RENAL CORTICAL AND MEDULLARY MICROVASCULAR BLOOD-FLOW AUTOREGULATION IN RAT

Previous studies have demonstrated the critical role of the afferent a rteriole in autoregulation of nephron blood flow in response to change s in perfusion pressure. The present study focused on the responses of postglomerular vascular segments to alterations in renal arterial pre ssure. Afferent arterioles, efferent arterioles and outer medullary de scending vasa recta of juxtamedullary nephrons were visualized using t he in vitro blood-perfused juxtamedullary nephron technique. Simultane ous measurements of inside vessel diameter and centerline erythrocyte velocity were made in order to determine single vessel blood flow. Blo od flow measured in afferent arterioles (N = 13) displayed efficient a utoregulation of blood flow and afferent arterioles responded actively with decreases in arteriolar diameter juring stepwise elevations of r enal perfusion pressure from 100 to 150 mm Hg. Similarly, blood flow m easured at efferent arterioles (N = 9) exhibited autoregulation during increases in renal perfusion pressure. However, efferent arteriolar d iameters were not altered during increases in perfusion pressure. Duri ng superfusion with the calcium channel blocker, diltiazem (10 mu M), which primarily dilates afferent arterioles, efferent arteriolar blood flow (N = 7) increased and responded to changes in perfusion pressure . Nevertheless, efferent arteriolar diameter remained unchanged and di d not respond to increases in perfusion pressure. Outer medullary desc ending vasa recta (N = 7) diameter, centerline erythrocyte velocity an d calculated blued flow were also not significantly altered following stepwise increases in pressure to 125 and 150 mm Hg. These data demons trate effective autoregulation of postglomerular blood flow, measured at efferent arterioles and at outer medullary descending vasa recta, o ver a perfusion pressure range of 100 to 150 mm Hg. There was no disso ciation of arteriolar and outer medullary descending vasa recta blood flow responses to increases in renal perfusion pressure indicative of efficient autoregulation in both cortical and medullary postglomerular circulations of the rat.