Attribution of specific roles to the two ubiquitously expressed PI 3-kinase 
(PI3K) isoforms p110α and p110β in biological functions they have been 
implicated, such as in insulin signalling, has been challenging. While p110α has 
been demonstrated to be the principal isoform activated downstream of the 
insulin receptor, several studies have provided evidence for a role of p110β. 
Here we have used isoform-selective inhibitors to estimate the relative 
contribution of each of these isoforms in insulin signalling in adipocytes, 
which are a cell type with essential roles in regulation of metabolism at the 
systemic level. Consistent with previous genetic and pharmacological studies, we 
found that p110α is the principal isoform activated downstream of the insulin 
receptor under physiological conditions. p110α interaction with Ras enhanced the 
strength of p110α activation by insulin. However, this interaction did not 
account for the selectivity for p110α over p110β in insulin signalling. We also 
demonstrate that p110α is the principal isoform activated downstream of the 
β-adrenergic receptor (β-AR), another important signalling pathway in metabolic 
regulation, through a mechanism involving activation of the cAMP effector 
molecule EPAC1. This study offers further insights in the role of PI3K isoforms 
in the regulation of energy metabolism with implications for the therapeutic 
application of selective inhibitors of these isoforms.