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In Meniere's disease, an inner ear disorder related to an endolymphatic hydrops, an alteration of the functioning of the endolymphatic sac has been proposed. The endolymphatic sac is assumed to be involved in the secretion/resorption of endolymph. The epithelial transport systems have been indirectly studied by the recording of the endolymphatic sac transepithelial potential (ESP) in control conditions and after the local injection of drugs such as diuretics that have been proposed in the treatment of Meniere's disease. The ESP was recorded, in vivo, in guinea pigs up to 150 minutes after the perisaccular injection of 5 μL of a 150 mmol/L (mM) NaCl solution containing various drugs known to inhibit ionic transport systems. The initial ESP was +8.4 ± 0.3 mV (mean ± SEM, n = 78). The basolateral injection of 5 μL of 150 mM NaCl induced an ESP decrease of 64% ± 6.0% (n = 12), 5 minutes after the end of the injection. Then ESP increased, returning to its initial value at 60 minutes and surpassing it at 120 minutes. Diuretics such as acetazolamide (10 -3 mol/L [M]), an inhibitor of carbonic anhydrase, and amiloride (10 -4 M), an inhibitor of Na channel or Na/H exchanger, decreased the ESP recovery. At variance, bumetanide (10 -6 M, 10 -4 M), the Na-K-Cl cotransport inhibitor, and chlorothiazide (10 -4 M), a Na-Cl cotransporter inhibitor, failed to alter the ESP as compared with the control group. Ouabain (10 -3 M), the Na + ,K + -adenosine triphosphatase (ATPase) inhibitor, prevented the ESP recovery otherwise observed 60 minutes after the NaCl injection. Bafilomycin A 1 , the inhibitor of the vacuolar-type H + -ATPase, prevented the recovery of the ESP with a log-dose/effect (10 -5 M, 10 -6 M, 10 -9 M). Disulfonic acid stilbene (DIDS) (10 -4 M), an inhibitor of transporters involving HCO 3 - , also prevented the ESP recovery. These results suggest that the genesis of the ESP was highly dependent on acid-base transport systems including carbonic anhydrase, a vacuolar-type H + -ATPase, and an anionic transport system blocked by DIDS. Further studies are needed to confirm the alteration of the acid-base balance in this epithelium and its possible involvement in the pathogenesis of Meniere's disease.

We evaluated, in renal epithelial cells with a proximal tubule phenotype, the effect of nitric oxide (NO) on ecto-5′-nucleotidase (5′-NU), the underlying mechanism and its functional consequence. Sodium nitroprusside (SNP, 1-1000 μM), a NO donor, inhibited 5′-NU activity in a time- and concentration-dependent manner. Consequently, NO blunted the inhibition by extracellular cyclic AMP (cAMP, 10-1000 μM) of sodium-phosphate cotransport, a pathway which involves degradation of adenosine monophosphate (AMP) by 5′-NU. SNP-induced inhibition of 5′-NU was not mediated by cyclic GMP, since it was not mimicked by atrial natriuretic peptide, and was reproduced by isosorbide dinitrate and sodium nitrate, two NO donors. SNP and genuine NO decreased the activity of 5′-NU in renal homogenates, and the effect of SNP was potentiated by dithiothreitol and glutathione, but not by nicotinamide adenine dinucleotide. In vivo in rats, kidney ischemia/reperfusion, which activates inducible NO-synthase, inhibited renal 5′-NU. This inhibition was prevented by N-nitro-L-arginine methyl ester, a NO-synthase inhibitor. These results indicate that: (i) NO-related activity inhibited the activity of an ecto-enzyme, 5′-NU, most likely through S-nitrosylation of the enzyme; (ii) inhibition of 5′-NU activity by NO, which can occur in vivo under pathophysiological conditions, affected the extent to which extracellular cAMP inhibited sodium-P cotransport.

