The digestive physiology of the Pacific hagfish, Eptatretus stoutii

• Author / Creator

  Weinrauch, Alyssa

• Hagfish are considered the oldest living representatives of the vertebrates and as such are of interest for studies in evolutionary biology. Moreover, hagfish occupy a unique trophic niche wherein they are active predators and benthic scavengers of a wide range of invertebrate and vertebrate prey, yet can tolerate nearly a year of fasting. Hagfish are also capable of acquiring nutrients across the integument in a manner similar to that of marine invertebrates, a hallmark of their position between the invertebrate and vertebrate lineages. Despite many interesting modes of feeding, relatively few studies have examined the digestive physiology of the hagfishes.Feeding and digestion cause many physiological perturbations, which result in elevated metabolic rate. I have characterised the increased metabolic demand (specific dynamic action) and determined that its peak in hagfish corresponds with the peak of acid-base alterations. Specifically, the rate of base efflux reaches a maximum 8 h post-feeding when the intestinal blood supply is significantly more alkaline. The offloading of base to the circulation (alkaline tide) is common of vertebrates that employ an acidic digestion in order to maintain cellular acid-base homeostasis. Likewise, the hagfishes employ a luminal acidification, despite lacking a stomach. I further investigated the cellular mechanisms responsible for hindgut luminal acidification and found that soluble adenylyl cyclase and cAMP are utilised to enhance proton secretion via proton-potassium exchangers (HKA), as is found in later-diverging vertebrates. Vacuolar-type ATPase (VHA), which energizes gastric alkalization in some invertebrates, was also present and used for luminal acidification. In addition to acidic digestion, hagfish also utilize digestive enzymes. I have characterised the distribution and post-feeding activities of six digestive enzymes (α-amylase, maltase, lipase, trypsin, aminopeptidase and alkaline phosphatase) along the hagfish digestive tract. Contrary to previous reports, enzymatic activity is not restricted to the hindgut and is present in the buccal cavity and pharyngocutaneous duct. Furthermore, enzyme activity is differentially regulated post-feeding, indicating the first instance of compartmentalization of gut function in the vertebrates. Once digested, nutrients are absorbed across the intestinal epithelium using specialized transport proteins. I have identified specific, carrier-mediated transport pathways for glucose, oleic acid, and L-alanyl L-alanine in the hagfish hindgut. Glucose uptake likely occurs via sodium-linked glucose transporters (SGLT) and glucose transporters (GLUT) as both sodium-dependent and -independent uptake were observed. Furthermore, specific pharmacological inhibitors of each transporter type effectively reduced glucose acquisition. Oleic acid acquisition was not altered with insulin application, consistent with the behaviour of the intestinal isoforms of mammalian fatty acid transport proteins (FATP). Feeding significantly increased the maximal uptake rate of oleic acid, suggesting an increased apical expression of transporters, possibly as a result of increased surface area. Indeed, post-prandial intestinal remodeling occurred wherein mucosal thickness and microvilli length were significantly increased. Microvilli length rapidly regressed to fasting conditions within 36 h of a feed, suggesting that intestinal regression is utilised to conserve energy in between feeds. Dipeptide (L-alanyl L-alanine) uptake, similarly increased post-feeding and had a sodium-dependent component. This could suggest a role for sodium-proton exchangers (NHE) as has been identified in mammals. Overall, the mechanisms of uptake and regulation of transporter function appear to be conserved between hagfish and later-diverging vertebrates. Hagfish therefore represent an excellent model for studies of comparative vertebrate digestive physiology as they diverged prior to the genome duplication, have relatively simpler intestinal morphology, and conserved mechanisms of digestion and nutrient acquisition.

• Subjects / Keywords

  • acidic digestion
  • nutrients
  • digestive enzymes
  • transport

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-7mxv-yc70

• License

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