Cyclic changes in the osmolality and electrolyte composition in the gastrointestinal tract of the rock hyrax

The rock hyrax has been shown to have a very unusual and complex digestive tract. The gastrointestinal tract is comparable to that of the simple and complex stomach of mammals as well as to that of birds. Determinations of osmolality and electrolytes have been made in different sections of the gut of the hyrax. However, with the exception of the elevated potassium levels observed in the cranial stomach, the hyrax poses no unusual osmotic or electrolyte concentrations when compared to man or other mammals. The greater cation concentration and hypertonicity of the cranial stomach was largely accounted for by the elevated potassium load in this gut segment. Sodium and potassium comprised the major cations present throughout the gastrointestinal tract. Chloride was the major anion observed in the foregut; however, the electroneutrality of the mid and hindgut of the hyrax was primarily maintained by the presence of large concentrations of volatile fatty acids produced in these portions of the gut. Volatile fatty acid also accounted for the high anion concentration and hypertonicity of the cranial stomach.