Adenosine is a paracrine/autocrine antilipolytic agent that binds to A, receptors of adipocyte membranes, thereby activating GTP-binding proteins, G,, which inhibit adenylate cyclase activity and hence lipolysis. The mechanism whereby GH facilitates lipolysis is not fully explained, however it is possible that GH exerts its effects by modulating the acute regulation of this process by other hormones rather than having a direct effect on lipolysis itself. Initially studies focused on modulation of response of adipocytes to catecholamines, but very recent studies [ I , 21 suggest that the G,-mediated antilipolytic system is a major target of GH action. We have investigated this further by (a) treating sheep with recombinant bovine growth hormone, (Monsanto) 10 mg per day for 7 days in vivo and (b) by maintaining sheep adipose tissue explants in culture with GH (metabolic activities and other processes are particularly well maintained in sheep adipose tissue during culture [3]. For culture, adipose tissue explants were preincubated for 24h with no hormones and then GH (100ng/ml) was added for the next 24h. In both cases, response to adenosine in vitro was determined using N6-phenylisopropyladenosine (PIA) as described previously [2] (in essence lipolysis was activated with 100 nM isoprenaline and inhibited with various concentrations Treatment of sheep in vivo with GH decreased maximum inhibition of isoproterenol-stimulated lipolysis by 100 nM PIA from 68.8 f 11.4 to 27.8 f 9.2% (P<0.05, results means f SEM of 5 observations). This effect of GH was not accompanied by any discernable change in the number of adenosine receptors or amounts of the various isoforms of G, (measured as described previously, [2]). This suggested that a decrease in G, activity was occumng (GH also decreased response to PGE, which activates G, but via a distinct receptor). Maintenance of sheep adipose tissue in culture for up to 48h in the absence of added hormones had no effect on response to PIA. Addition of GH for 24h again decreased (P < 0.05) maximum response to adenosine from 72.8 to 40.2% inhibition of isoprenaline-stimulated lipolysis (results are means of 4 observations, SED = 9.8). Again there was no change in either the number of adenosine receptors or amounts of G, isoforms (not shown). As there is evidence that G,2a can be phosphorylated by PKC and at least one other kinase [4] we tested the effect of protein kinase and phosphatase inhibitors in the tissue culture system. Addition of 100 pM H7 (a protein serine kinase inhibitor) completely abolished the effects of GH on response to PIA (Table 1) while having no apparent effect on control tissue. This is consistent with a kinase mediating the effects of GH on the G, system. Further evidence for this was obtained using the protein-serine phosphatase inhibitor, okadaic acid. Addition of this agent to the culture medium mimicked the effect of GH; effects of the two agents were additive (Table 1). Table 1. Effect of H7 (100 p M ) and okadacic acid (10 nM) on growth hormone induced inhibition of PIA action on isoprenaline-stimulated lipolysis Tissue was cultured for 24h without additions after which GH, H7 and okadaic acid were added, singly or in combination, for the next 24h. Lipolysis was measured in the presence of isoprenaline (100 nM) and adenosine deaminase (0.8 pg/ml) PIA (100 nM). Results are means of 4 observations; SED was 7.2 and 7.3 for the H7 and okadaic acid experiments respectively.