Glycerol (2 g/kg body weight), or 0.15 M NaCl for control animals, was administered to rats by i.v. injection. The dose was chosen in order to obtain an osmolarity increase in plasma of about 15 mosm/l 1 h after the glycerol administration, an increase which is similar to that observed in the human glycerol dehydration test. Endolymph and perilymph were sampled from the basal turn of the cochlea; cerebrospinal fluid (CSF) was sampled from cisterna magna. Plasma osmolarity, endocochlear potential, Na and K concentrations in endolymph, perilymph and CSF were determined 1 and 2 h after the glycerol injection. Compared with control animals, glycerol induced an increase in Na and K concentration in perilymph and endolymph, respectively, 1 and 2 h after the glycerol injection. No modification of the endocochlear potential was observed. These results are compatible with an increase in inner ear fluids osmolarity induced by glycerol.

We have evaluated the effect of lipopolysaccharides (LPS), endotoxins from gram negative bacteria, on sodium-coupled amino acid and phosphate transport by alveolar epithelial type II cells and on their alteration induced by oxidants. Alveolar type II cells were obtained by enzymatic digestion of rat lung and grown for 24 h prior to incubation with LPS and then exposed or not exposed to H2O2 (2.5 mM; 20 min). LPS (10 μg/ml, 24 h) induced a significant increase in the Na-dependent component of alanine and phosphate uptake while they decreased Na, K-ATPase activity measured by ouabain-sensitive 86Rb influx. We showed that this stimulatory effect i) was independent from macrophage products since it was not mimicked either by supernatant of LPS-treated alveolar macrophages or by pretreatment with tumor necrosis factor and/or interleukin 1 and ii) was dependent on protein synthesis since it was abolished by protein synthesis inhibitors cycloheximide and actinomycin D. Moreover, LPS blunted H2O2-induced decrease of Na-dependent alanine and phosphate uptake. This protective effect of LPS against H2O2 injury i) was independent of macrophage products, ii) was abolished by cycloheximide, and iii) was not associated with either changes in extracellular H2O2 clearance or catalase and glutathione peroxidase activities. We conclude that, in alveolar type II cells, LPS stimulate sodium-coupled transport by a process involving protein synthesis and partially prevent H2O2-induced decrease of Na-coupled transport without discernible change in antioxidant activities. © 1995 Wiley-Liss, Inc.

Platelet activating factor inhibits Cl and K transport in the medullary thick ascending limb. Since the kidney medulla was reported to generate platelet activating factor (PAF), we investigated a possible effect of this agent on the reabsorptive function of in vitro microperfused medullary thick ascending limbs from mouse kidney (mTAL). PAF, 10 -7 M in the bath, significantly decreased the net chloride flux (JCl) from 48.8 ± 7.1 to 27.4 ± 5.7 pmol/min. This effect was reversible, blocked by the antagonist BN 50730, and not reproduced by the inactive metabolite lyso-PAF. PAF inhibited the transepithelial potential difference with a threshold at 10 -9 M. In the presence of isoproterenol, the PAF-induced decrease of JCl was not significantly different from that observed in basal conditions; moreover, PAF did not modify the adenylate cyclase activity in isolated mTALs, either in basal condition or under stimulation by isoproterenol. The effect of PAF on JCl was not prevented by mepacrine, NDGA associated with proadifen, or adenosine desami-nase. When the apical Na-K-2Cl cotransport was blocked by furosemide or bumetanide, a net K secretion occurred (-1.1 ± 0.2 pmol/min), which was significantly decreased by PAF (-0.06 ± 0.3 pmol/min). Moreover, it was verified on isolated mTALs that PAF did not modify the Na,K-ATPase activity. It is concluded that PAF inhibits the reabsorptive function of the mTAL, as indicated by the decrease of Cl reabsorption and K secretion. This effect could not be accounted for by adenosine or arachidonic acid metabolite action, and was not mediated by an inhibition of the adenylate cyclase activity.

