Ca2+ oscillations in the rabbit renal cortical collecting system induced by Na+ free solutions

A nicardipine-sensitive Ca2+ entry contributes to the hypotonicity-induced increase in [Ca2+]i of principal cells in rat cortical collecting duct

We examined the mechanisms involved in the [Ca(2+)](i) response to the extracellular hypotonicity in the principal cells of freshly isolated rat cortical collecting duct (CCD), using Fura-2/AM fluorescence imaging. Reduction of extracellular osmolality from 305 (control) to 195 mosmol/kgH(2)O (hypotonic) evoked transient increase in [Ca(2+)](i) of principal cells of rat CCDs. The [Ca(2+)](i) increase was markedly attenuated by the removal of extracellular Ca(2+)(.) The application of a P(2) purinoceptor antagonist, suramin failed to inhibit the hypotonicity-induced [Ca(2+)](i) increase. The [Ca(2+)](i) increase in response to extracellular hypotonicity was not influenced by application of Gd(3+) and ruthenium red. On the other hand, a voltage-gated Ca(2+) channel inhibitor, nicardipine, significantly reduced the peak amplitude of [Ca(2+)](i) increase in the principal cells. In order to assess Ca(2+) entry during the hypotonic stimulation, we measured the quenching of Fura-2 fluorescence intensity by Mn(2+). The hypotonic stimulation enhanced quenching of Fura-2 fluorescence by Mn(2+), indicating that a Ca(2+)-permeable pathway was activated by the hypotonicity. The hypotonicity-mediated enhancement of Mn(2+) quenching was significantly inhibited by nicardipine. These results strongly suggested that a nicardipine-sensitive Ca(2+) entry pathway would contribute to the mechanisms underlying the hypotonicity-induced [Ca(2+)](i) elevation of principal cells in rat CCD.

Modeling stochastic Ca2+ release from a cluster of IP3-sensitive receptors

We focused our attention on Ca(2+) release from the endoplasmic reticulum through a cluster of inositol(1,4,5)-trisphosphate (IP(3)) receptor channels. The random opening and closing of these receptors introduce stochastic effects that have been observed experimentally. Here, we present a stochastic version of Othmer-Tang model (OTM) for IP(3) receptor clusters. We address the average behavior of the channels in response to IP(3) stimuli. In our stochastic simulation we found that the fraction of open channels versus [IP(3)] follows a Hill curve, whose associate Hill coefficient increases when intracellular Ca(2+) level increase. This finding suggests that feedback from cytosolic Ca(2+) plays a key role in the channel response to IP(3). We also study several aspects of the stochastic properties of Ca(2+) release and we compare with experimental observations.

Coupling of vasopressin-induced intracellular Ca2+ mobilization and apical exocytosis in perfused rat kidney collecting duct

Arginine vasopressin (AVP) regulates the osmotic water permeability of the kidney collecting duct by inducing exocytotic insertion of aquaporin-2 into apical membrane. The coupling between AVP-induced intracellular Ca2+ mobilization and apical exocytosis was investigated in isolated perfused rat inner medullary collecting duct (IMCD) segments using confocal fluorescence microscopy. Changes of [Ca2+]i in IMCD cells were measured with fluo-4. A novel confocal imaging technique using a styryl dye, FM1-43, was developed to monitor real-time exocytosis induced by arginine vasopressin. AVP (0.1 nM) triggered a rapid increase of [Ca2+]i in IMCD cells, followed by sustained oscillations. Ratiometric measurement of [Ca2+]i confirmed that the observed [Ca2+]i oscillation was a primary event and was not secondary to changes in cell volume. The frequencies of [Ca2+]i oscillations in each IMCD cell were independent and time variant. 1-Deamino-8-D-arginine vasopressin (a V2 receptor agonist, 0.1 nM) simulated the effects of AVP by triggering [Ca2+]i oscillations. In the absence of extracellular Ca2+, ryanodine (0.1 mM) inhibited AVP-induced Ca2+ mobilization. AVP (0.1 nM) triggered accumulative apical exocytosis in IMCD cells within 20 s after application. Pre-incubating the IMCD with an intracellular Ca2+ chelator, BAPTA, prevented AVP-induced intracellular Ca2+ mobilization, apical exocytosis, and increase of osmotic water permeability. These results indicate that AVP, via the V2 receptor, triggers a calcium signalling cascade observed as [Ca2+]i oscillations in the IMCD and that intracellular Ca2+ mobilization is required for exocytotic insertion of aquaporin-2.

