Cholinergic agonists increase cell calcium in rat medullary collecting tubules. A fura-2 study

Cellular localization of type 5 and type 6 ACs in collecting duct and regulation of cAMP synthesis

The cellular distribution of Ca(2+)-inhibitable adenylyl cyclase (AC) type 5 and type 6 mRNAs in rat outer medullary collecting duct (OMCD) was performed by in situ hybridization. Kidney sections were also stained with specific antibodies against either collecting duct intercalated cells or principal cells. The localization of type 5 AC in H(+)-ATPase-, but not aquaporin-3-, positive cells demonstrated that type 5 AC mRNA is expressed only in intercalated cells. In contrast, type 6 AC mRNA was observed in both intercalated and principal cells. In microdissected OMCDs, the simultaneous superfusion of carbachol and PGE(2) elicited an additive increase in the intracellular Ca(2+) concentration, suggesting that the Ca(2+)-dependent regulation of these agents occurs in different cell types. Glucagon-dependent cAMP synthesis was inhibited by both a pertussis toxin-sensitive PGE(2) pathway (63.7 +/- 4.6% inhibition, n = 5) and a Ca(2+)-dependent carbachol pathway (48.6 +/- 3.3%, n = 5). The simultaneous addition of both agents induced a cumulative inhibition of glucagon-dependent cAMP synthesis (78.2 +/- 3.3%, n = 5). The results demonstrate a distinct cellular localization of type 5 and type 6 AC mRNAs in OMCD and the functional expression of these Ca(2+)-inhibitable enzymes in intercalated cells.

Localization of mRNAs encoding Ca2+-inhibitable adenylyl cyclases along the renal tubule. Functional consequences for regulation of the cAMP content

Expression of Ca2+-inhibitable types V and VI adenylyl cyclases was studied by reverse transcription-polymerase chain reaction in rat renal glomeruli and nephron segments isolated by microdissection. Quantitation of each mRNA was achieved using a mutant cRNA which differed from the wild type by substituting two bases to create a new restriction site in the corresponding cDNA. Type VI mRNA was present all along the nephron but was more abundant in distal than in proximal segments. The expression of type V mRNA was restricted to the glomerulus and to the initial portions of the collecting duct. Expression of the Ca2+-insensitive type IV mRNA studied on the same samples was evidenced only in the glomerulus. The functional relevance of the expression of Ca2+-inhibitable isoforms was studied by measuring cAMP content in the microdissected outer medullary collecting duct which expressed both type V mRNA (2367 +/- 178 molecules/mm tubular length; n = 8) and type VI mRNA (5658 +/- 543 molecules/mm, n = 8). Agents known to increase intracellular Ca2+ in this segment induced a Ca2+-dependent inhibition on either arginine vasopressin- or glucagon-stimulated cAMP level. The characteristics of these inhibitions suggest a functional and differential expression of types V and VI adenylyl cyclases in two different cell types of the rat outer medullary collecting duct.

Inhibition of amiloride-sensitive apical Na+ conductance by acetylcholine in rabbit cortical collecting duct perfused in vitro

We examined effects of acetylcholine (ACh) on the electrical parameters and intracellular Ca2+ concentration ([Ca2+]i) in the isolated rabbit cortical collecting duct (CCD) perfused in vitro using the conventional microelectrode technique and microscopic fluorescence spectrophotometry. ACh (10(-8) to 10(-5) M) in the bath caused a positive deflection of the transepithelial voltage (VT) and an increase in [Ca2+]i. Carbachol also showed similar but smaller effects. The effects of ACh were antagonized by muscarinic receptor antagonists. ACh at 10(-6) M hyperpolarized the apical membrane voltage and increased the fractional resistance of the apical membrane of the collecting duct cells accompanied by a positive deflection of VT and an increase in transepithelial resistance, whereas it did not affect these parameters in the beta-intercalated cells. In the presence of 10(-5) M amiloride in the lumen, the effects of ACh were almost completely abolished. The ACh-induced increase in [Ca2+]i is accounted for by the release of Ca2+ from intracellular store and Ca2+ entry from the bath. In the absence of Ca2+ in the bath, the ACh-induced changes in electrophysiological parameters were significantly smaller than those observed in the presence of Ca2+. Both phorbol-12-myristate-13-acetate (PMA) and phorbol-12,13-dibutylate (PDBu), activators of protein kinase C (PKC), also inhibited the apical Na+ conductance. In the presence of PMA or PDBu in the bath, ACh did not show further inhibitory effect. 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of PKC, partially attenuated the effect of ACh. These observations indicate that ACh inhibits the apical Na+ conductance partly by both increasing [Ca2+]i and activating PKC. Such an action of ACh may partially explain its natriuretic effect.

