Rabbit calcium metabolism, "bladder sludge," and urolithiasis (Proceedings)
Calcium is the most abundant mineral in the mammalian body, and the major component of bones and teeth. In addition, calcium is involved in a variety of vital physiologic processes including blood coagulation, muscle contraction, membrane permeability, nerve conduction, enzyme activity, and hormone release. Calcium balance is achieved by the interaction of various control mechanisms which regulate its absorption and excretion. In most mammals, the interaction of these regulatory mechanisms maintains serum calcium levels within a narrow range. In contrast, rabbits have adapted a unique strategy in which most dietary calcium is absorbed in the intestine, and the excess is excreted in the urine. In this species, calcium levels may vary within a wide range, and in direct proportion to the dietary calcium intake. This unique calcium metabolism has implications for the health and husbandry of the rabbit.
Calcium metabolism in rabbits
Calcium metabolism in the rabbit differs in several respects from other mammals. In most species, calcium absorption is closely regulated to balance metabolic needs, and serum calcium concentrations are maintained within a narrow range, typically 1.25-1.6 mmol/L (5.0 – 6.4 mg/dL). Total serum calcium in the rabbit is 30-50% higher than in other mammals, and varies over a wide range from 3.25-3.75 mmol/L (13-15 mg/dL). The intestinal absorption of calcium in most mammals involves primarily vitamin D3-regulated active transport. In the rabbit, however, calcium is absorbed in direct proportion to the amount ingested in the diet rather than in accordance with metabolic need, and calcium absorption is relatively independent of vitamin D.Most mammalian species grow only 1-2 sets of teeth in their lifetime. In contrast, a rabbits' teeth are constantly erupting at a rate of approximately 2 – 2.4 mm/week. The rabbit's increased life-long demand for calcium compared to other mammals is met by its efficient intestinal calcium absorption. In addition, during normal dental wear, calcium is released from the teeth, swallowed, and reabsorbed from the intestine.
In the rabbit, urinary calcium excretion increases in parallel to dietary intake. The fraction of calcium that can be filtered out of the blood is higher than in other mammals. The fractional excretion of calcium for most mammals is less than 2%; the range in rabbits is 45%-60%. When the reabsorptive capacity of the kidney is reached, calcium precipitates as calcium carbonate in the alkaline urine of the rabbit, causing cloudy or sludgy urine. When metabolic demand for calcium is increased by growth, pregnancy, lactation, or metabolic disorders, less calcium is excreted and the urine appears clear.