Cell volume regulation in the renal papilla.

Urine osmolality and interstitial solute concentration in the renal medulla are determined by the body's hydration state. This implies that in the renal medulla extracellular solute concentrations, and hence the volume of medulla-resident cells, will vary drastically with the state of hydration. Medullary cells regulate their volume primarily by adjusting accumulation and release of low-molecular weight organic osmolytes appropriately. There is growing evidence that not only transcriptional processes, but also post-transcriptional mechanisms, such as targeting of transport molecules to the plasma membrane, contribute to the successful adaptation of medullary cells to extreme environmental conditions.