Reexamination of the split oil droplet method as applied to kidney tubules

The technique and evaluation of the split oil drop method of Gertz was reexamined in rats with a view to defining the correct dimension in which the results should be expressed so that the point of reference for reabsorptive capacity thus obtained be uninfluenced by the geometry of the tubule. 1. It was found that one tubular diameter had to be added to each length between the tops of the approaching menisci to produce a truly linear relationship between these lengths and time. This correction improved the coefficient of variation (reproducibility) of the reabsorptive half time (t1/2) and eliminated the correlation between the length of the split column and t1/2. 2. Furthermore a significant correlation was found between internal tubular radius and corrected t1/2's in a group of tubules with naturally varying diameters, indicating constancy of reabsorptive capacity per unit surface area. 3. The reproducibility of reabsorptive capacity per unit surface area, Jv(a) thus obtained (coefficient of variation of 17%, n=44) was found to compare very favourably with other micropuncture techniques measuring the same parameter. The reproducibility of this parameter when expressed per unit tubular length, Jv(l) was 23% (n=44). The accuracy of the method as measured by duplicate determinations was at 10% even better (n=38). From these considerations it was concluded that reabsorptive capacity of tubules should be expressed per unit area when comparisons between different tubules are to be made, and per unit length when comparisons of two or more determinations in the same tubule are to be compared. 4. The small difference in Jv found between measurements made with the Gertz method and microperfusion methods using TF/P inulin ratios was not due to the oil used nor was it due to a possible mechanical damage by the relatively large size of the double barrelled glass capillary usually employed in the Gertz method