PROXIMAL TUBULAR LITHIUM PLASMA CONCENTRATION RATIO DEPENDS ON TUBULAR FLUID-FLOW RATE/

Previous studies have given slightly different values of proximal tubu lar fluid/plasma lithium concentration ratios (TF/P)(L)i. The aim of t he present study was to investigate whether the technique of tubular f luid collection by micropuncture might contribute to this difference. Early distal and late proximal tubular fluid was collected by micropun cture during recording of early proximal hydrostatic pressure with var ying pressures applied to the collection pipette. Distal fluid collect ion with 2-3 mmHg suction was unable to influence distal flow, proxima l tubular pressure, nephron filtration rate (SNG FR). (TF/P)(L)i or (T F/P)(Na) significantly. as compared with values obtained without sucti on, in contrast, during late proximal collections, tubular pressure co uld 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 ni min(-1). (TF/P)(L)i was positively and significantly c orrelated to late proximal flow rate (P < 0.005, r = 0,65). independen t oi tubular pressure. (TF/P)(Na) was 1.03 and independent of flow rat e and pressure. With collection adjusted to a late proximal flow of 24 nL min(-1), (TF/P)(L)i would be about 1.15. At the more physiological flow rate of 12-14 ni min(-1), (TF/P)(L)i was not significantly diffe rent from 1.00. It is concluded that proximal ii transport is distinct from Na transport and lags behind Na and water transfer only when flo w rate is increased above normal. The result emphasizes the importance of the micropuncture fluid collection technique for estimates of prox imal (TF/P)(L)i.