Effects of osmolality on phosphoinositide hydrolysis in renal medulla

We have reported previously that carbachol stimulates hydrolysis of phosphoinositides (PIs) in the renomedullary slices when incubated in a buffer of 300 mOsm/kg of H2O. However, the mammalian renal medulla has a hypertonic environment that changes with the state of hydration of the animal. In order to determine if the change in renal osmolality produces a change in the response of the inner medulla to hormones and neurotransmitters, we determined the effects of osmolality on carbachol-stimulated hydrolysis of PIs in the inner medullary slices of the rabbit kidney. The hydrolysis was determined by incorporation of [3H]inositol into PIs and the release of [3H]inositol phosphates in the presence and absence of 1 mM carbachol. The osmolality of the incubation media was increased from 300 to 1200 mOsm/kg of H2O in increments of 300 mOsm/kg of H2O by addition of either urea, NaCl, mannitol or an equiosmolar mixture of urea and NaCl. Increasing the osmolality of the incubation media by any one of these solutes decreased carbachol-stimulated release of inositol phosphates in the inner medullary slices of the rabbit kidney. Our results suggest that the effect of carbachol on PI messenger system in the renal medulla in vivo will depend on the tissue osmolality that itself depends on the state of hydration of the animal.