No evidence of a countercurrent multiplier in the intestinal villus of the dog

Comparative pathophysiology and management of protein-losing enteropathy

Protein‐losing enteropathy, or PLE, is not a disease but a syndrome that develops in numerous disease states of differing etiologies and often involving the lymphatic system, such as lymphangiectasia and lymphangitis in dogs. The pathophysiology of lymphatic disease is incompletely understood, and the disease is challenging to manage. Understanding of PLE mechanisms requires knowledge of lymphatic system structure and function, which are reviewed here. The mechanisms of enteric protein loss in PLE are identical in dogs and people, irrespective of the underlying cause. In people, PLE is usually associated with primary intestinal lymphangiectasia, suspected to arise from genetic susceptibility, or “idiopathic” lymphatic vascular obstruction. In dogs, PLE is most often a feature of inflammatory bowel disease (IBD), and less frequently intestinal lymphangiectasia, although it is not proven which process is the true driving defect. In cats, PLE is relatively rare. Review of the veterinary literature (1977‐2018) reveals that PLE was life‐ending in 54.2% of dogs compared to published disease‐associated deaths in IBD of <20%, implying that PLE is not merely a continuum of IBD spectrum pathophysiology. In people, diet is the cornerstone of management, whereas dogs are often treated with immunosuppression for causes of PLE including lymphangiectasia, lymphangitis, and crypt disease. Currently, however, there is no scientific, extrapolated, or evidence‐based support for an autoimmune or immune‐mediated mechanism. Moreover, people with PLE have disease‐associated loss of immune function, including lymphopenia, severe CD4+ T‐cell depletion, and negative vaccinal titers. Comparison of PLE in people and dogs is undertaken here, and theories in treatment of PLE are presented.

Understanding gastrointestinal perfusion in critical care: so near, and yet so far

An association between abnormal gastrointestinal perfusion and critical illness has been suggested for a number of years. Much of the data to support this idea comes from studies using gastric tonometry. Although an attractive technology, the interpretation of tonometry data is complex. Furthermore, current understanding of the physiology of gastrointestinal perfusion in health and disease is incomplete. This review considers critically the striking clinical data and basic physiological investigations that support a key role for gastrointestinal hypoperfusion in initiating and/or perpetuating critical disease.

Nutrient-induced changes in intestinal blood flow in the dog

The cardiovascular response to feeding consists of two phases. The anticipation/ingestion phase is a brief, generalized cardiovascular response mediated by sympathetic neural activity. The digestive/absorptive phase is a longer, locally mediated vascular response to luminal nutrients. This post-prandial hyperaemia is localized primarily to the intestinal segment and tissue layer (mucosa) exposed to chyme. The stimuli for this hyperaemia are the digested products of food, and bile. Micellar fatty acids induce the greatest hyperaemia, followed by glucose. Individual amino acids have little effect, but by-products of protein digestion may increase blood flow. Bile enhances the glucose-induced hyperaemia and renders fatty acids and amino acids vasoactive. The mechanisms involved in the initiation and maintenance of the hyperaemia are complex and involve numerous factors that may vary with the type of nutrient. The factors involved include intestinal activity such as absorption, motility and secretion, tissue oxidative metabolism, adenosine, tissue oxygen tension, the enteric nervous system, gastrointestinal peptides such as vasoactive intestinal polypeptide, and paracrine substances such as histamine and prostanoids. The post prandial intestinal hyperaemia is probably the net result of the complex interaction of all these factors on the intestinal vascular smooth muscle.

Tissue osmolality in intestinal villi of four mammals in vivo and in vitro

Using a freezing point depression method osmolality in the intestinal tissue of four mammals (gerbils, guinea-pigs, rabbits and rats) was estimated in vivo, during fluid transport from an isotonic electrolyte-glucose solution. Net fluid transport was also measured. In gerbils, guinea-pigs and rabbits tissue osmolality was also estimated during in vitro conditions. A marked hyperosmolality was observed in vivo in the upper parts of the villi of all four mammals studied. The tissue osmolality was significantly higher than that seen in the same species during in vitro conditions. A villus hyperosmolality was observed also in species which exhibited a net fluid secretion (guinea-pig, rabbit ileum), indicating that the fluid secretion emanated from the intestinal crypts. Based on the results of the present experiments and on observations made in earlier experiments performed on the cat, it is proposed that the villus hyperosmolality is created by a countercurrent multiplier present in the intestinal villus. The hyperosmolar compartment in the villus tissue creates the force that drives fluid from lumen to tissue.