Whole kidney volume/pressure relationships.
We measured changes in kidney volume and intrarenal pressure produced by step-wise, steady state increases and decreases in ureteral pressure (UP). The purpose of such maneuvers, termed exercises, was to examine the relationship between changes in kidney volume and the changes in intrarenal pressure responsible for those in kidney volume. Changes in kidney volume were assessed from directly measured changes in kidney weight. Changes in the intrarenal pressure distending the renal capsule (renal subcapsular pressure [RSCP]) were measured by a strain guage diaphragm-type microtransducer placed between cortex and capsule. We measured these whole kidney volume/pressure events before and after saline loading and examined their relationship to changes in renal function following saline loading. We found that after four "exercises", the kidney became more complaint, i.e., occupied a larger volume at any given UP or RSCP, and that the compliance of the kidney was further increased following saline loading. However, about one hour after saline loading, RSCP returned to or below pre-saline loading levels and renal volume returned to pre-saline loading levels; nevertheless, the natriuresis persisted. Thus, an increase in renal volume is not necessary to sustain increased sodium and water excretion in post-saline loading. Finally, we found evidence that the renal capsule provides the major force opposing expansion of outer cortex when intrarenal pressure is increased.