Renomedullary blood flow and blood volume are increased during vasopressin escape

Abstract

Hyponatremia is a common electrolyte disorder usually caused by inappropriate vasopressin (AVP) levels relative to serum osmolality. The degree of hyponatremia is limited by “escape” from AVP-induced antidiuresis, characterized by increased flow of hypotonic urine independently of plasma AVP. We have hypothesized that increased renomedullary blood flow (BF) contributes to escape. We therefore investigated intrarenal BF and blood volume distribution in rats with and without escape. Sprague-Dawley rats (n=10) were infused with DDAVP (5 ng/h) to produce maximal antidiuresis. Half were fed liquid diet (AIN-76) to produce escape; half were fed solid diet to prevent escape. After 5 days, all rats were anesthetized and high resolution images (voxel size 97×97×600 μm) of renal BF were acquired using a Siemens Definition CT. Iopromide (0.15 ml) was injected iv and contrast intensity recorded for 30 s. Cortical and medullary BF were 709±41 and 251±50 ml/100ml/min in non-escaped rats. Cortical BF in escaped rats was similar (588±81 ml/100ml/min), but medullary BF was increased compared to non-escaped rats (666±105 ml/100ml/min). Blood volumes were similar in cortex (42±1 vs. 49±6 ml/100ml), but elevated in medulla of escaped rats (70±3 vs 18±3 ml/100ml). In conclusion, escape is accompanied by elevated renomedullary BF and volume which can explain the reduced ability to concentrate urine during AVP escape.