Effects of hyperosmolality on calcium mobilization in renal inner medulla: relationship to alterations in prostaglandin E synthesis.

Hypertonic NaCl and mannitol stimulate the release of [14C]arachidonate and iPGE from rat inner medullary slices, whereas hypertonic urea suppresses these same parameters. Since numerous observations indicate that arachidonate release and PG synthesis in this tissue are Ca2+-dependent processes, we examined the possibility that the divergent actions of these solutes were related to differential effects on Ca2+ availability in inner medullary slices. Inner medullary slices prelabeled with [45Ca2+] were initially perfused with 305 mOsm Krebs' buffer followed by perfusion with buffer whose osmolality was raised by the addition of 1000 mOsm NaCl, mannitol, or urea. The [45Ca2+] efflux rate coefficient, an index of Ca2+ mobilization, was determined from the release of [45Ca2+]. In Ca2+-replete buffer (1.5 mM Ca2+), addition of each solute markedly enhanced the efflux rate coefficient compared to that observed in the same slices during perfusion with 305 mOsm buffer. Addition of hypertonic NaCl, mannitol, or urea also clearly stimulated the efflux rate coefficient in slices initially perfused with Ca2+-free 305 mOsm buffer containing the Ca2+ chelator EGTA and/or La3+, which displaces superficially bound Ca2+. Moreover, each solute enhanced the efflux rate coefficient in mitochondrial fractions isolated from inner medullary slices that had been prelabeled with [45Ca2+] and initially perfused with 305 mOsm buffer. The latter observations support the ability of hypertonic NaCl, mannitol, and urea to mobilize Ca2+ bound to intracellular sites. Thus it is unlikely that the differential actions of these solutes on arachidonate release and iPGE synthesis in inner medullary slices are simple functions of their effects on Ca2+ availability. This is consistent with earlier findings which demonstrated that urea, but not NaCl or mannitol, directly inhibits Ca2+-dependent acyl hydrolase activity in plasma membrane and microsomal preparations of inner medullary slices. Accordingly, inhibitory effects of urea on PG synthesis may be related to its actions on this enzymatic step, rather than an effect on Ca2+ availability in inner medullary slices.