In order to evaluate the influence of membrane fluidization on three apical transport systems and on a basolateral enzyme, and to analyse the mechanisms involved, we studied, in cultured rabbit proximal tubular cells, the effect of increasing concentrations of the local anesthetic drug benzyl alcohol on Na(+)-dependent uptakes of phosphate (Pi), methyl alpha-D-glucopyranoside (MGP), and L-alanine, as well as on basal and stimulated cyclic AMP content. At 10 mM, benzyl alcohol increased the Vmax of Pi uptake by 31%, decreased that of MGP uptake by 24%, and did not affect alanine uptake. Km values were not affected. Benzyl alcohol, up to 40 mM, increased in a concentration-dependent manner basal, PTH-stimulated, and cholera toxin-stimulated, but not forskolin-stimulated cyclic AMP accumulation. In the presence of 40 mM benzyl alcohol, the magnitude of PTH-induced inhibition of Pi uptake was enhanced from 11% to 24%. It is concluded that: (i) fluidization of apical membranes affected differently Na+/Pi, Na+/MGP, and Na+/alanine cotransports, reflecting differences in the lipidic environments of these transport system; (ii) fluidization of basolateral membranes enhanced PTH-stimulated cyclic AMP generation through improved coupling between the receptor-GS complex and the catalytic subunit of adenylate cyclase; (iii) these variations may result in physiological and pathophysiological modulation of the renal handling of solutes and of the phosphaturic effect of PTH.

PTH-induced phosphaturia is exerted in part by cAMP added to the renal tubular lumen under the influence of the hormone. Modulation of renal phosphate transport by luminal cAMP requires degradation of the nucleotide into adenosine by brush-border membrane ectoenzymes, among them ecto-5'-nucleotidase (5'-NU). Hormonal modulation of 5'-NU activity was evaluated in cultured opossum kidney cells. PTH (1-100 nM) stimulated 5'-NU in a time-, concentration-, and protein synthesis-dependent manner. The effect of PTH-(1-34) was mimicked by PTH-(3-34), which does not activates adenylate cyclase, and by phorbol 12-myristate 13-acetate (PMA), but not by forskolin or (Bu)2cAMP. Down-regulation or pharmacological inhibition of protein kinase-C (PKC) abolished the effect of PTH fragments and PMA. PTH fragments increased intracellular Ca2+ and translocated PKC activity to the membrane. PTH or PMA did not affect 5'-NU messenger RNA content. Inhibition of sodium-phosphate cotransport by extracellular cAMP was decreased by 5'-NU inhibition and was magnified by PTH. These results indicate that 1) PTH stimulates 5'-NU activity in renal proximal tubular cells in a manner involving PKC activation and de novo protein synthesis; and 2) this effect participates in PTH modulation of renal phosphate transport.

To the Editor: Dipyridamole, a widely used vasodilatory drug, enhances renal tubular reabsorption of phosphate by decreasing adenosine uptake by tubular cells.1 In rats, dipyridamole prevents phosphate excretion induced by cyclic AMP and decreases the phosphaturic effect of parathyroid hormone.1 This demonstrates that adenosine, a local product of the degradation of nephrogenous cyclic AMP, is associated with phosphaturia. We sought to determine whether dipyridamole might decrease urinary phosphate excretion in humans. We evaluated the short-term effect of dipyridamole in 48 people (22 men and 26 women) with a mean age of 52 years (range, 26 to 74). Twelve were normal . . .

