Growth hormone decreases the response to anti-lipolytic agonists and decreases the levels of G_i2 in rat adipocytes

The effect of growth hormone (GH) in vivo on the Gi-mediated anti-lipolytic signalling system of rat adipocytes has been investigated. Lowering of serum GH levels, by treatment of rats with an antiserum (anti-rGH) specific for rat GH, increased the sensitivity of adipocytes to the anti-lipolytic agonists N6-phenylisopropyladenosine (PIA) and prostaglandin E1. This occurred in the absence of any change in PIA binding to adipocyte membranes. Immunoblot analysis of adipocyte membranes revealed that lowering of serum GH resulted in at least a 3-fold increase in the levels of alpha-subunit of Gi2, but had no effect on the alpha-subunits of Gi1 and Gi3 nor on the 42 and 45 kDa forms of the alpha-subunit of Gs. Replenishment of serum GH, by concurrent administration of ovine GH to rats, prevented all of these effects of anti-rGH. It is concluded that GH down-regulates the amount of Gi2 alpha-subunit in adipocyte membranes, resulting in a decrease in the sensitivity of the cells to anti-lipolytic agonists.