OBJECTIVES

Leptin is an adipocyte-specific protein that functions as an adipostat to control food intake and to balance the whole energy system in mammals; it is also involved in the modulation of neuroendocrine system and gonadal activity. Leptin plasma levels appear to be related to body fat percentage, glucose blood concentration and several hormones as GH and sexual steroids. Scarce data are available in dog: leptin was identified and cloned very recently and our previous studies indicate that hormone's secretion is pulsatile in the bitch, as in humans and rodents. The aim of this study was to investigate the possible sexual dimorphism in leptin blood levels as well as its correlation with GH concentration and animal size.

MATERIALS

130 dogs (60 males; 70 females), healthy, normal weight, aging 2 - 7 years, admitted to veterinary clinic for routine examination were used. Dogs were divided into 4 groups on the basis of sex and size: medium size (10-25 Kg; M=38, F=41) and large size (>25 Kg; M=27, F=24). In the presence of the owner, a single blood sample (1.5 ml) was collected from the cephalic vein of each animal in untreated tubes. Serum was harvested and assayed for leptin by an EIA kit (Cayman Chemical Co) and for GH by a validated ELISA modified for canine species. All animals were overnight fasted; females included in this study were in anestrous phase, determined on the basis of progesterone serum concentration (RIA), anamnesis and clinical signs.

RESULTS

All values are given as mean +/- SEM; the significance of differences between groups was assessed by Student's t-test. P<0.05 was considered significant. We did not observe any significant difference in GH levels among the groups: medium breed: 2.51 +/- 0.12 (males) and 2.63 +/- 0.17 ng/ml (females); large breed: 2.59 +/- 0.21 (males) and 2.72 +/- 0.23 ng/ml (females). A significant (p<0.05) positive correlation was found between leptin serum concentration and sex: medium breed: 0.6 +/- 0.04 (males) vs 1.5 +/- 0.08 ng/ml (females); large breed: 0.5 +/- 0.02 (males) vs 1.3 +/- 0.05 ng/ml (females). Leptin levels did not appear to be influenced by the size of animals.

CONCLUSION

This study shows that leptin levels in blood are significantly higher in bitches than in males; according to previous studies, we did not find any correlation between GH plasma levels and animal size, even if a single blood sample is obviously not fully reliable for GH evaluation. Our data agree well with those reported in humans and rats and contribute to support the hypothesis that leptin plays a significant role in modulating of female reproductive axis. It is well known, for example, that this peptide is crucial for female puberty's onset and gonadal development. Further studies are required to better define its role in reproductive system as well as its relation with metabolic hormones involved in the control of the hypothalamus-pituitary-gonadal axis.