To evaluate a possible modulation by membrane fluidity of hormonal, cAMP-mediated effects on renal epithelial cells, we studied the effect of the neutral local anesthetic, benzyl alcohol, on membrane fluidity and on basal and stimulated intracellular cAMP content in intact MDCK cells. Benzyl alcohol induced a dose-dependent decrease of lipid order which was measured by steady-state fluorescence anisotropy using trimethylammonium-diphenylhexatriene and propionyl-diphenylhexatriene as fluorescent probes. Benzyl alcohol induced a 2-fold increase in basal cAMP content, likely as a consequence of increased prostaglandin synthesis since this effect was abolished by indomethacin. The effect of benzyl alcohol on stimulated cAMP synthesis depended on the nature of the ligand: 10 mM benzyl alcohol increased significantly the stimulatory effect of prostaglandin E2, glucagon and forskolin but not of vasopressin. At higher concentrations (40 mM), benzyl alcohol did not affect significantly the glucagon-stimulated cAMP content, while it inhibited significantly the prostaglandin E2-, forskolin- and vasopressin-stimulated cAMP synthesis. The 40 mM benzyl alcohol-induced inhibition was reversed by 1 mM Mn2+, which is known to block the inhibitory GTP-binding protein Ni. These results suggest that: (i) the various components of the adenylate cyclase-cAMP system and their coupling are affected differently by changes in membrane fluidity, which might reflect differences in their lipid environment, (ii) changes in membrane fluidity can modulate responses of renal tubular cells to hormones, and thus tubular functions.

The effect of somatostatin and alpha 2-adrenergic agonists on cyclic AMP accumulation was examined in MDCK cells, grown in defined medium. These hormones inhibited vasopressin-induced cyclic AMP formation, without affecting either the basal or the glucagon- and prostaglandin E2-stimulated level. Pretreating the cells with pertussis toxin, or incubating them with MnCl2 at a low concentration reversed the effect of somatostatin and alpha 2-agonists. These results suggest that somatostatin and norepinephrine could selectively modulate the renal effect of vasopressin, via the inhibitory regulatory subunit (Ni) of adenylate cyclase.

The Ca++ messenger system is nearly universally present in the control of cell functions by extracellular messengers. It elicits different kinds of responses: either brief, as in neurotransmission and in skeletal muscle contraction, or sustained, as in smooth muscle contraction and in the regulation of transepithelial transport systems. In this latter function the Ca++ messenger system interacts with other cellular modulation systems such as cAMP production and arachidonic acid cascade. In the vestibular apparatus, the Ca++ messenger system is involved in the transduction processes in that it controls both the release of neurotransmitter at the basal pole of the hair cell and the electrical resonance of the cell. It also figures in the contractile properties of cochlear outer hair cells. The Ca++ messenger system as well as adenylate cyclase and prostaglandins may play an important role in the modulation of endolymph secretion.

