Secretion of Inulin by the Kidney of the Crayfish. ∗

Abstract

Cumulative evidence indicates that inulin is not secreted by nor passively reabsorbed through the vertebrate nephric tubule (Smith 1 ). In vertebrates, the inulin-clearance, therefore, is independent of the concentration of inulin in the plasma and is used as a measure of the rate of glomerular filtration.

In an attempt to find whether filtration occurs through the nephron of the crayfish by the use of inulin, the writer obtained most unexpected results as regards the inulin-clearance. This is the first invertebrate kidney which has been studied from the standpoint of “clearances”. Inulin was measured by a micro-adaptation of the Shaffer-Hartmann-Somogyi Method.

Nowhere in the crayfish nephron 2 is there a tenuous syncytium such as constitutes the glomerular capsule of the vertebrate nephron.

The renal arterioles open freely into the cavernous haemocoele which follows the numerous involutions of the renal epithelium. The kidneys of the crayfish consist of a pair of relatively large nephrons, each of which ends blindly in a highly lobulated coelomosac.

Contrary to the condition in the vertebrate aglomerular kidney, inulin, xylose, and glucose appear in the urine of the crayfish after being injected into the blood stream. This, however, is not proof of filtration because the inulin-clearance is not independent of the concentration of inulin in the plasma. The inulin-clearance: plasma-inulin curve (Fig. 1), which resembles an adsorption isotherm, shows that inulin is outwardly secreted. This is further indicated by the fact that the shape of the curve in Fig. 1 is determined practically entirely by the urine/plasma ratio for inulin (Fig. 2).

The sudden decline in the inulin-clearance with augmentation of the plasma-inulin (Fig. 1) indicates that inulin is not appreciably filtered. Filtration, therefore, apparently does not occur through the crayfish nephron.