The activation of protein kinase C prevents PGE2-induced inhibition of AVP-dependent cAMP accumulation in the rat outer medullary collecting tubule

Previous studies have demonstrated that prostaglandin E2 (PGE2) inhibits arginine vasopressin-(AVP)dependent adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in microdissected rat outer medullary collecting tubules (OMCD), by a mechanism unrelated to the inhibition of cAMP synthesis. The potential role of the activation of protein kinase C (PKC) to explain the negative regulation elicited by PGE2 was investigated in this study. Single OMCD samples were pre-incubated (10 min, 30 degrees C) in the presence or absence of either activators of PKC, phorbol 12-myristate 13-acetate (PMA), 1-oleoyl-2-acetyl-glycerol (OAG), dioctanoylglycerol (DOG) or an inhibitor of PKC, staurosporine (SSP). These compounds were present also with the agonists tested during the incubation period (4 min, 35 degrees C). In contrast to PGE2, activators of PKC did not decrease AVP-dependent cAMP accumulation (mean +/- SEM): 1 nM AVP = 47.1 +/- 6.8 fmol.mm-1 x 4 min-1; AVP+0.3 microM PGE2 = 20.1 +/- 2.7, P < 0.01 versus AVP; AVP + 10 nM PMA = 42.0 +/- 4.7, NS versus AVP; AVP + 50 micrograms/ml OAG = 44.1 +/- 4.8. NS versus AVP, N = 5 experiments. However, 10 nM PMA prevented PGE2-induced inhibition: AVP + PGE2 = 44.2 +/- 3.5% of the response to AVP and 90.3 +/- 3.2% without and with PMA respectively, N = 16. Similar results were obtained with either 50 micrograms/ml OAG or 25 micrograms/ml DOG (AVP + PGE2 + OAG = 92.9 +/- 6.6% of the response to AVP, N = 8; AVP + PGE2 + DOG = 94.1 +/- 5.3%, N = 7).(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin increases [Ca2+]i in M-1 mouse cortical collecting duct cells by a dual mechanism

We tested the effects of endothelin-1 (ET-1) on intracellular calcium concentration ([Ca2+]i) of cultured M-1 mouse cortical collecting duct cells. [Ca2+]i was measured using fura 2 and a fluorescent imaging system. At a concentration of extracellular calcium ([Ca2+]o) of 1 mM, ET-1 (10(-12) to 10(-7) M) increased [Ca2+]i. A second application of ET-1 had no effect on Ca2+. In contrast, application of arginine vasopressin after an initial exposure to ET-1 induced a second Ca2+ response. In the absence of extracellular Ca2+ (1 mM EGTA) ET-1 also elicited a Ca2+ peak, indicating participation of Ca2+ release from intracellular stores in the initial Ca2+ peak. At [Ca2+]o of 10 mM, ET-1 also induced an intracellular Ca2+ peak but [Ca2+]i remained significantly elevated. The Ca2+ plateau phase was abolished by nickel (10 or 100 microM) and nifedipine (0.1 or 1 microM). We conclude that ET-1 mediates an increase in [Ca2+]i by Ca2+ release from intracellular stores and activation of a nickel- and nifedipine-sensitive Ca2+ entry mechanism.

Agonist-induced intracellular Ca2+ transients in HT29 cells

In the present study we have investigated the mechanism of intracellular Ca2+ activity ([Ca2+]i) changes in HT29 cells induced by adenosine triphosphate (ATP), carbachol (CCH), and neurotensin (NT). [Ca2+]i was measured with the fluorescent Ca2+ indicator fura-2 at the single-cell level or in small cell plaques with high time resolution (1-40Hz). ATP and CCH induced not only a dose-dependent [Ca2+]i peak response, but also changes of the plateau phase. The [Ca2+]i plateau was inversely dependent on the ATP concentration, whereas the CCH-induced [Ca2+]i plateau increased at higher CCH concentrations. NT showed (from 10(-10) to 10(-7) mol/l) in most cases only a [Ca2+]i spike lasting 2-3 min. The [Ca2+]i plateau induced by ATP (10(-6) mol/l) and CCH (10(-5) mol/l) was abolished by reducing the Ca2+ activity in the bath from 10(-3) to 10(-4) mol/l (n = 7). In Ca(2+)-free bathing solution the [Ca2+]i peak value for all three agonists was not altered. Using fura-2 quenching by Mn2+ as an indicator of Ca2+ influx the [Ca2+]i peak was always reached before Mn2+ influx started. Every agonist showed this delayed stimulation of the Ca2+ influx with a lag time of 23 +/- 1.5 s (n = 15) indicating a similar mechanism in each case. Verapamil (10(-6)-10(-4) mol/l) blocked dose dependently both phases (peak and plateau) of the CCH-induced [Ca2+]i increase. Short pre-incubation with verapamil augmented the effect on the [Ca2+]i peak, whereas no further influence on the plateau was observed. Ni2+ (10(-3) mol/l) reduced the plateau value by 70%.

