Effects of antidiuretic hormone on intrarenal red cell and plasma volumes in the water diuretic rat.

The intrarenal distribution of 51Cr-labelled red cells and 125I-immunoglobulin M (125I-IgM) was studied in water diuretic control rats and in water diuretic rats infused with argenine vasopressin (ADH) 1 muU per minute and gram rat. The IgM volume after 1 min equilibration time was taken to be a measure of plasma volume. Intrarenal haematocrits were calculated from the red cell and plasma volumes and were expressed as fractions of total body haematocrit. Infusion of ADH provoked a rise in urine osmolality from below 200 to values ranging from 800 to 2000 mosm per kg with only insignificant elevation of arterial blood pressure. Calculated red cell volume was significantly increased in all renal zones whereas calculated regional plasma volumes were not significantly changed by ADH. In all renal zones ADH administration thus involved significant increments in haematocrit; the relative increment was most pronounced in the inner medulla. The results give no direct evidence in favour of or against vasoconstrictive effects of ADH in the rat kidney. However, it may be argued that the haematocrit changes are compatible with increased haemodynamic resistance and reduction of blood flow in all renal zones (especially in the medulla) involving decreased linear velocities and less pronounced tendency to axial accumulation of red cells. ADH may increase haemodynamic resistance in two ways, first by acting as a vasoconstrictor and, second, by indirectly increasing blood viscosity in the medulla by increasing tissue osmolality.