Rabbits (Oryctolagus cuniculus) increase caecal calcium absorption at increasing dietary calcium levels.

Area of Science:

• Comparative physiology
• Gastrointestinal biology
• Nutrient absorption

Background:

• Hindgut fermenting herbivores often absorb more calcium (Ca) than needed, excreting the surplus via urine.
• The reasons for this surplus Ca absorption, potentially to aid hindgut microbial phosphorus availability, remain unclear.

Purpose of the Study:

• To investigate the mechanisms and dietary influences on active calcium absorption in rabbits (Oryctolagus cuniculus).
• To test the hypothesis that herbivores deliberately manage Ca absorption to benefit their gut microbiome.

Main Methods:

• Ussing chamber experiments were conducted on rabbits fed four different diets.
• Active Ca absorption was measured across different intestinal segments.
• The expression of calbindin-D9K (CB) and vitamin D receptor (VDR) proteins was quantified.

Main Results:

• Rabbits increased active Ca absorption with higher dietary Ca levels, contrary to expectations of passive uptake dominance.
• Ca absorption was significantly higher in the caecum than the duodenum.
• Increased dietary Ca correlated with higher levels of CB and VDR, but their distribution suggests other regulatory factors.

Conclusions:

• Rabbit Ca absorption is actively regulated even at high dietary levels, supporting the deliberate removal hypothesis.
• The high caecal absorption is unusual for mammals and warrants further investigation.
• Additional unstudied factors significantly influence Ca absorption in rabbits, highlighting a complex interplay in herbivore digestion.