Decreased osteopontin expression in the rat kidney on a sodium deficient diet

Osteopontin (OPN) is a secreted phosphoprotein that is constitutively expre ssed in the normal kidney and is induced by various experimental and pathol ogic conditions. Several possible functions of OPN have been suggested, how ever the mechanism and significance of OPN expression are still uncertain. Since high sale concentration or salt crystal have been known to enhance OP N expression in intact kidney or cultured renal cells, in me present study we examined whether or not a low salt condition had an effect on OPN expres sion in the kidney. Adult male Sprague-Dawley rats were fed either a normal sodium or a sodium deficient dec for 1 week. Kidneys were processed for in situ hybridization using a digoxigenin-labeled riboprobe and for immunohis tochemistry using antibodies to OPN, renin, and Na-K-ATPase. In rats fed a normal sodium diet, OPN mRNA and protein were expressed only in the descend ing thin limbs of Henle's loop (DTL) and in the papillary and pelvic surfac e epithelium (PSE). In rats fed a sodium deficient diet, there was a marked decrease in OPN immunoreactivity in the DTL, bur no changes in PSE. In con trast, no changes were observed in OPN mRNA expression in the DTL by in sit u hybridization, indicating chat decreased OPN protein expression was a res ult of translational regulation. As expected, rats fed a sodium deficient d iet were associated with increased immunoreactivity for Na-K-ATPase and ren in compatible with activation of the renin-angiotensin system. These result s suggest that dietary sodium may be involved in the regulation of OPN expr ession in the DTL of the rat kidney.