Linda A. Scharschmidt

Prostaglandin synthesis by rat glomerular mesangial cells in culture. Effects of angiotensin II and arginine vasopressin.

Arginine vasopressin (AVP) and angiotensin II (ANG II) reduce the glomerular filtration rate and ultrafiltration coefficient. Vasodilatory prostaglandins (PG) antagonize these effects. AVP and ANG II also cause mesangial cell contraction. Therefore, possible PG stimulation by these peptides and two vasopressin analogues was studied in cultured rat glomerular mesangial cells. The effect of altered calcium availability on PG production was also studied. Glomeruli from 75-100-g Sprague-Dawley rats were cultured in supplemented nutrient media for 28 d and experiments were performed on the first passage. Mesangial cell morphology was confirmed by electron microscopy. Cells produced PGE2 much greater than PGF2 alpha greater than 6-keto-PGF1 alpha greater than thromboxane B2 when incubated with the divalent cation ionophore, A23187, or arachidonic acid (C20:4). ANG II and AVP selectively stimulated PGE2 at threshold concentrations of 10 nM ANG II and 100 pM of AVP. The effects of the antidiuretic analogue 1-desamino-8-D-arginine vasopressin (dDAVP) and the antipressor analogue [1-(beta-mercapto-beta beta-cyclopentamethylene propionic acid)-4-valine, 8-D-arginine]-vasopressin (d[CH2]5VDAVP), were studied. Neither compound stimulated PGE2 and preincubation with d(CH2)5VDAVP abolished, and dDAVP blunted, AVP-enhanced PGE2 production. Incubation in verapamil, nifedipine, or zero calcium media blocked peptide-stimulated PGE2 production. Increasing extracellular calcium or adding A23187 increased PGE2 synthesis. Selective stimulation of PGE2 by ANG II or AVP in mesangial cells suggests a hormone-sensitive phospholipase and a coupled cyclooxygenase capable of synthesizing only PGE2. Since neither vasopressin analogue stimulated PGE2, but both blocked AVP-enhanced PGE2 production, we conclude that these cells respond to the pressor activity of AVP. This is a calcium-dependent process. Selective stimulation of PGE2 by ANG II and AVP may modulate their contractile effects on the glomerulus.

Nonsteroidal Anti‐Inflammatory Drugs and Renal Function

T he control of normal renal function, including the glomerular filtration rate (GFR) and renal blood flow (RBF), is not dependent on basal prostaglandin synthesis in the kidney. This belief is based on the observations showing no changes of GFR or RBF after inhibition of prostaglandin synthesis with NSAIDs in normal animals or normal human volunteers.6 This is not to say that the basal level of intrarenal prostaglandin synthesis is physiologically unimportant and does not exert a modulatory function. It is likely that loss of the modulatory effect of the vasodilatory prostaglandins, particularly in the glomerulus and in the renal vasculature, can be compensated for by other vasoregulatory mechanisms under physiologic but not pathophysiologic conditions. Experimental animals with cardiovascular, hepatic, or renal damage will respond to NSAIDs with acute decrements of both GFR and RPF.5 We and others attribute these alterations in renal function after administration of NSAID to the vasoconstrictor actions of angiotensin II (ANG II), arginine vasopressin (AVP), and norepinephrine, which are no longer opposed by vasodilatory prostaglandins. ANG II, AVP, and norepinephrine stimulate renal prostaglandin synthesis in cultured renal cells as well as after intraarterial infusion into the kidney. This compensatory release of PGE2 and PGI2 counteracts the vasoconstrictor action of these hormones.78 Pretreatment with NSAID potentiates renal vasoconstriction secondary to norepinephrine, AVP, and ANG 11.910 These experiments reinforce our belief that vasoconstrictor hormones, which stimulate renal cortical and medullary PGE2 and PGI2, elicit an important modulatory feedback pathway, and nonsteroidal anti-inflammatory drugs, by inhibiting the modulatory effects of prostaglandins, accentuate the vasoconstriction.

Glomerular prostaglandins, angiotensin II, and nonsteroidal anti-inflammatory drugs

The glomerulus can, in part, regulate its own flow and filtration characteristics, both of which are determinants of the glomerular filtration rate. This occurs in part as the result of interactions between vasoconstrictors, e.g., angiotensin II (AII), and the vasodilatory prostaglandins E2 or I2. It is well accepted that these prostaglandins modulate the constrictor effects of AII on systemic and renal vasculature. Experimental data accumulated from micropuncture studies, analyses of isolated glomeruli in vitro, and glomerular mesangial cell cultures also support the hypothesis that AII-stimulated production of vasodilatory prostaglandins attenuates AII-induced constriction at the glomerular level as well. These studies help to explain the deleterious actions of nonsteroidal anti-inflammatory drugs on glomerular filtration in clinical conditions associated with a decreased effective blood volume and, therefore, activation of AII and other neurohormonal constrictors. These results have also furthered our understanding of the role of prostaglandins in maintaining renal function in human and experimental renal diseases that may be associated with enhanced hormonal constrictor activity.

