Background:

The Na⁺-coupled phosphate transporter NaPiIIa is the main carrier accomplishing renal tubular phosphate reabsorption. It is driven by the electrochemical Na⁺ gradient across the apical cell membrane, which is maintained by Na⁺ extrusion across the basolateral cell membrane through the Na⁺/K⁺ ATPase pump. The operation of NaPiIIa thus requires energy in order to avoid cellular Na⁺ accumulation and K⁺ loss with eventual decrease of cell membrane potential, Cl^- entry and cell swelling. Upon energy depletion, early inhibition of Na⁺-coupled transport processes may delay cell swelling and thus foster cell survival. Energy depletion is sensed by the AMP-activated protein kinase (AMPK), a serine/threonine kinase stimulating several cellular mechanisms increasing energy production and limiting energy utilization. The present study explored whether AMPK influences the activity of NAPiIIa.

Methods:

cRNA encoding NAPiIIa was injected into Xenopus oocytes with or without additional expression of wild-type AMPK (AMPK^α1-HA+AMPK^β1-Flag+AMPK^γ1-HA), of inactive AMPK^αK45R (AMPK^α1K45R+AMPK^β1-Flag+AMPK^γ1-HA) or of constitutively active AMPK^γR70Q (AMPK^α1-HA+AMPK^β1-Flag+AMPKγ1^R70Q). NaPiIIa activity was estimated from phosphate-induced current in dual electrode voltage clamp experiments.

Results:

In NaPiIIa-expressing, but not in water-injected Xenopus oocytes, the addition of phosphate (1 mM) to the extracellular bath solution generated a current (I_p), which was significantly decreased by coexpression of both wild-type and constitutively active AMPK but not of AMPK^αK45RI_p in NaPiIIa and AMPK expressing oocytes was significantly increased by AMPK inhibitor Compound C (20 µM). . AMPK significantly decreased the maximal transport rate.

Conclusions:

The AMP-activated protein kinase AMPK is a powerful regulator of NaPiIIa and thus of renal tubular phosphate transport.