Low Tonicity Mediates a Downregulation of Cyclooxygenase-1 Expression by Furosemide in the Rat Renal Papilla

Abstract

Abstract. It is well known that loop diuretics enhance the renal excretion of prostanoids; therefore, this study aimed to characterize the influence of loop diuretics on the intrarenal expression of cyclooxygenases, which are the key enzymes for prostanoid formation. Male Sprague-Dawley rats were infused with furosemide (12 mg/kg per d) for 6 d, and the expression of cyclooxygenase-1 and -2 (Cox-1 and Cox-2) was analyzed in the different kidney zones. Furosemide increased Cox-2 mRNA expression approximately twofold in the cortex, but it left Cox-1 mRNA expression unaltered there. In the outer medulla, furosemide changed neither Cox-1 nor Cox-2 mRNA expression. In the inner medulla, however, furosemide decreased Cox-1 and Cox-2 mRNA levels to approximately 30% and 60% of their control levels, respectively. The downregulation of mRNA was paralleled by a decrease of Cox protein in the collecting ducts and interstitial cells. Moreover, tissue prostaglandin E 2 (PGE 2 ) concentrations in the papilla were markedly decreased by furosemide to about 30% of the control level. Furosemide lowered urine osmolality from 1550 mosmol/kg to 480 mosmol/ kg; therefore, further consideration was given to the influence of tonicity as a possible mediator of the effects of furosemide on the Cox expression. Water loading was therefore used to reduce the medullary tonicity by a second maneuver. Water loading led to a similar reduction in papillary Cox mRNA expression and PGE 2 content like furosemide. To investigate the influence of the osmolarity on the expression of Cox and the production of PGE 2 under defined in vitro conditions, inner medullary collecting duct cells were incubated with culture medium containing graded amounts of NaCl ranging from 200 mmol/L to 600 mmol/L, and Cox-1 and Cox-2 mRNA abundance were determined after 24 h an 48 h. Cox-1 and Cox-2 mRNA abundance changed in parallel with the osmolarity. The data suggest that loop diuretics decrease the expression of cyclooxygenases and consequently tissue PGE 2 concentrations in the kidney inner medulla. This effect could be related to the breakdown of the papillary osmotic gradient induced by loop diuretics.

Altered expression of COX-1, COX-2, and mPGES in rats with nephrogenic and central diabetes insipidus

“…and body fluid volume on COX expression (3,44). Thus these findings support that COX-2-derived PGs participate in the regulation of urinary water excretion.…”

“…The downregulation of mRNA and protein levels was paralleled by a decrease in tissue PGE 2 concentrations in the papilla (3). Moreover, in vitro studies demonstrated that the expression of COX-1 mRNA changed in parallel with the changes in osmolality of the culture medium for IMCD cells (3), suggesting that the changes in interstitial osmolality may be a possible mediator of the effects of furosemide on the COX-1 expression. Similarly, lithium treatment reduces inner medullary interstitial osmolality (4) and urinary osmolality compared with controls.…”

“…Particularly, COX-2 is thought to play an important role in the survival of inner medullary interstitial cells during increased interstitial osmolality, whereas expression of inner medullary COX-1 remains unchanged in this condition (48). Moreover, COX-1 and COX-2 in the inner medulla are downregulated in response to low interstitial osmolality induced by furosemide administration or water loading (3). Nevertheless, the role of COX-2 as an inducible stress enzyme has been questioned by the data showing that COX-2 appears to be expressed constitutively in the rat and mouse macula densa region of cortical thick ascending limb and inner medullary interstitial cells (2).…”

Renal cortical regulation of COX-1 and functionally related products in early renovascular hypertension (rat)

“…Changes in PGES expression were parallel to COX-1, and these parameters were further coordinated with an ϳ65% decrease in PGE 2 urinary output. Currently available data provide no specific mechanisms for these changes, and most data on COX-1 have come from studies in the renal inner medulla where tonicity appears to play a role in its regulation (6,23). Yet, the present conditions cannot easily be related with cortical cell volume or transport status.…”

mPGES-1-derived PGE2 mediates dehydration natriuresis

“…PGE 2 synthesis in the collecting duct cells is modulated by osmolality (20). Along this line, it is consistently demonstrated that renal medullary COX-2 expression is induced by WD (13, 45); the opposite is true in that the COX-2 expression is reduced by water loading or chronic furosemide infusion that eliminates renal medullary osmotic gradient (4). Extensive in vitro studies have defined the signaling transduction pathway leading to osmotic regulation of COX-2 in collecting duct and renal medullary interstitial cells that primarily involve the activation of MAPK and NF-B (13,46).…”