Role of prostaglandin E (PGE) in the modulation of the action of vasopressin on water flow in the urinary bladder of the toad and mammalian kidney

Vaptans or voluntary increased hydration to protect the kidney: how do they compare?

The adverse effects of vasopressin (AVP) in diverse forms of chronic kidney disease have been well described. They depend on the antidiuretic action of AVP mediated by V2 receptors (V2R). Tolvaptan, a selective V2R antagonist, is now largely used for the treatment of patients with autosomal dominant polycystic kidney disease. Another way to reduce the adverse effects of AVP is to reduce endogenous AVP secretion by a voluntary increase in fluid intake. These two approaches differ in several ways, including the level of thirst and AVP. With voluntary increased drinking, plasma osmolality will decline and so will AVP secretion. Thus, not only will V2R-mediated effects be reduced, but also those mediated by V1a and V1b receptors (V1aR and V1bR). In contrast, selective V2R antagonism will induce a loss of fluid that will stimulate AVP secretion and thus increase AVP's influence on V1a and V1b receptors. V1aR is expressed in the luminal side of the collecting duct (CD) and in inner medullary interstitial cells, and their activation induces the production of prostaglandins, mostly prostaglandin E2 (PGE2). Intrarenal PGE2 has been shown to reduce sodium and water reabsorption in the CD and increase blood flow in the renal medulla, both effects contributing to increase sodium and water excretion and reduce urine-concentrating activity. Conversely, non-steroidal anti-inflammatory drugs have been shown to induce significant water and sodium retention and potentiate the antidiuretic effects of AVP. Thus, during V2R antagonism, V1aR-mediated actions may be responsible for part of the diuresis observed with this drug. These V1aR-dependent effects do not take place with a voluntary increase in fluid intake. In summary, while both strategies may have beneficial effects, the information reviewed here leads us to assume that pharmacological V2R antagonism, with resulting stimulation of V1aR and increased PGE2 production, may provide greater benefit than voluntary high water intake. The influence of tolvaptan on the PGE2 excretion rate and the possibility to use somewhat lower tolvaptan doses than presently prescribed remain to be evaluated.

Inhibition of basolateral cAMP permeability in the toad urinary bladder

1. The effect of sulphonylurea drugs on hydrosmotic flow across toad urinary bladder epithelium was re-evaluated in the present study. Glibenclamide, added to the basolateral medium, significantly enhanced the osmotic flow induced by low doses of antidiuretic hormone (ADH) or forskolin (FK), while it inhibited the effect of exogenous cyclic adenosine monophosphate (cAMP) or its non-hydrolysable bromo derivative, 8-Br-cAMP, added to the basolateral medium. These opposite effects of glibenclamide on the transepithelial osmotic flow can be explained by a reduction of cAMP permeability across the basolateral membrane of the epithelium. The decrease in cAMP permeability leads, according to the direction of the cAMP gradient, to firstly an enhanced osmotic flow when cAMP is generated intracellularly by addition of ADH and FK, glibenclamide reducing cAMP exit from the cell, and secondly a decreased osmotic flow in response to cAMP (and 8-Br-cAMP) added to the basolateral medium, glibenclamide inhibiting, in this case, their entry into the cell. 2. The demonstration that glibenclamide actually inhibits the basolateral cAMP permeability rests on the fact that firstly it decreases the release of cAMP into the basolateral medium by about 40 %, at each concentration of ADH or forskolin tested, secondly it increases the cAMP content of paired hemibladders incubated in the presence of ADH or FK, when intracellular degradation was prevented by phosphodiesterase inhibition, and thirdly it decreases also the uptake of basolateral 8-Br-[3H]cAMP into paired toad hemibladders. 3. Taken together, the present data demonstrate that glibenclamide inhibits the toad urinary bladder basolateral membrane permeability to cAMP, most probably by a direct interaction with a membrane protein not yet indentified but distinct from the sulphonylurea receptor.

