose production), but plasma ghrelin is positively associated with whole-body insulin sensitivity (mainly reflecting muscle insulin action) being increased in lean individuals or after diet-induced weight loss and reduced in obesity or after diet-induced weight gain. To investigate potential mechanisms, we measured in vivo effects of sustained ghrelin administration at a non-orexigenic dose on skeletal muscle and liver insulin signaling at the AKT level and adipokine expression changes.
Research methods and procedures: Young-adult male rats received 4-day, twice daily subcutaneous ghrelin (200 mug/injection) or saline. We measured skeletal muscle (mixed, gastrocnemius; oxidative, soleus) and liver protein levels of activated [phosphorylated (P)] and total (T) AKT and glycogen synthase kinase (GSK; reflecting AKT-dependent GSK inactivation) and epididymal adipose tissue adipokine mRNA.
Results: Ghrelin increased body weight (+1.4%) and blood glucose (both p < 0.05 vs. saline) but not food intake, plasma insulin, or free fatty acids. Ghrelin, however, enhanced P/T/AKT and P/T/GSK ratios and glucose transporter-4 mRNA in soleus (p < 0.05), but not in gastrocnemius, muscle. In contrast, ghrelin reduced hepatic P/T-AKT and P/T-GSK. No alterations occurred in adiponectin, leptin, or resistin transcripts or plasma adiponectin.
Discussion: Despite moderate weight gain and in the absence of insulin-free fatty acid changes, sustained ghrelin administration enhanced oxidative muscle AKT activation. Reduced liver AKT signaling could potentially contribute to concomitant blood glucose increments. These findings support ghrelin as a novel tissue-specific modulator of lean tissue AKT signaling with insulin-sensitizing effects in skeletal muscle but not in liver in vivo.