Photoperiod-dependent fat pad mass and cellularity changes after partial lipectomy in Siberian hamsters.

Long day (LD)-housed Siberian hamsters show compensatory mass increases in nonexcised white adipose tissue (WAT) after partial lipectomy, whereas hamsters exposed to short days (SDs) for 22 wk do not. The purpose of this experiment was to determine the cellularity changes underlying lipectomy-induced WAT compensation and whether the duration of SD exposure affects this compensation. Male Siberian hamsters were epididymal (E) or inguinal (I) WAT lipectomized (x) or sham-lipectomized (Sham) and either remained in LDs or were transferred to SDs and killed 6 or 12 wk later. In LDs, lipectomized hamsters showed compensatory mass increases in retroperitoneal WAT (RWAT) due to hyperplasia. IWAT mass also was increased by approximately 40% in LD-housed EWATx hamsters because of nonsignificant increases in adipocyte size and number at weeks 6 and 12, respectively. SD-housed hamsters responded to lipectomy by delaying the SD-associated body fat loss so that RWAT mass was reduced only one-third as much in lipectomized as in Sham hamsters, and the IWAT adipocytes of EWATx hamsters were larger than in Sham hamsters at week 6. At week 12, there was little indication of fat pad compensation by SD-housed hamsters. Collectively, the results of the present experiment and our previous study (16) suggest that the inhibitory effect of SDs on fat pad compensation after lipectomy increases with prolonged SD exposure.