Diurnal variations of serum leptin in dogs: effects of fasting and re-feeding K. Ishioka a, * , H. Hatai a , K. Komabayashi a , M.M. Soliman a , H. Shibata b , T. Honjoh b , K. Kimura a , M. Saito a a Laboratory of Biochemistry, Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060-0818, Japan b Morinaga Institute of Biological Science, 2-1-1 Shimosueyoshi 2, Tsurumi, Yokohama 230-8504, Japan Accepted 24 December 2003 Abstract Leptin is a protein synthesized and secreted primarily by adipocytes, and plays a key role in the regulation of energy balance. We have reported that serum leptin is elevated in obese dogs. In the present study, we examined diurnal variations of serum leptin in the dog, with special references to feeding and fasting cycles. Four male beagles were accustomed to feed once a day at 10:00 h, and blood samples were taken every 3 h for 24–36 h. Serum leptin concentration showed clear diurnal variations, being lowest before food intake (2.3 Æ 0.5 ng/mL) at 09:00 h, and highest (10.5 Æ 2.4 ng/mL) at 18:00 h. Such diurnal variations disappeared when the dogs were fasted. Serum insulin also showed diurnal variation with higher levels at 12:00–15:00 h. When insulin or glucose was injected in the fasted dogs to mimic the post-prandial insulin rise, serum leptin concentration was significantly increased in 4–8 h, but in both cases to a lesser extents than those after food intake. The results indicate that serum leptin concentrations change diurnally in association with feeding–fasting cycles in the dog, partially due to changes in insulin secretion. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Dogs; Food intake; Glucose; Insulin; Leptin 1. Introduction Leptin, the ob gene product, is a 16 kDa protein synthesized and secreted primarily by adipocytes (Friedman and Halaas, 1998). In humans and rodents, serum leptin concentration is known to positively cor- relate with body fat content, being higher in obesity (Maffei et al., 1995; Mizuno et al., 1996). In addition, serum leptin concentration shows diurnal changes in association with feeding–fasting cycles (Boden et al., 1996; Schoeller et al., 1997; Ahima et al., 1998). These variations of serum leptin may be due to the net effects of various neuroendocrine and nutritional factors such as glucose, insulin, glucocorticoids and catecholamines, which regulate synthesis of leptin in adipocytes (Wa- bitsch et al., 1996). In companion animals, as in human medicine, obesity has become the most common nutritional disorder (Edney and Smith, 1986; Markwell et al., 1990). How- ever, there have been a limited number of reports on leptin in companion animals such as the dog and cat. Recently, we have cloned canine (Iwase et al., 2000a) and feline (Sasaki et al., 2001) leptin cDNAs, produced recombinant leptin in Escherichia coli, and established enzyme-linked immunosorbent assay (ELISA) methods for serum leptin in the dog (Iwase et al., 2000b) and cat (Shibata et al., 2003). Using our ELISA methods, we have confirmed a highly positive relationship between serum leptin concentration and body fat content in dogs (Ishioka et al., 2002a; Sagawa et al., 2002) and cats (Shibata et al., 2003), suggesting that plasma leptin is a quantitative diagnostic marker of adiposity and obesity. In the present study, we examined diurnal variations of The Veterinary Journal 169 (2005) 85–90 The Veterinary Journal www.elsevier.com/locate/tvjl * Corresponding author. Tel.: +81-11-706-5204; fax: +81-11-757- 0703. E-mail address: katsumi@vetmed.hokudai.ac.jp (K. Ishioka). 1090-0233/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2004.01.003