Comparison of Glomerular and Mesangial Prostaglandin Synthesis and Glomerular Contraction in Two Rat Models of Diabetes Mellitus

Enhanced prostaglandin production is postulated to contribute to altered vascular reactivity and glomerular hyperfiltration in early insulin-deficient diabetes mellitus. Rats with streptozocin-induced diabetes (STZ-D) show glomerular hyperfiltration and develop renal disease. BB rats with genetic diabetes (BB-D) also hyperfilter but have only minor renal lesions. We therefore compared glomerular and mesangial prostaglandin E2 (PGE2) production and glomerular contractility in response to pressors as a reflection of in vitro vascular reactivity in these models. Glomeruli isolated from rats with 3 wk of STZ-D produced significantly more PGE2 under basal and ionophore A23187-stimulated conditions than those from control rats. Glomeruli from BB-D rats under basal and stimulated conditions, however, generated amounts of PGE2 that were comparable to either those of nondiabetic litter mates or of normal Wistar rats. Mesangial cells cultured from glomeruli of STZ-D, BB-D, and control rats all had identical prostaglandin profiles judged by conversion of [14C]arachidonic acid. They also produced comparable amounts of PGE2 under basal conditions and after stimulation with angiotensin II or A23187, as determined by radioimmunoassay. Planar surface area of glomeruli isolated from control rats showed a dose-dependent decrease in response to angiotensin II (10−11-10−9 M). This response to angiotensin II was at least as great in glomeruli from STZ-D rats. Contraction of glomeruli from control and STZ-D rats was also comparable after vasopressin or norepinephrine. Similarly, glomeruli from BB-D and BB control rats contracted in a comparable fashion to angiotensin II and norepinephrine. We conclude that in vitro PGE2 production is enhanced in glomeruli from STZ-D rats but not in those from BB-D rats, as compared with their respective controls. Because the contractile response of isolated glomeruli to pressors was comparable in all groups, our in vitro results argue against a major role of prostaglandins in the hyperfiltration of diabetes mellitus.

Mechanisms of the Nephrotoxicity of Non-steroidal Anti-inflammatory Drugs

Renal cortical prostaglandin synthesis, particularly by arterioles and glomeruli, is important to preserve renal blood flow (RBF) and glomerular filtration rate (GFR). Glomeruli and arterioles synthesize not only the vasodilatory prostaglandins PGE2 and PGI2, but also the vasoconstrictor, thromboxane A2. The primary renal cortical actions of these prostaglandins are renal vasodilatation and maintenance of GFR (PGE2 and PGI2) or renal vasoconstriction and reduction of GFR (thromboxane A2). Vasodilatory renal prostaglandins are relatively unimportant under normal circumstances but play a modulatory role after ischemia or in the presence of increased concentrations of vasoconstrictor substances such as angiotensin II (ANG II), vasopressin or norepinephrine. ANG II and vasopressin stimulate the synthesis of PGE2 in rat glomerular epithelial and mesangial cells maintained in cell culture. These stimulatory actions of constrictor peptides are dependent upon calcium entry into the cells since removal of extracellular calcium or co-incubation with verapamil or nifedipine block the prostaglandin stimulatory capacity of ANG II or vasopressin. In vivo indomethacin potentiates the actions of ANG II on the kidney, particularly the reduction of RBF and GFR. Isolated rat glomeruli contract in response to ANG II and this contractile effect, which reflects reduction in glomerular filtration surface area, can be potentiated by cyclooxygenase blockade. Conversely, arachidonic acid reduces the glomerular contractile effect of ANG II. The importance of renal prostaglandins in support of RBF and GFR has been studied in dogs after chronic bile duct ligation (CBDL). CBDL dogs have significant increase in renal PGE2 and PGI2 which maintain RBF and GFR since cyclo-oxygenase inhibition resulted in a 50% decrease in both RBF and GFR. Indomethacin, ibuprofen, naproxen, and sulindac sulfide had comparable effects. The pro-drug, sulindac sulfoxide, was tested in normal volunteers and found to spare renal prostaglandin synthesis whereas indomethacin reduced renal synthesis of PGE2 and PGF2 alpha by more than 50%. In vitro, sulindac sulfide is a potent inhibitor of renal prostaglandin synthesis by kidney cells in culture. It is, therefore, concluded that renal prostaglandins play an important vasoregulatory role. Furthermore, sulindac sulfoxide may spare renal cyclo-oxygenase and thereby preserve renal function.

Priapism and Dialysis

Ninety-three dialysis units located in the tri-state area were surveyed for the prevalence of priapism. Seventeen of 3,337 male patients experienced an episode of priapism. All of these episodes occurred either during or 2-7 h after dialysis, suggesting a cause-and-effect relationship. The use of heparin during dialysis seemed to play a role in its induction; none of the patients on peritoneal dialysis experienced priapism, and heparin was not used in their exchanges. Eleven of 17 patients also received androgen therapy which might have been a contributing factor. Additional considerations included dialysis-induced hypoxemia and acidosis which have been known to precipitate priapism in patients with sickle cell disease or trait. In our study, 2 of 10 patients, including our case report, were black males with sickle cell trait. The calculated prevalence in the male population is 1:196, and in blacks and Caucasians 1:128 and 1:293, respectively. Except for 2 cases of sickle cell trait in the 10 black males, no particular subpopulation at risk was identified. In view of the absence of a proven etiologic agent and the relatively low prevalence of priapism we do not recommend changes in dialysate, anticoagulant, or the use of androgen therapy.