Stationary perfusion methods.

The so-calledl split-drop method was developed to measure transtubular isotonic water reabsorption from tubular convolutions at kidney surfaces. In principle the metlhod involves intratubular injection of viscous castor oil in amounts stufficient to block intratubular flow rate; the oil column remains in place. Subse(Itently the oil coltumn is split into two oil columns by the injection of a small amotunt of isotonic saline soltution. Since this saline droplet will be reabsorbecl from the tubuilar lutmen, the two oil columns will move togetlher until they again merge. If the ttubuilar diamneter remains constant during reabsorption, the distance between the two oil columns, as a measture of intratublular volume, will (lecrease exponentially witlh time. Because of this exponential volume decrease, transtuibtular fluii(d reabsorption is expressed as lhalf time of volume reabsorption insteadl of as milliliters per tunit time andlunit tubular lengtlh or as unit ttubtular area. This limitatioin has been a disadvantage of this metlhodl from the very beginning. Half time of voluLme reabsorption under control conditions lhas been found in several laboratories to be about 10 sec. in proximal convolutions of rat kidneys and to be prolonged after administration of such inhibitors of tlle transepithelial so(lium ptump as dituretics. Althouglh the measurements of half time have been repeatable within small limits, doubts arose as to wlhetlher this metlhod produced results consistent with fluid reabsorption under free flow condlitions. One of the most frequent arguments against this metlhod lhas concerned the influence of intratuLbular diameter on fluii(d reabsorption. The column of the saline droplet between the two adjacent oil coltumns lhas a diameter of about 30 tu compared to an intratubular cliameter of al)out 90 [c under free flow conditions. For several reasons, such as inaccuracy of the microplhotographs or the effects of the brush border, a direct evaluation of the influence of the tubular diameter on volume reabsorption is difficult to perform and tlherefore the clata are conflicting.