Mechanism of action of aspirin-like drugs

Nonsteroid antiinflammatory drugs (NSAIDs) or aspirin-like drugs act by inhibiting the activity of the cyclooxygenase (COX) enzyme. Two isoforms of COX exist, COX-1, which is constitutively expressed, and COX-2, which is an inducible isoform. Prostaglandins synthesized by the constitutively expressed COX-1 are implicated in the maintenance of normal physiological function and have a 'cytoprotective' action in the stomach. COX-2 expression is normally low but is induced by inflammatory stimuli and cytokines. It is thought that the antiinflammatory actions of NSAIDs are caused by the inhibition of COX-2, whereas the unwanted side effects, such as gastrointestinal and renal toxicity, are caused by the inhibition of the constitutively expressed COX-1. Individual NSAIDs show different selectivities against the COX-1 and COX-2 isoforms. NSAIDs that are selective towards COX-2, such as meloxicam, may have an improved side-effect profile over current NSAIDs. In addition to their use as antiinflammatory agents in the treatment of rheumatoid arthritis and osteoarthritis, selective COX-2 inhibitors may also be beneficial in inhibiting colorectal tumor cell growth and in delaying premature labor.

Analgesic effect and tolerance of Voltaren and Ketogan in acute renal or ureteric colic

Fifty-six patients with renal or ureteric colic were entered into a randomised, prospective, double-blind investigation of the analgesic efficacy and tolerance of Voltaren versus Ketogan, both administered intramuscularly. There were no significant differences regarding pain-relief but side effects were fewer in patients treated with Voltaren.

Intracellular Ca2+ concentration and the antidiuretic hormone-induced increase in water permeability: effects of ionophore A23187 and quinidine

The hydroosmotic responses induced by oxytocin and 8-bromo-cyclic AMP, in frog and toad urinary bladders, were recorded minute by minute. 3HHO and 45Ca unidirectional fluxes as well as prostaglandin B2 liberation were also measured. It was observed that: (1) Addition of the calcium ionophore A23187 or quinidine to the serosal bath inhibited the response to oxytocin, but not to 8-bromo-cyclic AMP, while increasing prostaglandin E1 liberation into the serosal but not into the mucosal bath. (2) Addition of A23187 to the mucosal bath induced a transient and temperature-dependent inhibition of the response elicited by 8-bromo-cyclic AMP. The time-course of this reduction in water permeability and its sensitivity to medium temperature were similar to those observed after the withdrawal of agonist, but clearly different of those observed after intracellular acidification. (3) The hydroosmotic response was also transitorily inhibited when the Ca2+ concentration was step-changed in the mucosal bath. (4) When added to the mucosal or to the serosal baths, the ionophore increased either the apical or the laterobasal Ca2+ permeabilities. It is concluded that manipulation of intracellular Ca2+ interferes with the hydroosmotic response at two different levels. (1) A first target point located 'pre-cyclic-AMP production'. This effect would be mediated by prostaglandin liberation. (2) A second target point located after cyclic AMP production and before the 'temperature-dependent rate-limiting step'. This effect is probably related to the mechanism controlling the insertion and removal of water channels.

Prostaglandin synthetase inhibition of renal pelvic smooth muscle in the rabbit

The prostaglandin synthetase inhibitors diclofenac and indomethacin relieve pain in renal colic probably by decreasing renal pelvic pressure. Decreased diuresis and effects on the oedema around the obstructing stone are plausible explanations. In the present study on rabbit renal pelvic tissue strips it was found that both indomethacin and diclofenac decreased the spontaneous phasic activity of the strips. ED50 was 5.4 +/- 0.7 X 10(-5) M for indomethacin and 2.4 +/- 0.5 X 10(-5) M for diclofenac (P less than 0.001). With the prostaglandin synthetase inhibitors still in the tissue bath the activity was regained when either prostaglandin E2 or F2 alpha was added, prostaglandin E2 being more potent in this respect. It was concluded that prostaglandin synthetase inhibitors seem to reduce smooth muscle activity in the renal pelvis, a mechanism which might contribute to the pain-relieving ability of these drugs in renal colic.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

Modification of sodium transport in freshwater mussels by prostaglandins, cyclic AMP and 5-hydroxytryptamine: effects of inhibitors of prostaglandin synthesis

Pondwater acclimated unionid mussels, Ligumia subrostrata, experienced an increased Na influx, compared to controls, when injected with the phospholipase A2 inhibitor dexamethasone, or the cyclooxygenase inhibitors N-(2,6-dichloro-m-tolyl) anthranilic acid (meclofenamate) and indomethacin. Prostaglandin E2 (PGE2) or PGF2 alpha injections inhibited Na transport by depressing Na influx with no change in Na efflux. Prostaglandin E2 injections inhibited the indomethacin and dexamethasone dependent increase in Na transport. The PGE2 inhibition of Na influx was reversed by the administration of dibutyryl cAMP. Injections of serotonin (5-HT) elevated Na influx in mussels and the stimulation of Na transport by 5-HT could be potentiated by the injection of meclofenamate.

