Eliminating the antilipolytic adenosine A1 receptor does not lead to compensatory changes in the antilipolytic actions of PGE2 and nicotinic acid

Aim: We examined whether compensatory changes after adenosine A(1) receptor knockout [A(1)R (-/-)] eliminate the antilipolytic actions mediated by this receptor. Methods: Lipolysis experiments were performed on adipocytes prepared from the wild type A(1)R (+/+), A(1)R (-/-) and heterozygous mice. Gene expression was assayed with cDNA microarray technique and real-time PCR; protein expression with immunoblotting. Results: The A(1)R was the only adenosine receptor involved in lipolysis. The effects of adenosine deaminase and 2-chloroadenosine were abolished in A1R (-/-) mice. The IC50 value of 2-chloroadenosine doubled from 16.6 to 33.6 nM when half of the A(1)Rs were eliminated. Adrenergic alpha(2) agonists had no effects on lipolysis. Prostaglandin E-2 (PGE(2)) inhibited lipolysis with an IC50 value of 5.8 nM (4.7-7.2 nM) in the A(1)R (+/+) mice and 10.6 nM (9.0-12.6 nM) in the A(1)R (-/-) mice. Nicotinic acid inhibited lipolysis with an IC50 value of 0.30 mu M (0.19-0.46 mu M) in the A(1)R (+/+) mice and 0.24 mu M (0.16-0.37 mu M) in the A(1)R (-/-) mice. G(i)alpha(1) mRNA was significantly up-regulated in adipose tissue from A(1)R (-/-) mice. However, immunoblotting showed that G(i)alpha(1) was not up-regulated at the protein level. Conclusion: The A(1)R mediates the antilipolytic actions of adenosine. Deletion of the A(1)R in mice does not result in compensatory increases in G-protein-mediated antilipolytic actions of PGE(2) or nicotinic acid.