Effects of osmolality and oxygen availability on soluble cyclic AMP-dependent protein kinase activity of rat renal inner medulla

Abstract

The renal inner medulla is ordinarily exposed to osmolalities that are much higher and to O2 tensions that are lower than those in other tissues. The effects of media osmolality and O2 availability on basal and arginine vasopressin(AVP)-responsive soluble cyclic (c)AMP-dependent protein kinase activity were examined in slices of rat inner medulla. Increasing total media osmolality from 305 to 750 or 1,650 mosM by addition of urea plas NaCl to standard Krebs-Ringer bicarbonate buffer significantly reduced basal cAMP content and protein kinase activity ratios. This occurred in the presence or absence of O2. Incubation of slices in high osmolality buffer also blunted increases in inner medullary slice cAMP and protein kinase activity ratios induced by O2. These changes reflected predominantly an action of the urea rather than the NaCl content of high osmolality buffers. In contrast to effects on basal activity, high media osmolality significantly enhanced activation of inner medullary protein kinase by AVP. Conversely, increases in media O2 content suppressed AVP stimulation of enzyme activity. This inhibitory effect of O2 was best expressed at low osmolality. Naproxen and ibuprofen, inhibitors of prostaglandin biosynthesis, reduced basal kinase activity ratios and increased AVP responsiveness in the presence, but not in the absence, of O2. Exogenous prostaglandins (PG) modestly increased (PGE2 and PGE1) or did not change (PGF2alpha) cAMP and protein kinase activity ratios in O2-deprived inner medullary slices. Protein kinase activation by PGE2 was not observed in oxygenated inner medulla with high basal activity ratios. The stimulatory effects of PGE2 and PGE1 on protein kinase activity observed in O2-deprived slices were additive with those of submaximal or maximal AVP. PGE2, PGE1, and PGF2alpha all failed to suppress AVP activation of protein kinase. Thus, enhanced endogenous PGE production may contribute to the higher basal protein kinase activity ratios induced by O2. However, the results do not support a role for PGE2, PGE1, or PGF2alpha in O2-mediated inhibition of AVP responsiveness. The present data indicate that both solute content and O2 availability can alter the expression of AVP action on cAMP-dependent protein kinase activity in inner medulla. AVP activation of protein kinase is best expressed when osmolality is high and O2 availability is low, conditions that pertain in inner medulla during hydropenia.