Analysis of increased myocardial contractility during sodium acetate infusion in humans. To analyze the reported effects of acetate on left ventricular (LV) contractility during dialysis, LV function was studied before and after a 20-min sodium acetate (Na Ac) infusion (0.06 mmoles · kg -1 · min-1) in seven patients with heart rate (HR) controlled by atrial pacing. Angiographically determined LV volumes and LV pressures were used to calculate the LV function indices. A plasma acetate concentration of 3.13 ± 1.05 (SD) mmoles · liter-1 induced an increase in cardiac index from 3.8 ± 0.6 to 4.4 ± 0.6 (SD) liter · min-1 · m-2 (P < 0.01) and a rise in total body O2 consumption from 7.47 ± 1.28 to 8.67 ± 1.66 mmoles · min-1 · m-2 (P < 0.05); there was no alteration of the volume elastic constant, of the end diastolic stress, and of the end systolic stress. There was an increase of the ejection fraction from 0.44 ± 0.10 to 0.51 ± 0.09 (P < 0.01), the maximum velocity of shortening (Vmax, sec-1) from 1.37 ± 0.25 to 1.55 ± 0.28 (P < 0.05), and of the end systolic stress–end systolic volume ratio (g · cm-2 · ml-1) from 2.99 ± 0.76 to 3.40 ± 0.98 (P < 0.01). Hence, the enhancement of these indices was not the consequence of any alteration of HR, preload, or afterload. In seven other subjects, HR, LV end diastolic and systolic pressures, positive dP/dtmax, and Vmax were measured before, during, and after a 6-min infusion of Na Ac into the left coronary artery (0.45 mmoles · min-1); none of these parameters varied significantly during the procedure. It is concluded that the enhancement of myocardial contractility by acetate did not result from a direct effect of acetate on myocardium, but was more likely mediated by an increased peripheral metabolism.Etude de l'augmentation de la contractilité myocardique par l'acétate chez les hommes. Afin d'analyser les effets de l'acétate sur la contractilité du ventricule gauche (VG), la fonction VG a été étudiée avant et après 20-min de perfusion d'acétate de sodium (Na Ac) (0,06 mmoles · kg 1 · min-1) chez sept sujets dont la fréquence cardiaque (FC) était contrôlée par stimulation auriculaire. Les volumes VG calculés par angiographie et les pressions VG ont été utilisés pour calculer les indices de fonction ventriculaire gauche. Une concentration plasmatique d'acétate de 3,13 ± 1,05 mmoles · litre-1 a entrainé une augmentation de l'index cardiaque de 3,8 ± 0,6 à 4,4 ± 0,6 (SD) litre · min-1 · m -2 (P < 0,01) et une élévation de la consommation d'oxygène de 7,47 ± 1,28 à 8,67 ± 1,66 mmoles · min-1 · m -2 (P < 0,05); il n'y a pas eu de modification de la constante d'élasticité volumique, de la contrainte télédiastolique et de la contrainte télésystolique. La fraction d'éjection s'est élevée de 0,44 ± 0,10 à 0.51 ± 0,09 (P < 0,01), la vitesse maximum de raccourcissement (Vmax, sec-1) s'est élevée de 1.37 ± 0,25 à 1.55 ± 0.28 (P < 0,05), le rapport contrainte-volume télésystolique (g · cm-2 · ml-1) s'est élevé de 2,99 ± 0,76 à 3,40 ± 0,98 (P < 0,01). L'augmentation de tous ces indices ne résultait donc pas de modifications de la FC, de la pré-charge ou de la post-charge. Chez sept autres sujets, la FC, la pression télédiastolique et de la pression systolique du VG, la dP/dtmax et la Vmax ont été mesurés avant, pendant et après une injection continue d'acétate de sodium pendant 6-min dans l'artère coronaire gauche (0,45 mmoles · min-1); aucun des paramètres n'a varié de façon significative lors de ce protocole. Il apparaît donc que l'augmentation de la contractilité myocardique induite par l'acétate ne résulte pas d'un effet direct de l'acétate sur le myocarde, mais qu'elle est plus probablement secondaire à une élévation du métabolisme périphérique.

Phosphate handling by the rat nephron during saline diuresis. Inorganic phosphate handling by the nephron was investigated using renal micropuncture techniques in fasted rats undergoing sustained saline diuresis and in controls. The early distal and the late accessible proximal convolutions of the same nephron were micropunctured. Despite the presence of decreased fractional volume reabsorption at the end of the proximal tubule during saline diuresis, the tubular fluid to plasma phosphate ratio was unaffected. Fractional phosphate delivery at this site was not significantly higher. Fractional phosphate delivery at the early distal and late proximal segments of the same nephron were significantly correlated in both groups. The slopes of the regression lines were not different. The delivery of phosphate to the early distal tubule, for a given value of delivery from the late proximal, was lower during saline diuresis. Mean fractional phosphate reabsorption between the late proximal and the early distal tubules was twice as high during saline diuresis as in controls. This observation may account for the decreased fractional phosphate excretion in ureteral urine observed in fasting animals. Clearance studies indicated a different behavior between fed and fasted rats undergoing saline diuresis. Fractional phosphate excretion increased in the former and decreased in the latter.

