Composition of Tubular Fluid in the Macula Densa Segment as a Factor Regulating the Function of the Juxtaglomerular Apparatus

In micropuncture experiments on the rat kidney, solutions of various electrolyte composition were injected toward the macula densa of single nephrons. In kidneys with a high renin content, the transient collapse of the proximal convolution of the same nephron occurred when sodium concentration was increased to 150 mM/L. This collapse indicated that glomerular filtration rate was interrupted. An increase in tubular fluid osmolality without a concomitant increase in sodium concentration had no effect on the filtration rate. This sodium-specific reaction was absent or markedly diminished in kidneys depleted of renin. These data demonstrate the operation of a sodium-sensitive feedback mechanism at the level of the juxtaglomerular apparatus in each single nephron unit. The physiological meaning of this mechanism may be to adjust glomerular filtration rate, and thereby tubular sodium load, to the reabsorptive capacity of the nephrons for sodium chloride; it may therefore be an intrarenal sodium-conserving mechanism.