Mathematical aspects of renal function: reabsorption of individual solutes as interdependent processes

The estimation of the plasma free fraction of cyclosporine in rabbits and heart transplant patients: the application of a physiological model of renal clearance

We estimated the free fraction (fu) of cyclosporine (CyA) in the plasma from concentrations of CyA in urine (Cu) and plasma (Cp), urine flow rate (UF), and glomerular filtration rate in rabbits and in heart transplant patients. Following intravenous administration of CyA (5-30 mg kg-1) in ten NZW rabbits and oral administration of CyA (4.8-12.1 mg kg-1) in nine heart transplant patients, CyA concentrations in urine and plasma were measured by HPLC. The ratios of Cu to Cp and UF data were fitted to a physiological model of renal clearance using NONMEM. The free fraction of cyclosporine in the rabbits and the heart transplant patients was 0.0122 and 0.14, respectively. Because of the relatively low permeability of CyA across the tubular epithelium, no apparent equilibrium between Cu and Cp at any urine flow rate was reached and, therefore, the Cu to Cp ratio will not be equal to fu.

A comparison of the effects of glucose ingestion and NH4Cl acidosis on urinary calcium and magnesium excretion in man

Both glucose ingestion and NH(4)Cl acidosis have been reported to augment urinary calcium (U(Ca) V) and magnesium (U(Mg) V) excretion. Both also cause acidification of the urine and an increase in renal acid excretion. To examine whether a common mechanism of action was involved, the effects of glucose ingestion and NH(4)Cl acidosis on U(Ca) V and U(Mg) V were tested in the same subjects. Glucose ingestion caused significant increases in both U(Ca) V and U(Mg) V. During stable NH(4)Cl acidosis, U(Ca) V increased significantly, while U(Mg) V was unaffected. When a glucose load was given during acidosis, the separate effects of acidosis and glucose on U(Ca) V were additive, whereas U(Mg) V increased less than observed during normal acid-base balance. Although renal acid excretion increased and the urine was acidified after glucose in the normal steady state, when glucose was administered during NH(4)Cl acidosis urine pH rose and there was no change in renal acid excretion. We concluded that NH(4)Cl acidosis and glucose ingestion reduce the renal tubular reabsorption of magnesium and (or) calcium, but they act through separate mechanisms.