Interactions between temperature and human leptin physiology in vivo and in vitro.

To investigate the possibility that environmental temperature may exert physiologically significant direct, local effects on subcutaneous adipose tissue temperatures, and its secretion of leptin, we exposed healthy males ( n=12) to repeated cold-water immersion (study 1), and also incubated surgically removed human subcutaneous adipose tissue samples ( n=7) at 27 degrees, 32 degrees and 37 degrees C (study 2). In vivo immersions were conducted over 15 days (60-90 min at 18 degrees C). Regional body temperatures and plasma leptin concentrations were measured before and during immersion. Acute cold exposure suppressed plasma leptin concentration (25 min: -14%, 60 min: -22%, P=0.0001), whilst repeated cold-water immersion was associated with an increase of plasma leptin concentration relative to test day 1 (+19% day 8, +13% day 15, overall P=0.03). Leptin secretion in vitro decreased 3.7-fold as the incubation temperature decreased from 37 degrees to 27 degrees C ( P=0.001). In a compartmental model of leptin turnover in vivo, the measured (local) temperature effect on leptin secretion in vitro was more than able to account for the observed cold-induced decrease in leptin concentration in vivo. We therefore conclude that acute and repeated cold-water immersions have separate and opposing effects on circulating leptin concentrations in humans. Under our experimental conditions, the local effects of reduced subcutaneous adipose tissue temperature may be a more important contributor to the acute effects observed in vivo, than the sympathetically mediated suppression of leptin secretion.