Abstract Acetate arguably is the most important substrate for de novo fatty acid biosynthesis in bovine adipose tissue. However, including glucose in incubation media in vitro strongly stimulates fatty synthesis from acetate; additionally, glucose carbon is essential for triacylglycerol synthesis from acetate. Our laboratory is unique in that we utilize bovine intramuscular (i.m.) and subcutaneous (s.c.) adipose tissue explants dissected from beef longissimus muscle immediately post-exsanguination to study bovine adipose tissue metabolism. Four decades ago, using this experimental technique, we established that glucose is a primary substrate for fatty acid biosynthesis in i.m. adipose tissue. Recent research in our laboratory has addressed the hypothesis that acetate and/or long-chain fatty acids (LCFA) differentially affect lipid filling of i.m. and s.c. adipose tissue. We had previously demonstrated that, whereas s.c. adipose tissue is highly responsive to isoproterenol (a non-selective ß-adrenergic agonist), i.m. adipose tissue is relatively refractory to isoproterenol. This subsequently was confirmed in our laboratory utilizing primary cultures of bovine s.c. and i.m. adipocytes. We currently are focusing on the G-coupled protein receptors-43 (GPR43) and GPR120. Several previous studies from our laboratory have documented GPR43 expression in i.m. and s.c. adipose tissue, and we more recently demonstrated GPR120 expression in bovine adipose tissues. Acetate is a ligand for GPR43, whereas LCFA are ligands for GPR120. The binding of ß-adrenergic agonists to ß-adrenergic receptors increases the production of cyclic adenosine monophosphate (cAMP), whereas binding of acetate and LCFA to GPR43 and GPR120, respectively, decreases cAMP production. Thus, binding of circulating acetate and LCFA to their respective GPR receptors putatively should lead to increase lipid filling of bovine adipose tissues. A current study from our laboratory demonstrated that acetate had no effect on forskolin-stimulated cAMP production in i.m. or s.c. adipose tissue explants unless the culture medium also included oleic acid (18:1n-9). A subsequent study confirmed that oleic acid and palmitic acid (16:0), but not acetate, strongly depressed forskolin-stimulated cAMP production in s.c. adipose tissue explants; oleic and palmitic acid were much less effective in depressing cAMP production in i.m. adipose tissue explants. Unusually, GPR43 expression was detected in neither s.c. adipose tissue nor i.m. adipose tissue in our most recent study, whereas GPR120 expression was readily detected in both adipose tissue types. We conclude the following from these recent studies: LCFA binding to GPR120 is more effective than acetate binding to GPR43 in promoting lipid filling in bovine adipose tissue; and LCFA binding to GPR120 putatively would promote greater lipid filling in s.c. adipose tissue than in i.m. adipose tissue.
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