Intracellular Phosphate Dynamics in Muscle Measured by Magnetic Resonance Spectroscopy during Hemodialysis

Abstract

Of the 600-700 mg inorganic phosphate (Pi) removed during a 4-hour hemodialysis session, a maximum of 10% may be extracted from the extracellular space. The origin of the other 90% of removed phosphate is unknown. This study tested the hypothesis that the main source of phosphate removed during hemodialysis is the intracellular compartment. Six binephrectomized pigs each underwent one 3-hour hemodialysis session, during which the extracorporeal circulation blood flow was maintained between 100 and 150 ml/min. To determine in vivo phosphate metabolism, we performed phosphorous ((31)P) magnetic resonance spectroscopy using a 1.5-Tesla system and a surface coil placed over the gluteal muscle region. (31)P magnetic resonance spectra (repetition time =10 s; echo time =0.35 ms) were acquired every 160 seconds before, during, and after dialysis. During the dialysis sessions, plasma phosphate concentrations decreased rapidly (-30.4 %; P=0.003) and then, plateaued before increasing approximately 30 minutes before the end of the sessions; 16 mmol phosphate was removed in each session. When extracellular phosphate levels plateaued, intracellular Pi content increased significantly (11%; P<0.001). Moreover, βATP decreased significantly (P<0.001); however, calcium levels remained balanced. Results of this study show that intracellular Pi is the source of Pi removed during dialysis. The intracellular Pi increase may reflect cellular stress induced by hemodialysis and/or strong intracellular phosphate regulation.

Keywords: chronic dialysis; hyperphosphatemia; intracellular pH; intracellular signal; ion transport; phosphate uptake.

Figure 1.

Good performance of dialysis. (A) Urea and (B) bicarbonates kinetics during dialysis. Values are means±SDs. Urea decreased over the time, and bicarbonates increased over the time.

Figure 2.

Calcium in the effluent was almost identical to the calcium contained in the dialysis solution. Calcemia does not modify over the time. (A) Kinetics of calcemia during dialysis sessions. (B) Calcium balance. Calcium balance was measured with the formula (Ca_e − Ca_b)(V_e − UF)+(Ca_e × UF), where Ca_e is the calcium in the effluent, Ca_b is the calcium in the dialysis solution, V_e is the volume of effluent, and UF is the ultrafiltration.

Figure 3.

Intra-cellular phosphate concentration increase during extracellular phosphate plateaued. Removed phosphate, extracellular Pi kinetics, and intracellular PCr-to-Pi ratio kinetics. (A) The mean removed phosphate was measured in the effluent bags throughout the sessions. (B) Extracellular phosphate decreased during the first 1 hour, plateaued, and then, rose before the end of dialysis. (C) The PCr-to-Pi ratio plateaued and then, decreased (−11%; P<0.001; i.e., increase of intracellular phosphate) when extracellular phosphate began to plateau.

Figure 4.

βATP decreased and intracellular pH increases during dialysis. (A) The Pcr/βATP ratio increases significantly (6%) during dialysis (meaning a decrease of βATP), whereas (B) pHi (+1%; P<0.001) increases significantly.

Figure 5.

Different peaks of molecules containing phosphate during MRS acquisition. ³¹P MRS acquisition with (A) the schematization of the surface coil sensitivity superimposed to the anatomic acquisition and (B) the acquired MR spectra showing Pi, PCr, and α-, β-, and γATP resonance peaks. L, left; R, right; I, inferior; S, superior.