The pivotal role of nucleotides in cell function and metabolism has been acknowledged for more than three decades since the milestone work of Sutherland [1]. Intracellular nucleotides are involved in each and every event of cell life including synthesis of nuclear material, phosphorylation processes, or transduction of extracellular signals. As regards to this last point, a huge number of studies has been devoted to identifying the key role of GTP-binding proteins or G proteins, on the one hand, and of cyclic nucleotides cAMP and cGMP, on the other hand, in the early steps of cell signaling. In epithelial cells such as renal tubular cells, generation of these mediators as a hallmark of cell-hormone interaction has been investigated in detail [reviewed in 2]. More recently, experimental evidence was provided showing that extracellular nucleotides are able to affect virtually every organ and/or tissue system. These aspects have been extensively reviewed [3—7] and will not be discussed here. Instead, attention will be focused on the effect of extracellular nucleotides, primarily cAMP, on renal tubular transport of inorganic phosphate (Pi). The initial observations that systemic infusion of cAMP to parathyroidectomized rats induced phosphaturia, thus mimicking the proximal effect of parathyroid hormone (PTH), drew attention on the effects of extracellular cAMP on renal function [8, 9]. A striking feature was that other renal effects of PTH (modulation of calcium or magnesium excretion), which take place beyond the proximal tubule, or renal effects of other peptidic hormones that act through the cAMP-protein kinase A pathway in distal parts of the nephron, such as antidiuteric hormone, were not reproduced by cAMP infusion [9]. Along the same line, glucagon, infused or injected at large pharmacological doses that induced a rise of plasma cAMP concentration from hepatic origin and urine excretion of the nucleotide [8, 10], was shown to increase phosphate excretion [8]. These data raised the possibility that handling (metabolism and/or transport) of extracellular cAMP by proximal cells was involved in the effect of the cyclic nucleotide on Pi transport. In contrast with the extreme experimental conditions described above, physiological situations are characterized by a fairly constant systemic cAMP concentration, while luminal cAMP concentration varies with the parathyroid status [10—12]. These observations suggest that luminal cAMP may act as local paracrine modulator of Pi transport and raise three questions: (a) How does cAMP reach the extracellular space under normal situations; (b)

Localization of parathyroid-hormone-independent sodium bicarbonate inhibition of tubular phosphate reabsorption. We investigated phosphate transport along superficial nephrons in two groups of acutely parathyroidectomized (APTX) rats. Animals of group 1 were infused with 0.5 M sodium chloride; and those of group 2, with 0.5 M sodium bicarbonate at the same rates. Compared to the sodium chloride infusion, the sodium bicarbonate infusion was associated with a significant increase in urinary excretion of phosphate: the fractional phosphate excretion was 2.3 ± (SD) 1.3% in the sodium chloride group and 14.4 ± 3.2% in the sodium bicarbonate group, P < 0.01, whereas the fractional sodium excretion was identical, 7.4 ± 0.60% and 7.5 ± 0.50%. Micro-puncture studies performed at the late accessible proximal and early accessible distal sites of the same superficial nephrons indicate that the reabsorptive capacity for phosphate (absolute reabsorption/absolute delivered phosphate per nephron segment) is decreased during sodium bicarbonate infusion in the convoluted proximal tubule as well as in the loop (segment located between late proximal and early distal accessible convolutions) and the terminal nephron. Such an effect is independent of both parathyroid hormone secretion and extracellular fluid volume expansion.Localisation de l'inhibition par le bicarbonate de sodium, indépendamment de l'hormone parathyroidiene de la réabsorption tubulaire du phosphate. Le transport de phosphate le long des néphrons superficiels a été étudié dans deux groupes de rats en situation de parathyroïdectomie aiguë (APTX). Les animaux du groupe 1 ont été perfuses avec 0,5 M chlorure de sodium et ceux du groupe 2 avec 0,5 M bicarbonate de sodium au même débit. Par comparaison avec l'effet du chlorure de sodium, la perfusion de bicarbonate de sodium est contemporaine d'une augmentation significative de l'excrétion urinaire de phosphate: l'excrétion fractionnelle est de 2,3 ± 1,3% (moyenne ± déviation standard) dans le groupe chlorure de sodium et 14,4 ± 3,2% dans le groupe bicarbonate de sodium, P < 0.001, alors que l'excrétion fractionnelle du sodium est identique, 7,4 ± 0,60% et 7,5 ± 0,50%. Les microponctions réalisées à la fin du tube proximal et au début du tube distal accessibles indiquent que la capacité de réabsorption pour le phosphate (réabsorption absolue/débit absolu délivré à chaque segment) est diminuée au cours de la perfusion de bicarbonate de sodium dans le tube contourné proximal de même que dans l'anse (segment compris entre la fin du proximal et le début du distal accessibles) et dans le néphron terminal. Cet effet est indépendant de la sécrétion d'hormone parathyroïdienne et de l'expansion des liquides extra-cellulaires.

