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

Abstract

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 A(1)R (-/-) mice. The IC(50) 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 IC(50) 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 IC(50) value of 0.30 microm (0.19-0.46 microm) in the A(1)R (+/+) mice and 0.24 microm (0.16-0.37 microm) 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(ialpha1) 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.