If sodium is transported across the tubule membrane down the concentration gradient established via the $Na^+$/$K^+$ exchanger via symport with glucose and antiport with hydrogen, and water is transported via aquaporins...how is this coupled? They don't have the same transporter. This seems like they are independent.
Is this related to concentration gradients (more $Na^+$ in epithelial cells --> water wants to follow osmotically)? If so, why is $Na^+$ controlling whether water is reabsorbed or secreted, and not some other ion?