Mechanisms for compensatory growth of adipose tissue in lipectomized rats
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Studies were designed to examine the regulation of body weight/fat by lowering body weight/fat through surgical removal of bilateral epididymal fat pads of adult male Wistar rats. Food intake during the first 4 weeks and energy expenditure on day 7-10 and day 28-31 post surgery were not different between lipectomized and sham operated rats. The carcass composition of lipectomized and sham operated rats was not significantly different 16 weeks post surgery, indicating a compensatory adipose tissue growth in lipectomized rats. The compensatory growth of adipose tissue was fat pad specific: both mesenteric and retroperitoneal fat pads, but not inguinal or perirenal fat pads were significantly heavier in lipectomized rats than in sham operated rats. Lipectomized rats had more total, small and large adipocytes at 4 weeks, and more total and large adipocytes at 16 weeks, than sham operated rats in the retroperitoneal fat depot, suggesting that both hyperplasia and hypertrophy contributed to the compensatory growth. Leptin and insulin did not appear to facilitate signaling of the loss of fat content, as serum concentrations of these putative adiposity signals were not different between lipectomized and sham operated rats 2 or 4 weeks after lipectomy. In vitro testing indicated that serum from lipectomized rats stimulated the proliferation of preadipocytes more than that from sham operated rats. In contrast, media conditioned by exposure to fat pads from lipectomized rats did not increase preadipocyte proliferation. This suggests that the blood borne factors stimulating proliferation of adipocytes are from tissues other than fat tissue. On the other hand, serum from lipectomized rats did not induce differentiation of preadipocytes more than that from sham operated rats, indicating that mechanism(s) other than blood borne factors may induce hypertrophy in compensating adipose depots. The possible involvement of the sympathetic nervous system in compensatory growth was tested by measuring norepinephrine concentrations in retroperitoneal fat pads, and no significant difference was observed between lipectomized and sham operated rats. Rats restore body fat level reduced by lipectomy through compensatory adipose tissue growth indicating that body weight/fat is regulated. Compensatory growth of adipose tissue is mediated, in part, by blood borne factors.