Thesis (M. Med. Sc.)--University of Hong Kong, 2006.

Horses with high energy requirements are generally fed two large concentrate meals per day, either in the form of grain or pellets. The postprandial elevation of blood glucose resulting from this type of feeding has the potential to alter production of hormones such as leptin. Leptin is an adipose-derived protein that promotes satiety in normal animals. The objective of this study was to determine if feeding large amounts of concentrate twice each day would alter serum leptin concentration. Nine horses were placed into three groups (A, B, and C) and each group was rotated through three feeding schedules (2x, 3x, and 4x) in a 3 x 3 Latin square design. Horses were fed twice per day on the 2x schedule, three times per day on the 3x schedule, and four times per day on the 4x schedule. Horses were fed the same total amount of concentrate per day throughout the study, although meal size varied with the number of times the horse was fed per day. Horses were weighed and scored for body condition on the first day of each period. Each treatment period lasted for 11 days. Blood was drawn on days one, four, and seven of each period and leptin concentration was determined by radioimmunoassay. On the afternoon of the tenth day of each period, horses were fitted with jugular catheters and blood was drawn every two hours for 24-hours to determine the circadian rhythm of leptin secretion. Additionally, blood was taken 30 minutes prior to and every 30 minutes after the morning meal to determine postprandial plasma glucose concentrations. Mean and peak glucose values were higher on the 2x schedule than the 3x or 4x schedules (P < 0.05). Leptin concentration was highest in horses on the 3x schedule, although when these data were normalized to baseline (day one) values, leptin was highest on the 2x schedule (P < 0.05). Serum leptin concentration was highly correlated with body condition score (P < 0.01), but not gender (P = 0.82), and leptin increased throughout the study (P < 0.05). Data from the 24-hour collection showed that serum leptin concentration varied with time in horses on the 2x but not the 4x schedule (P < 0.05). Linear regression of data from the 2x schedule indicates that the pattern of change may be modeled by a quadratic equation (P < 0.05). This study demonstrates that feeding horses large carbohydrate meals twice per day disrupts the normal pattern of leptin in the horse, possibly affecting appetite and other physiological processes.

Horses with high energy requirements are generally fed two large concentrate meals per day, either in the form of grain or pellets. The postprandial elevation of blood glucose resulting from this type of feeding has the potential to alter production of hormones such as leptin. Leptin is an adipose-derived protein that promotes satiety in normal animals. The objective of this study was to determine if feeding large amounts of concentrate twice each day would alter serum leptin concentration. Nine horses were placed into three groups (A, B, and C) and each group was rotated through three feeding schedules (2x, 3x, and 4x) in a 3 x 3 Latin square design. Horses were fed twice per day on the 2x schedule, three times per day on the 3x schedule, and four times per day on the 4x schedule. Horses were fed the same total amount of concentrate per day throughout the study, although meal size varied with the number of times the horse was fed per day. Horses were weighed and scored for body condition on the first day of each period. Each treatment period lasted for 11 days. Blood was drawn on days one, four, and seven of each period and leptin concentration was determined by radioimmunoassay. On the afternoon of the tenth day of each period, horses were fitted with jugular catheters and blood was drawn every two hours for 24-hours to determine the circadian rhythm of leptin secretion. Additionally, blood was taken 30 minutes prior to and every 30 minutes after the morning meal to determine postprandial plasma glucose concentrations. Mean and peak glucose values were higher on the 2x schedule than the 3x or 4x schedules (P < 0.05). Leptin concentration was highest in horses on the 3x schedule, although when these data were normalized to baseline (day one) values, leptin was highest on the 2x schedule (P < 0.05). Serum leptin concentration was highly correlated with body condition score (P < 0.01), but not gender (P = 0.82), and leptin increased throughout the study (P < 0.05). Data from the 24-hour collection showed that serum leptin concentration varied with time in horses on the 2x but not the 4x schedule (P < 0.05). Linear regression of data from the 2x schedule indicates that the pattern of change may be modeled by a quadratic equation (P < 0.05). This study demonstrates that feeding horses large carbohydrate meals twice per day disrupts the normal pattern of leptin in the horse, possibly affecting appetite and other physiological processes.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 82-107). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 82-107). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 103-104). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 103-104). Also available on the Internet.

Leptin, the protein product of obese gene, is a 16-kD adipokine with regulatory functions on food intake and energy metabolism. At present, limited information is available on leptin functions and regulation in lower vertebrates mainly due to the fact that the primary structure of leptin is highly diversified from fish to mammals. Leptin in teleost fish is even more complicated as leptin isoforms have been reported presumably as a result of whole-genome duplication that occurred during the evolution of modern-day bony fish. Using grass carp (Ctenopharyngodon idella) as a model, I sought to investigate the physiological functions and endocrine regulation of leptin in bony fishes. As a first step, the structural identities of two leptins, namely leptin A and B, were established by 5’/3’RACE. The two isoforms share low levels of amino acid sequence homology with mammalian leptins but their deduced 3D-protein models are highly comparable to that of the human counterpart. In grass carp, leptin A and B are widely expressed with highest levels of expression detected in the liver with leptin A as the dominant form. To study the biological actions of grass carp leptins, recombinant proteins of leptin A and B were produced in Escherichia coli and found to inhibit both basal and NPY-stimulated food consumption and feeding behavior in goldfish by both intraperitoneal and intracerebroventricular injection. In addition to the anorexic effects observed, the effects of leptin on pituitary hormone secretion and synthesis were also examined in primary culture of carp pituitary cells. Using reverse transcription-polymerase chain reaction coupled to laser captured microdissection, leptin receptor expression was detected in somatotrophs, gonadotrophs and lactotrophs. Furthermore, leptin A and B were both effective in increasing basal secretion, cell content and transcript expression of growth hormone, luteinizing hormone and prolactin in carp pituitary cells. In the same study, parallel rises in somatolactin α and β mRNA levels without major changes in transcript expression of other pituitary hormones were also noted. These stimulatory effects were mediated by differential coupling with Janus kinase-2 (JAK2)/ signal transducers and activators of transcription (STATs), mitogen-activated protein kinase (MAPK) and/or phosphoinositide 3-kinase (PI3K)/Akt pathways. Although leptin A and B exhibited similar effects on feeding and pituitary hormone expression, their endocrine regulation appears to be quite different. In primary culture of carp hepatocytes, insulin could reduce leptin A but not leptin B mRNA levels through MAPK but not PI3K/Akt pathway. Glucagon, in contrast, could trigger leptin A but not leptin B mRNA expression via the cAMP/PKA cascades and this stimulatory effect could be negated by co-treatment with insulin. At the hepatic level, SLα could also induce leptin A but not leptin B mRNA expression via JAK2 activation of PI3K/Akt cascades. Parallel treatment with SLβ, however, was found to up-regulate leptin B but not leptin A transcription by MAPK coupling to JAK2. These results suggest that the two leptin isoforms identified in grass carp are responsible for similar biological functions but under differential regulation by various endocrine factors coupled to a number of post-receptor signaling mechanisms. / published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / Zoology / Master / Master of Philosophy

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