Zonation of Ca2+ transport and enzyme activity in the caeca of rainbow trout – a simple structure with complex functions

Trout caeca are vermiform structures projecting from the anterior intestine of the gastrointestinal tract. Despite their simple gross morphology, these appendages are physically distinct along the anterior-posterior axis, and ultrastructural evidence suggests zonation of function within the structures. Individual caeca from three sections (anterior, middle and posterior) were removed from the intestine of freshwater rainbow trout and investigated for ion transport and enzyme activity. Ca²⁺ absorption appeared as a combination of active and passive movement, with Michaelis-Menten kinetics observable under symmetrical conditions, and was inhibited by several pharmacological agents (ouabain, La³⁺ and a calmodulin antagonist). There was a decrease in ion transport function from adjacent to the intestine (proximal) to the distal tip of each caecum, along with decreasing transport from anterior to posterior for the proximal portion alone. Feeding increased the J_Max and K_M for Ca²⁺ absorption within all sections, whereas ion-poor water (IPW) exposure further increased the J_Max and K_M for Ca²⁺ transport in the anterior and middle sections. Increased Na⁺/K⁺-ATPase (NKA) and citrate synthase (CS) activity rates paralleled trends seen in Ca²⁺ transport. Feeding in freshwater and IPW exposure increased the glycolytic capacity of the caeca via increased pyruvate kinase (PK) and decreased lactate dehydrogenase (LDH) activity, while amino acid metabolism increased with IPW exposure through increased glutamate dehydrogenase (GDH) activity. Overall, feeding and IPW exposure each altered ionoregulation within the caeca of freshwater rainbow trout in a zone-specific pattern, with the anterior and proximal portions of the caeca being most affected. Increased carbohydrate and protein metabolism fueled the increased ATP demand of NKA through CS.