Final answer:
Calcium transport across an enterocyte's basolateral border is facilitated by calcium-binding proteins, which are influenced by PTH and calcitriol, and occurs alongside the action of an electrochemical gradient created by the Na+/K+ ATPase pump.
Step-by-step explanation:
Calcium crosses the basolateral border of an enterocyte primarily via the action of calcium-binding proteins. These proteins facilitate the movement of calcium inside the cell and help in its exocytosis across the basolateral membrane into the interstitial space. This is influenced by the presence of parathyroid hormone (PTH), which when bound to its receptors on the cells of the distal convoluted tubule (DCT), promotes the insertion of calcium channels on the luminal surface, enhancing calcium recovery from the forming urine. Moreover, the active efflux of Na+ from the cell by the Na+/K+ ATPase creates an electrochemical gradient that attracts Ca++ into the cell. Additionally, calcitriol, the active form of vitamin D, plays a critical role by inducing the production of calcium-binding proteins, essential for calcium transportation into the cell.