Regulation of vasopressin action by prostaglandins. Evidence for prostaglandin synthesis in the rabbit cortical collecting tubule

The present studies examined whether vasopressin increases prostaglandin biosynthesis in isolated rabbit cortical collecting tubules (CCT) and whether endogenous prostaglandin biosynthesis plays a role in modulating the response of this nephron segment to vasopressin. Three groups of studies were performed. In the first group, CCT and proximal straight tubules (PST) were incubated with [(3)H]arachidonic acid, and metabolites were separated and identified using silica gel thin-layer chromatography. CCT were capable of producing all of the major prostaglandins (PG) (PGE(2) > thromboxane B(2)[TxB(2)] > PGF(2alpha) > PGI(2)). PST produced significantly lesser quantities of these lipids. In the second group, radiolabeled arachidonic acid was incorporated into the phospholipid pool of both CCT and PST, vasopressin was added to the incubation medium, and metabolities were separated and identified as above. Vasopressin stimulated the release of all of the major prostaglandins in CCT but had no effect on PST. PGE release into the incubation medium, as assessed by a radioreceptor assay, increased 108%, and a vasopressin analogue, 1-desamino-8-d-arginine vasopressin, had a quantitatively similar effect. In the third group, a submaximal dose of vasopressin was administered to isolated, perfused CCT studied in the presence and absence of indomethacin to assess whether endogenous prostaglandins play a role in modulating the antidiuretic response to vasopressin. Studies were performed in rabbits on a normal diet and in desoxycorticosterone acetate (DOCA)- or KCl-loaded animals. In the state of mineralocorticoid excess, basal prostaglandin synthesis was 63% lower, and vasopressin-stimulated prostaglandin synthesis 76% lower, than the synthesis observed in rabbits on a normal diet. Cyclooxygenase inhibition exposed a significant hydroosmotic response to a submaximal dose of vasopressin in CCT from DOCA- or KCl-loaded animals. With arachidonic acid in the bath, the same dose of vasopressin failed to elicit a hydroosmotic response in CCT from rabbits on a normal diet even in the presence of a cyclooxygenase inhibitor. However, removal of exogenous arachidonic acid, with a consequently lower rate of prostaglandin synthesis, allowed the cyclooxygenase inhibitor to enhance the hydroosmotic response to vasopressin in these tubules.We conclude from these studies that the rabbit CCT has the capacity to synthesize all of the major prostaglandins and that the rate of synthesis of these lipids is enhanced by vasopessin. Prostaglandin synthesis by the CCT is postulated to modulate the antidiuretic action of vasopressin via a closed feedback loop. The effectiveness of this feedback regulation is dependent upon the mineralocorticoid status of the animal, which determines the level of basal and vasopressin-stimulated prostaglandin synthesis by the CCT.

Mechanisms for the effects of acetylcholine on sodium transport in frog skin

In frog skin (Rana temporaria) acetylcholine applied to the serosal surface produces either a sustained inhibiton or sustained stimulation of short-circuit current (SCC). The former effect is accompanied by a reduction and the latter by an increase in total tissue conductance. Both effects of acetylcholine can be accounted for, within experimental error, by changes in net sodium flux across the tissue. By use of selective agonists and antagonists it is concluded that acetylcholine interacts with muscarinic receptors in the serosal membrane. The effects of cholinoceptor agents are also seen with isolated epithelium. The stimulatory effect of acetylcholine is potentiated by theophylline and blocked by inhibitors of prostaglandin synthetase and by mepacrine. It is suggested that acetylcholine stimulates transport by liberating prostaglandins which may then activate adenylcyclase. The inhibitory effect of acetylcholine is correlated with a reduction in cyclic AMP content of the epithelium. Calcium appears to be an important determinant of the type of response seen eith acetylcholine, but the mechanism is not known.