Effects of Saline Loading on Jejunal Absorption of Calcium, Sodium, and Water, and on Parathyroid Hormone Secretion in the Rat

To investigate whether intestinal calcium absorption parallels that of sodium following extra-cellular fluid volume expansion, the effects of saline loading on intestinal transport of calcium, sodium and water were studied in rats by perfusing jejunal loops in situ. After calcium-free saline infusion net calcium absorption was reversed similar to that of sodium and water and net secretion occurred. Concurrently, blood-to-lumen (b-l) calcium flux, measured using 45Ca, increased significantly (P less than 0.001). Following expansion with calcium-containing Ringer a similar reversal of net calcium, sodium and water flux was also observed. Again the b-l calicum flux increased but to a significantly lesser extent (P less than 0.05). Plasma ionized calcium remained unchanged after calcium-rich Ringer loading, but decreased significantly (P less than 0.001) when calcium was omitted from the solution. Plasma immunoreactive parathyroid hormone unchanged after expansion with the calcium containing solution but increased following calcum-free infusion. It is concluded that after extracellular fluid volume expansion: 1. net jejunal calcium absorption is decreased; 2. the decrease parallels that of sodium and water; 3. b-l calcium transport is enhanced to a greater degree of calcium-free Rnger infusion than by a calcium-rich solution. This difference could be the result of increased parathyroid hormone secretion.