A. The Elements of Renal Phosphate Transport

B. Renal Phosphate Transport as a Whole

C. Measurement of Phosphate Reabsorption

D. The Physiology of Phosphate Excretion

E. Diseases of the Kidney and Serum Phosphate Homeostasis

A. The Elements of Renal Calcium Transport

B. Renal Calcium Transport as a Whole

C. Measurement of Calcium Reabsorption

D. The Physiology of Calcium Excretion

E. Diseases of the Kidney and Calcium Excretion

A. Parathyroid Hormone

B. Calcitonin

References

1 Glomerular Filtration

Micropuncture studies in amphibians and rats, although beset by many technical difficulties, should provide a direct comparison of the simultaneous concentrations of phosphate in serum and in the glomerular filtrate. The results of many such studies seem to indicate that these concentrations do not differ. Walser (1961a) has pointed out that the phosphate concentrations in glomerular filtrate and in serum can only be equal when about 13% of the serum phosphate is not filterable. One reason is that in measuring serum phosphate concentrations, the volume occupied by proteins is not taken into account. In addition, the presence of serum proteins on only one side of the ultrafiltering membrane will induce an electrochemical gradient across the membrane, and, as a result, the distribution of ions along the two sides of the membrane will be unequal (the Donnan equilibrium). He further reviewed existing in vitro ultrafiltration studies of serum phosphate and found that many were unreliable because factors, such as pH, pCO2, or temperature, had not been taken into account. However the best available controlled studies and his own studies in man show that, whatever the absolute value of the serum phosphate concentration, the ultrafiltrates of serum have approximately the same phosphate concentration as the serum itself. On this basis, he reasoned that on the average 13% of the serum phosphate is protein-bound and nonfilterable (Walser, 1961a) (Table I). Therefore, it just so happens that, despite considerable protein binding, the phosphate concentration in glomerular filtrate equals the serum phosphate concentration [PO4 (w/v)]. This is true for a wide range of serum phosphate concentrations. The filterable fraction of phosphate does apparently not change when phosphate is infused to raise its concentration in the blood to 10 mg per 100 ml. However, rapid and marked elevations of calcium and phosphate levels can result in formation of nonfilterable colloidal complexes of calcium phosphate (McLean and Hinricks, 1938). The filtration rate of phosphate in the kidney (filtered load, LPO4 weight/time) can, therefore, be calculated as the product of serum phosphate concentration and glomerular filtration rate [G.F.R. (volume/time)] [Eq. (1)]

(1)

Henceforth in this chapter the amount filtered per unit time will be designated as filtered load (LPO4) and defined as [PO4] × G.F.R. Moreover, the serum phosphate concentration [PO4] can be considered as equal to the amount of phosphate filtered per unit volume of glomerular filtrate [Eq. (2)].

(2)

2 Tubular Reabsorption and Secretion

Renal tubular handling of phosphate is still not completely understood. In man and in mammals the rate of phosphate excretion [UPO4V (weight/time)] is always less than the filtration rate. There is, therefore, a net reabsorption of phosphate. The net reabsorption rate [TPO4 (weight/time)] is defined as the difference between filtered load and excretion rate and can only be measured indirectly [Eq. (3)].

(3)

The occurrence of net phosphate reabsorption does not, however, preclude secretion of phosphate somewhere along the renal tubules, provided the reabsorption rate exceeds the secretion rate. Micropuncture studies in rats and dogs as well as stop-flow studies in dogs suggest a localization of phosphate reabsorption in the first part of the proximal tubules (Strickler et al., 1964; Malvin et al., 1958; Agus...