Do herbivorous minnows have “plug-flow reactor” guts? Evidence from digestive enzyme activities, gastrointestinal fermentation, and luminal nutrient concentrations

Few investigations have empirically analyzed fish gut function in the context of chemical reactor models. In this study, digestive enzyme activities, levels of gastrointestinal fermentation products [short chain fatty acids (SCFA)], luminal nutrient concentrations, and the mass of gut contents were measured along the digestive tract in herbivorous and carnivorous minnows to ascertain whether their guts function as “plug-flow reactors” (PFRs). Four of the species, Campostoma anomalum, C. ornatum, C. oligolepis, and C. pauciradii, are members of a monophyletic herbivorous clade, whereas the fifth species, Nocomis micropogon, is a carnivore from an adjacent carnivorous clade. In the context of a PFR model, the activities of amylase, trypsin and lipase, and the concentrations of glucose, protein, and lipid were predicted to decrease moving from the proximal to the distal intestine. I found support for this as these enzyme activities and nutrient concentrations generally decreased moving distally along the intestine of the four Campostoma species. Furthermore, gut content mass and the low SCFA concentrations did not change (increase or decrease) along the gut of any species. Combined with a previous investigation suggesting that species of Campostoma have rapid gut throughput rates, the data presented here generally support Campostoma as having guts that function as PFRs. The carnivorous N. micropogon showed some differences in the measured parameters, which were interpreted in the contexts of intake and retention time to suggest that PFR function breaks down in this carnivorous species

Digestion and Metabolism

In sheep and other ruminants, the exposure of ingested food to the metabolic activities of ruminal bacteria, protozoa and fungi (see Mackie et al., Chapter 4, this volume) has profound implications for the digestion and metabolism of food.Plant carbohydrates, usually the major sources of energy in ruminant diets, are largelyfermented to short-chain fatty acids (SCFA). These consist mainly of acetate, propionate andbutyrate and are readily absorbed from the rumen and metabolised in tissues to supportmaintenance and production. The degradation of ß-linked glucose polymers (cellulose andhemicellulose) from plant cell walls is of obvious nutritional benefit, since these materials arenot hydrolysed by the endogenous enzymes of the host animal. In contrast, the readyfermentation of starch and other α-linked glucose polymers, which are potential sources ofglucose if they reach the small intestine (SI), implies that only small amounts of glucose areabsorbed from the SI (see p.106)

The recycling of carbon in glucose, lactate and alanine in sheep

Pregnant ewes with catheters implanted in an artery and the uterine and recurrent tarsal veins were infused at a constant rate with U−¹⁴C-labelled glucose, alanine or bicarbonate. Measurements were made of the overall and local fractional contribution of glucose and alanine to CO₂ production and of the extent of interconversion of these metabolites. In the whole animal, by coupling the results with the authors’ previous study of lactate metabolism, a solution was obtained to an open unrestricted 4-compartment model of the exchange of carbon between glucose, lactate, alanine and CO₂. A more limited study was made with non-pregnant sheep because complete data for lactate interactions with alanine were not available. Our analysis of glucose/lactate/alanine/CO₂ interactions in pregnant sheep suggests that about two-thirds of the glycogenic carbon was oxidised fairly directly to CO₂. There was relatively little recycling of glucose carbon through lactate and alanine so that most of the remaining glycogenic carbon was stored as product with relatively long turnover time. It is possible that much of this was in the form of muscle glycogen, and analysis of glycogenic carbon exchange across the hind limb muscle was consistent with this conclusion. In non-pregnant ewes, the findings, although incomplete, suggested that there were no great differences from the findings in pregnant ewes.Derek B. Lindsay, Patrick J. Barker, Andrew J. Northrop, Brian P. Setchell, Graham J. Faichne