The studies on leptin, an endocrine factor secreted from adipose tissue, in domestic animals

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It has been known that the increase in body fat mass stimulated leptin secretion from adipocytes in rats and humans. Leptin suppressed food intake and increased basal metabolic rate in these animals. However, the action of leptin and the regulation of leptin secretion did not clarified in ruminants. In the first experiment, we intracerebroventricularly injected mouse leptin into sheep. The administration of leptin reduced feed intake and body weight and increased body temperature in sheep, which suggests that leptin suppresses food intake and increases basal metabolic rate in sheep. In the second experiment, we collected blood from sheep at 15-minutes intervals for 24 hours and plasma leptin concentration was determined. Similar to humans, plasma leptin level showed pulsatility in sheep. However, feeding and lightening condition did not affect plasma leptin level. Additionally, plasma leptin level was not affected by plasma cortisol. We suggest that ruminants do not show diurnal rhythms of plasma leptin because postprandial changes in blood glucose and insulin was slight in ruminants. In the third experiment, we collected blood in fattening beef cattle and serum leptin concentrations were determined. Serum leptin level was increased during fattening periods. There was a positive correlation between serum leptin level and marbling score. However the relationship between plasma leptin and subcutaneous fat thickness was not found. These results suggest that leptin adversely affects productivity of beef cattle during late fattening period but beef marbling may be estimated by plasma leptin level in fattening cattle. In the fourth experiment, we tried to manipulate leptin secretion by a dietary factor using isolated rat adipocyte. Vanadium increased leptin secretion from adipocyte. Additionally, the action of vanadium on leptin secretion was additive to the action of insulin, which suggests that vanadium enhances insulin-inducing leptin secretion.