Developmental aspects of the Adipose tissue renin-angiotensin system: Therapeutic implications

Adipose tissue possesses considerable growth potential, and excess adipose tissue mass is associated with noninsulin dependent diabetes mellitus (NIDDM). Adipose tissue is a significant source of angiotensinogen, the precursor of angiotensin II (All), a vasoactive peptide with in vitro mitogenic effects. The renin-angiotensin system remains only partially identified in adipose tissue, however, and knowledge of whether All has a physiologic role in this tissue is dependent in part upon other components of the system being characterized. We have chosen to further investigate aspects of the renin-angiotensin system in separate adipose tissue depots of rats during growth and development of the tissue. Either epididymal or retroperitoneal adipose tissue was examined in young lean, or adult obese male Sprague-Dawley rats. Adipose tissue was subjected to collagenase digestion, yielding separate vascular and fat cell fractions. Each fraction was analyzed for angiotensin converting enzyme (ACE) activity and All receptor binding capacity. ACE activity was significantly greater when expressed per gram of adipose tissue in the young lean rats. Vascular and fat cell fractions exhibited All receptors of high affinity, with a greater density of receptors observed in the epididymal adipose tissue fractions. Incubation of adipocytes with exogenous All was associated with the release of prostaglandin into the medium, and analyses of adipocyte cytosol indicated the presence of angiotensinogen. Growing rats given oral losartan, an All receptor antagonist, once daily (15 mg/kg) for 2 weeks exhibited reductions in body weight gain, fat mass, fat cell volume, and All receptor number. These data further document the presence of a peripheral renin-angiotensin system in anatomically distinct adipose tissue depots at different stages of growth. While the characterization of the effects of All upon adipose tissue development remains incomplete, future investigations with agents affecting the adipocyte renin-angiotensin system could provide additional information on the role of this system in adipose tissue growth.