Regulation of lipolysis by somatotropin: functional alteration of adrenergic and adenosine signaling in bovine adipose tissue.

To investigate the cellular mechanisms of somatotropin (ST) action on adipose tissue lipolysis, experiments were conducted using adipose tissue taken from lactating cows treated with excipient or ST (40 mg/day). Stimulation of lipolysis in vitro by the effectors isoproterenol with or without adenosine deaminase, dibutyryl cAMP with or without isobutylmethylxanthine, and forskolin was not altered by ST treatment. Conversely, the response to the antilipolytic effector, phenylisopropyladenosine (PIA), was significantly reduced in adipose tissue explants from ST or fasted cows. The different responses to adrenergic-stimulating agents (in vivo) and PIA (in vitro) were not due to differences in the abundance of alpha, beta or gamma subunits of the stimulatory (Gs) and inhibitory (Gi) subunits of the heterotrimeric G-proteins which bind to the beta-adrenergic and adenosine receptors respectively. However, the functionality of Gi proteins, as assessed by their ability to be ADP-ribosylated by pertussis toxin, was significantly reduced in ST-treated but not fasted cows. These data highlight differential regulation of signaling proteins by ST and fasting, both of which result in enhanced in vivo response to adrenergic stimulation of lipolysis.