Warm Exposure

Warm exposure entails a reduction of the orthosympathetic stimulus, resulting in BAT inac-tivation. Morphologically, this corresponds to a transformation of brown fat cells into cells similar to white adipocytes. During this transformation, we have observed a reduced genic expression of UCP1 and increased genic expression of leptin [55]. This suggests that such morphological transformation is accompanied by a new functional situation, with adipocytes losing their thermogenic ability and acquiring the properties of white fat cells, including production of such an important hormone as leptin. This is in line with the observation that classic multilocular brown adipocytes subjected to adrenergic stimulation express UCP1 but not leptin [56, 55], whereas cold exposure and sympathetico-mimetic drugs reduce leptinaemia [57] and induce the transformation of white into brown adipocytes [58] (see below 'Transdifferentiation').

It is interesting to note that the energy required for basic metabolic activities is consider-

Fig. 7. Delipidised (slimmed-down) white adipocyte. Slimmed adipocytes (asterisks) show distinctive morphological features: external lamina, pinocytotic vesicles, cytoplasmic invaginations, cytoplasm rich in organelles including mitochondria, endoplasmic reticulum, Golgi complex and lipid droplets of variable size. Lipid droplet size depends on the stage of delipidisation. Cap, capillary. Transmission electron microscopy 5500x

Fig. 7. Delipidised (slimmed-down) white adipocyte. Slimmed adipocytes (asterisks) show distinctive morphological features: external lamina, pinocytotic vesicles, cytoplasmic invaginations, cytoplasm rich in organelles including mitochondria, endoplasmic reticulum, Golgi complex and lipid droplets of variable size. Lipid droplet size depends on the stage of delipidisation. Cap, capillary. Transmission electron microscopy 5500x ably reduced in animals maintained in ther-moneutral conditions. Indeed, in an adult mouse subjected to prolonged fasting (48 h), the whole

WAT shrinks almost to disappearance at around 10°C, whereas it is nearly unchanged if the animal is kept at 28°C [39].

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