When tubular phosphate reabsorption was first studied with micropunc-ture techniques by Strickler et al. (1964), one would have expected the question “Where is phosphate reabsorbed in the nephron?” to be answered quickly, easily, and definitively. Sixteen years later, and despite a considerable number of publications on the subject, it still appeared timely to the editors of this volume to ask for a discussion of site(s) of phosphate reabsorption along the nephron. Is this the consequence of a failure of the micropuncture technique, of inadequate protocol design, or of insufficient interpretation of the data? A critical evaluation of the available data on the sites of phosphate reabsorption along the nephron is presented in this chapter.

To evaluate a possible modulation by protein kinase C of hormonal, cAMP-mediated effects on renal epithelial cells, we studied the effect of protein kinase C activators and of bradykinin on intracellular cAMP accumulation in MDCK cells. A 15-min pretreatment of cells with phorbol 12-myristate 13-acetate or 1-oleoyl-2-acetylglycerol induced a dose-dependent inhibition of vasopressin-stimulated cAMP synthesis, but not of basal or glucagon-, prostaglandin E2-, and forskolin-stimulated cAMP generation. 4 alpha-Phorbol 12,13-didecanoate, inactive on protein kinase C, did not affect cAMP accumulation. Bradykinin (0.1-10 microM) also inhibited the stimulatory effect of vasopressin on cAMP synthesis in a concentration-dependent manner, but affected neither basal cAMP content, nor its stimulation by glucagon, prostaglandin E2 and forskolin. The effect of activators of protein kinase C and of bradykinin occurred while renal prostaglandin synthesis was blocked with indomethacin. The inhibitory effect of protein kinase C activators and bradykinin on cAMP generation was reversed by the protein kinase C inhibitor H7, was enhanced by monensin, one effect of which is to block the recycling of membrane receptors, and persisted when the GTP-binding protein N1 was blocked with 1 mM Mn2+. Our data suggest that: protein kinase C can modulate the tubular effects of vasopressin by inhibiting cAMP generation; this effect is not mediated by renal prostaglandins, and might result from a direct action on the vasopressin receptor, or on its coupling with Ns; the modulation by bradykinin of vasopressin effects are likely to be exerted, at least partly, through activation of protein kinase C.

Antiarrhythmic drugs with local anesthetic properties modify the physical state of membrane phospholipids and could change adenylate cyclase activity and, thus, influence cardiac ischemic arrhythmias. Adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in cardiomyocytes cultured from newborn rat and fluorescence anisotropy of sarcolemma-enriched membranes were investigated in the presence of a neutral anesthetic drug benzyl alcohol and of a cationic anesthetic drug lidocaine. Benzyl alcohol increased in a dose-dependent manner both sarcolemma fluidity and isoproterenol- or cholera toxin-stimulated cAMP accumulation. In contrast, benzyl alcohol inhibited cAMP accumulation in forskolin-stimulated cells. Lidocaine induced a dose-related inhibition of isoproterenol-, forskolin-, and cholera toxin-stimulated cAMP accumulation without eliciting any change in sarcolemma fluidity. The inhibitory effect of lidocaine on isoproterenol-stimulated cAMP accumulation was reversed when cells were pretreated with pertussis toxin. These data suggest that the inhibitory effect of lidocaine on cAMP synthesis might involve a polar interaction with the Gi regulatory subunit of adenylate cyclase. Such an effect could contribute, in vivo, to both the antiarrhythmic and the negative inotropic effect of lidocaine.

In mammals inorganic phosphate (Pi) is filtered at the glomerulus and reabsorbed in the tubules (1).