The tissue distribution of ovine glucose transporter RNA : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science, Lincoln University

Abstract

The control of nutrient partitioning between tissues in an animal is of importance since it determines the potential of tissues to perform their particular functions. Various nutrient partitioning agents, such as insulin, GH and IGF-I have been identified and their partitioning effects have been extensively investigated. Glucose has been identified as one of the most important metabolites that is regulated by these partitioning agents. For example, the entry of glucose into cells is highly controlled by a family of glucose transporters which have tissue-specific distribution. Several research groups have proposed that nutrient partitioning agents regulate nutrient partitioning by acting directly or indirectly on the glucose transporters. To investigate the nutrient partitioning effects of GH, a trial was set up with 64 sheep genetically selected for either slow or fast glucose clearance. It has previously been shown that these animals exhibit altered nutrient partitioning such that animals from the slow glucose clearance line are leaner than animals from the fast glucose clearance line. A GH peptide vaccine was administered to 16 animals from the slow and to 16 animals from the fast glucose clearance line. The other animals were controls ( 16 slow and 16 fast glucose clearance respectively). After 5 months the animals were slaughtered and samples taken for analysis of carcass and non-carcass composition, pH, tenderness, and RNA distribution between tissues. Physiological and biochemical data indicated that there was no significant effect of the GH peptide on partitioning of nutrients between adipose and muscle tissues over the experimental period. Growth rate and insulin sensitivity were not affected by the administration of the GH peptide. A Growth Hormone Releasing Hormone (GRF) challenge increased circulatory GH in all animals, but there was no additional effect of the GH peptide on this challenge. A qualitative study of the distribution of glucose transporters in animals from the two selection lines was undertaken. Southern blot analysis illustrated that sheep possessed the genes for at least 4 glucose transporters. A PCR amplification of a portion of exon 11 of ovine GLUT4 produced a 329 nucleotide product which was sequenced. 84% homology occurred between human and ovine GLUT4 in this region. Northern blot analysis (with human cDNA glucose transporter probes) showed that GLUTl was present in brain, skin and muscle, GLUT2 in small intestines, GLUT3 in brain and GLUT4 in small intestines. Apart from the GLUT4 result, this pattern of distribution in the sheep is similar to the pattern observed in humans, rats, mice and cows. A novel ovine GLUTl probe was also used to examine the samples and showed that GLUTl was predominantly located in ovine brain, and that skin and adipose tissues expressed substantially less GLUTl. The distribution of glucose transporters in the ovine was shown to be similar to that of the bovine. Both patterns share some similarities with the human distribution pattern. A model was proposed in which GLUTl and GLUT4 levels in adipose and muscle tissues could account for the altered nutrient partitioning previously shown in the two lines of sheep selected on glucose clearance.