PGE2-induced inhibition of AVP-dependent cAMP accumulation in the OMCD of the rat kidney is cumulative with respect to the effects of alpha 2-adrenergic and alpha 1-adenosine agonists, insensitive to pertussis toxin and dependent on extracellular calcium

The accumulation of cyclic adenosine 3',5'-phosphate (cAMP) elicited by antidiuretic hormone (arginine vasopressin, AVP) in the medullary collecting tubule (OMCD) microdissected from the rat kidney is inhibited by different factors: the A1 agonist of adenosine (-)-N6-(R-phenylisopropyl) adenosine (PIA), an alpha 2-adrenergic agonist clonidine (CLO), and prostaglandin E2 (PGE2). The negative regulation elicited by PGE2 was further characterized by measuring summation of inhibition with other inhibitors, by testing the effect of pertussis toxin and by studying the part played by extracellular calcium. Inhibitors were used at concentrations inducing maximum effects. The simultaneous addition of 0.3 microM PGE2 with either 0.1 microM PIA or 1 microM CLO led to an inhibition of the response to AVP (80.0 +/- 3.5%, SEM, N = 7 and 92.6 +/- 0.8%, N = 5, respectively) greater than those elicited by each agent alone. In contrast, PIA and CLO added together induced an inhibition similar to that due to CLO alone. The action of PGE2 in combination with either PIA or CLO corresponded to a partial summation fitting with the values calculated by assuming a cumulative inhibition. Preincubation of OMCD samples with pertussis toxin (100 ng/ml or 1 micrograms/ml) relieved the inhibitory effects of CLO and PIA but did not affect the action of PGE2. PGE2-induced inhibition was prevented in a calcium-free medium [0 Ca2+ + 0.1 mM [ethylene-bis (oxyethylene-nitrilo)] tetraacetate (EGTA)]: values were 67.0 +/- 2.1% and 5.8 +/- 8.7% (+/- SEM) in 2 mM Ca2+ and 0 Ca2+ medium, respectively, N = 7.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine and kinin augmentation of Cl- secretion stimulated by prostaglandin in a canine renal epithelial cell line

1. The actions of kinins and of acetylcholine upon transepithelial ion transport in a renal-derived cultured epithelium (Madin-Darby canine kidney cells, MDCK) have been investigated. 2. In voltage-clamped epithelial layers mounted in Ussing chambers and with prior stimulation of inward short-circuit current (SCC) by 5 or 10 microM-prostaglandin E1 (PGE1), both bradykinin (1 microM) and acetylcholine (0.1 mM) stimulate an additional, but transient, inward SCC. In the absence of PGE1 minimal effects of both bradykinin and acetylcholine upon SCC are observed. The SCC response to bradykinin and acetylcholine are attenuated with prior stimulation by 10 microM-adrenaline. 3. Measurements of bradykinin and acetylcholine-stimulated inward SCC with cation and anion replacement of the bathing media and the use of the Cl channel blocker 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) together with bumetanide to inhibit Na(+)-K(+)-Cl- 'co-transport', are consistent with the bradykinin- and acetylcholine-stimulated SCC being the result of basal to apical Cl- secretion. 4. Bradykinin (1 microM) is capable of stimulation of inward SCC from both epithelial surfaces, whilst acetylcholine is only effective from the basolateral surface. Kallidin (lys-bradykinin) was similar in effect to bradykinin from both epithelial surfaces whereas bradykinin (1-8) was ineffective, suggesting that B2 bradykinin receptors mediate the effect of bradykinin upon SCC. Dose-response relationships show that the response to kallidin and bradykinin was of higher sensitivity for additions to the apical cell aspects. 5. The data are discussed in relation to a model for epithelial Cl- secretion, and to the mechanism of natriuresis observed with kinins and acetylcholine in vivo.

Synchronous transporting activity in epithelial cells in relation to intracellular calcium concentration

Cultured monolayers of human sweat-gland epithelia have been used to measure electrogenic sodium transport, as short-circuit current, and intracellular Ca2+ concentration ([Ca]i) from Fura-2 fluorescence. The sodium currents in response to the agonists lysylbradykinin, histamine and carbachol show oscillatory behaviour in the 1-2 per minute frequency range. The oscillations can be terminated either by using specific antagonists or with amiloride, which prevents sodium entry into the epithelium. Oscillatory behaviour is also seen when [Ca]i is measured and occurs in the same frequency range. Sodium transport in these cultured epithelia is thought to result from an increase in [Ca]i, which in turn activates calcium-sensitive potassium channels, so increasing the electrochemical gradient for sodium entry. The oscillatory behaviour implies that the epithelial cells behave in synchrony to increase [Ca]i, so inducing synchronous changes in sodium current. It is shown that the behaviour is not unique to sodium-absorbing epithelia, and the possible utility of synchronous behaviour in epithelial tissues is discussed.