Adenosine-stimulated Ca2+ reabsorption is mediated by apical A1 receptors in rabbit cortical collecting system

Confluent monolayers of immunodissected rabbit connecting tubule and cortical collecting duct cells, cultured on permeable supports, were used to study the effect of adenosine on net apical-to-basolateral Ca2+ transport. Apical, but not basolateral, adenosine increased this transport dose dependently from 48 +/- 3 to 110 +/- 4 nmol.h-1.cm-2. Although a concomitant increase in cAMP formation suggested the involvement of an A2 receptor, the A2 agonist CGS-21680 did not stimulate Ca2+ transport, while readily increasing cAMP. By contrast, the A1 agonist N6-cyclopentyladenosine (CPA) maximally stimulated Ca2+ transport without significantly affecting cAMP. Adenosine-stimulated transport was effectively inhibited by the A1 antagonist 1,3-dipropyl-8-cyclopenthylxanthine but not the A2 antagonist 3,7-dimethyl-1-propargylxanthine, providing additional evidence for the involvement of an A1 receptor. Both abolishment of the adenosine-induced transient increase in intracellular Ca2+ concentration by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and downregulation of protein kinase C (PKC) by prolonged phorbol ester treatment were without effect on adenosine-stimulated Ca2+ transport. The data presented suggest that adenosine interacts with an apical A1 receptor to stimulate Ca2+ transport via a hitherto unknown pathway that does not involve cAMP formation, PKC activation, and/or Ca2+ mobilization.

Hypoxia decreases calcium influx into rat proximal tubules

Renal ischemia results in adenosine triphosphate (ATP) depletion, particularly in cells of the proximal tubule (PT), which rely heavily on oxidative phosphorylation for energy supply. Lack of ATP leads to a disturbance in intracellular homeostasis of Na+, K+ and Cl-. Also, cytosolic Ca2+ levels in renal PTs may increase during hypoxia [1], presumably by a combination of impaired extrusion and enhanced influx [2]. However, Ca2+ influx was previously measured using radiolabeled Ca2+ and at varying partial oxygen tension [2]. We have now used to Mn2(+)-induced quenching of fura-2 fluorescence to study Ca2+ influx in individual rat PTs during normoxic and hypoxic superfusion. Normoxic Ca2+ influx was indeed reflected by the Mn2+ quenching of fura-2 fluorescence and this influx could be inhibited by the calcium entry blocker methoxyverapamil (D600; inhibition 50 +/- 2% and 35 +/- 3% for 10 and 100 mumol, respectively). La3+ completely blocked normoxic Ca2+ influx. Hypoxic superfusion or rat PTs did not induce an increase in Ca2+ influx, but reduced this influx to 79 +/- 3% of the normoxic control. We hypothesize that reducing Ca2+ influx during hypoxia provides the cell with a means to prevent cellular Ca2+ overload during ATP-depletion, where Ca2+ extrusion is limited.

