published_or_final_version / Pharmacology / Master / Master of Philosophy

published_or_final_version / Physiology / Master / Master of Philosophy

published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

published_or_final_version / Medicine / Doctoral / Doctor of Philosophy

published_or_final_version / Pharmacology / Master / Master of Philosophy

published_or_final_version / Physiology / Doctoral / Doctor of Philosophy

No description available.

No description available.

Laboratory experiments have suggested that the coexistence of certain heteromyid rodent species is facilitated through differential use of seed dispersions (clump sizes). Field experiments with the Bannertail kangaroo rat, Dipodomys spectabilis, and Merriam's kangaroo rat, D. merriami, indicate their use of different clump sizes is not sufficiently specialized to permit coexistence. Field experiments based on the assumption that these species interact primarily through aggression demonstrated that during at least one season of the year D. spectabilis excludes the smaller D. merriami from its foraging range. Preliminary characterization of spatial heterogeneity of resource productivity implies that the competitive coexistence of these species is regional and is dependent upon the existence of areas with low productivity that are not economically usable by D. spectabilis. These areas of low productivity may serve as a competitive refuge for D. merriami, facilitating its persistence. A brief review of the evidence for the occurrence of competition between heteromyid species is presented, as is a selected review of the evidence supporting the various hypotheses explaining the competitive coexistence of heteromyid rodents.

The suckling rat has provided a useful model for these studies of lipid digestion and absorption. In adults dietary triacylglycerols are predominantly hydrolyzed by the "classic" mechanism involving pancreatic lipase (E.C. 3.1.1.3) and biliary micelle-mediated product uptake. Unlike affluent man, adult laboratory rats normally consume a low-fat diet (< 20% of energy). However suckling rats, like most neonates, consume a milk diet that is normally high in fat. Suckling rats receive ~90% of non-protein energy from milk triacylglycerols, although rats have very low levels of "classic" pancreatic lipase before weaning. Dissertation studies demonstrate that other lipases promote efficient gastrointestinal triacylglycerol utilization in suckling rats. Nearly half of milk triacylglycerols are hydrolyzed to diacylglycerol within the stomach; and results strongly support that lingual lipase is the significant source of this activity. Furthermore, enzyme studies show considerable similarity between this lipase from rat tongue and so-called pregastric esterases of suckling ruminant species. Of fatty acids released in stomach, nearly three-quarters are of medium-chain length. These have high aqueous solubility and are rapidly absorbed from the upper gastrointestinal tract. Remaining triacylglycerols, diacylglycerols, and long-chain fatty acids enter the intestinal lumen and mix with lipid-rich bile. Suckling rats have higher biliary concentrations of bile salts, phospholipids and cholesterol than do adults. Bile salt enterohepatic circulation clearly occurs in the suckling rat, at least as early as 10 days. The bile acid β-muricholate is elevated during the suckling period, compared to post-weaning. Results show that further lipolysis occurs along the intestine, possibly through the action of lingual lipase and others from pancreas, producing mostly long-chain free fatty acids, monoacylglycerol, and some lysophosphatidylcholine, derived largely from bile. This composition closely resembles adult intestinal contents, but is much higher in both dietary and biliary lipid constituents. Alteration of milk triacylglycerol fatty acids produced no apparent impairment in gastrointestinal lipid utilization by sucklings, but resulted in hyperlipemia and increased carcass fatness. These results suggest gastrointestinal events do not limit triacylglycerol utilization in the suckling rat, regardless of composition; but post-absorptive metabolism may be different when sucklings consume altered milk lipids.