Phosphorus handling is a hot topic in renal physiology, especially considering the advanced being made in bone metabolism suggesting that vascular calcifications may play some role in the high mortality rate of chronic kidney disease patients. But how is it regulated at a molecular level?
The predominant phosphorus uptake channel in the proximal tubule is Napi channel, a Na-phosphorus cotransporter. Its regulation appears to be largely due to regulated trafficking. At basal conditions, the transporter is expressed on the apical surface of proximal tubule cells. However, when the cell receives signals from either PTH or FGF23 (which are secreted in response to elevated phosphorus levels), it will internalize Napi via clathrin-coated pits and it will ultimately be degraded; ultimately this will lead to decreased renal phosphorus reabsorption and a gradual decrease in phosphorus levels.