Proximal tubular lithium/plasma concentration ratio depends on tubular fluid flow rate

Previous studies have given slightly different values of proximal tubular fluid/plasma lithium concentration ratios (TF/P)Li. The aim of the present study was to investigate whether the technique of tubular fluid collection by micropuncture might contribute to this difference. Early distal and late proximal tubular fluid was collected by micropuncture during recording of early proximal hydrostatic pressure with varying pressures applied to the collection pipette. Distal fluid collection with 2-3 mmHg suction was unable to influence distal flow, proximal tubular pressure, nephron filtration rate (SNGFR), (TF/P)Li or (TF/P)Na significantly, as compared with values obtained without suction. In contrast, during late proximal collections, tubular pressure could be controlled. Within the range 2 mmHg below to 6 mmHg above free flow pressure maintained during collection, proximal flow rate varied from 9 to 44 nL min-1. (TF/P)Li was positively and significantly correlated to late proximal flow rate (P < 0.005, r = 0.65), independent of tubular pressure. (TF/P)Na was 1.03 and independent of flow rate and pressure. With collection adjusted to a late proximal flow of 24 nL min-1. (TF/P)Li would be about 1.15. At the more physiological flow rate of 12-14 nL min-1, (TF/P)Li was not significantly different from 1.00. It is concluded that proximal Li transport is distinct from Na transport and lags behind Na and water transfer only when flow rate is increased above normal. The result emphasizes the importance of the micropuncture fluid collection technique for estimates of proximal (TF/P)Li.