Culture of rat renal medullary tissue in media made hyperosmotic with NaCl and urea.

During antidiuresis, the rat kidney maintains a variable and steep osmotic gradient from the cortex (300 mOsm) to the inner medulla (at least 2,600 mOsm). Therefore, cells in the renal medulla must be able to adapt to a variably hyperosmotic environment. We have examined the ability of tissue fragments taken from various points on the cortical-medullary axis to survive and grow when cultured in media made hyperosmotic with urea and NaCl. Survival and growth were measured by the explants' ability to produce epithelial outgrowths. At osmotic concentrations of 1,100 and 1,200 mOsm, only explants from the inner medulla produced epithelial outgrowths. At 700 mOsm, all explants produced outgrowths but outgrowth size was a function of position on the cortical-medullary axis, with inner medullary fragments producing the largest outgrowths. Growth was most rapid at all osmolalities when the Na+:urea ratio was 1:1. These results are consistent with the hypothesis that renal medullary cells are adapted to elevated concentrations of Na+ and urea. Both explants and epithelial outgrowths were examined using light and electron microscopy. Physical continuities between the epithelial outgrowths and collecting duct epithelium in the explants, as well as the ultrastructural characteristics of the outgrowths at 700 mOsm, indicated that the outgrowths may have originated from collecting duct epithelium.

Rat renal papillary tissue explants survive and produce epithelial monolayers in culture media made hyperosmotic with sodium chloride and urea.

The capacity of papillary cells to adapt to elevated osmotic concentrations is unusual among mammalian cells. This capacity was evaluated by using primary tissue culture. Viability and growth of cells in rat renal papillary tissue explants were assessed after culture in media adjusted with urea and sodium chloride to various osmotic concentrations between 300 and 1,500 mOsm/kg water. The survival of cells, including cells resembling those of the collecting ducts and the loop of Henle, was greatest in medium adjusted to 1,000 mOsm with equiosmolar amounts of the two solutes. At 1,500 mOsm only cuboidal tubular epithelium resembling collecting duct epithelial cells survived. In contrast, cells of cortical tissue survived and grew at 300 and 640 mOsm, but not at 1,000 mOsm or above. Epithelial monolayers appeared to proliferate from collecting ducts and spread over the surface of the explants as well as onto the glass surface in the culture dish. Epithelial growth of medullary tissue was most rapid at 300 mOsm and was slower at 700 and 1,000 mOsm. Monolayers did not form at 1,500 mOsm; however, epithelial overgrowth of explants did occur. Hydropenia in the donor animal did not significantly affect the viability or growth of cultured papillary tissue. Explants cultured for 5 days at 300 mOsm followed by a stepwise increase in medium osmolality to 1,100 or 1,500 mOsm and cultured for 3 more days showed low or no survival whereas explants cultured at 700 mOsm survived such increases. Explants cultured for 5 days at 1,500 mOsm survived and grew monolayers when lowered to 300 mOsm. Poor viability and no epithelial proliferation were observed in explants cultured in medium adjusted to 900 mOsm with either urea or sodium chloride alone, suggesting that a mixture of the two solutes in the extracellular space, as found in vivo, may be essential in achieving elevated osmolalities.