β ₂-Adrenoceptors and non-β-adrenoceptors mediate effects of BRL37344 and clenbuterol on glucose uptake in soleus muscle: studies using knockout mice

Background and purpose: In previous work, 10 pM BRL37344 and 10 pM clenbuterol stimulated glucose uptake in mouse soleus muscle. Ten nM BRL37344 also stimulated uptake but 100 nM clenbuterol inhibited uptake. Antagonist studies suggested that the opposite effects of 10 nM BRL37344 and 100 nM clenbuterol are mediated by the β ₂-adrenoceptor. BRL37344 and clenbuterol have been studied in muscles that lack β ₃-, β ₂- or all three β-adrenoceptors. Effects of β-adrenoceptor antagonists on responses to the agonists have been studied further using muscles from wild-type mice. Experimental approach: Soleus muscles of wild-type or β-adrenoceptor knockout mice were incubated with 2-deoxy[1- ¹⁴C]-glucose, and β-adrenoceptor ligands. Formation of 2-deoxy[1- ¹⁴C]-glucose-6-phosphate was measured. Key results: Concentration-response relationships were similar for BRL37344 and clenbuterol in normal muscle and muscle lacking β ₃-adrenoceptors. Ten pM BRL37344 and clenbuterol stimulated glucose uptake in muscle lacking β ₂-adrenoceptors or all three β-adrenoceptors, but 10 nM BRL37344 did not stimulate uptake in either case, and 100 nM clenbuterol stimulated, rather than inhibited, uptake in muscle lacking β ₂- adrenoceptors. One hundred nM clenbuterol also stimulated glucose uptake in normal muscle when β ₂-adrenoceptors were blocked with ICI118551, and this was not prevented by antagonism of β ₁- or β ₃-adrenoceptors. Conclusions and implications: Ten nM BRL37344 and 100 nM clenbuterol have opposite effects on glucose uptake but both effects are mediated by the β ₂-adrenoceptor - apparently an example of agonist-directed signalling. Ten pM BRL37344, 10 pM clenbuterol and 100 nM clenbuterol in the presence of ICI118551 stimulate glucose uptake via β-adrenoceptor-independent mechanisms, demonstrating unknown properties for the agonists.