Co-ordinated control of apical calcium influx and basolateral calcium efflux in rabbit cortical collecting system

Transcellular Ca2+ transport in the distal nephron involves passive Ca2+ influx at the apical membrane, diffusion through the cytosol and active extrusion across the opposing basolateral membrane. The molecular identity of the apical Ca2+ entry step is still elusive, but its regulatory aspects have been analyzed in the present study. To this end, rabbit connecting and cortical collecting tubular cells were cultured on permeable and transparent supports and the apical Ca2+ influx was deduced from Mn2+ quenching of Ca2+ independent Fura-2 fluorescence, while the intracellular Ca2+ concentration ([Ca2+]i) was measured simultaneously. In parallel experiments, transcellular Ca2+ transport was determined isotopically as 45Ca2+ flux from the apical to basolateral compartment. Decreasing the apical pH from 7.4 to 5.9 inhibited transcellular Ca2+ transport by 53 +/- 1%, whereas apical Ca2+ influx was reduced by 39 +/- 7% and [Ca2+]i decreased by 18 +/- 3%. Reversal of the Na+/Ca2+ exchanger by iso-osmotic replacement of Na+ by N-methyl-D-glucamine in the basolateral compartment resulted in 50 +/- 5% inhibition of Ca2+ transport, 46 +/- 3% reduction of apical Ca2+ influx and 60 +/- 3% increase in [Ca2+]i. In the absence of basolateral Ca2+, however, this manoeuvre decreased [Ca2+]i by 21 +/- 8%, while Ca2+ transport and apical Ca2+ influx were reduced by the same magnitude as in the presence of Ca2+, that is by 53 +/- 6% and 45 +/- 4%, respectively. Stimulation of adenylyl cyclase with forskolin (10(-5) M) increased transcellular Ca2+ transport by 108 +/- 40%, stimulated apical Ca2+ influx by 120 +/- 17% and increased [Ca2+]i by 110 +/- 2%. In conclusion, the apical Ca2+ influx is regulated by apical pH, intracellular cAMP and basolateral Na+/Ca2+ exchanger activity, and is coupled in an 1:1 fashion to the rate of transepithelial Ca2+ transport.

Calcium dynamics and homeostasis in a mathematical model of the principal cell of the cortical collecting tubule

Calcium (Ca) dynamics are incorporated into a mathematical model of the principal cell in the cortical collecting tubule developed earlier in Strieter et al. (1992a. Am. J Physiol. 263:F1063-1075). The Ca components are modeled after the Othmer-Tang model for IP(3)-sensitive calcium channels (1993, in Experimental and Theoretical Advances in Biological Pattern Formation, 295-319). There are IP(3)-sensitive Ca channels and ATP-driven pumps on the membrane of the endoplasmic reticulum. Calcium enters the cell passively down its electrochemical gradient. A Ca pump and Na/Ca exchange in the basolateral membrane are responsible for the extrusion of cytoplasmic calcium. Na/Ca exchange can also operate in reverse mode to transport Ca into the cell. Regulatory effects of cytoplasmic Ca on the apical Na channels are modeled after experimental data that indicate apical Na permeability varies inversely with cytoplasmic Ca concentration. Numerical results on changes in intracellular Ca caused by decreasing NaCl in the bath and the lumen are similar to those from experiments in Bourdeau and Lau (1990. Am. J Physiol. 258:F1497-1503). This match of simulation and experiment requires the synergistic action of the Na/Ca exchanger and the Ca regulated apical Na permeability. In a homogeneous medium, cytoplasmic Ca becomes oscillatory when extracellular Na is severely decreased, as observed in experiments of cultured principal cells (Koster, H., C. van Os and R. Bindels. 1993. Kidney Int.43:828-836). This essentially pathological situation arises because the hyperpolarization of membrane potential caused by Na-free medium increases Ca influx into the cell, while the Na/Ca exchanger is inactivated by the low extracellular Na and can no longer move Ca out of the cell effectively. The raising of the total amount of intracellular Ca induces oscillatory Ca movement between the cytoplasm and the endoplasmic reticulum. Ca homeostasis is investigated under the condition of severe extracellular Ca variations. As extracellular Ca is decreased, Ca regulation is greatly impaired if Ca does not regulate apical ionic transport. The simulations indicate that the Na/Ca exchanger alone has only limited regulatory capacity. The Ca regulated apical sodium or potassium permeability are essential for regulation of cytoplasmic Ca in the principal cell of the cortical collecting tubule.

Simplification and analysis of models of calcium dynamics based on IP3-sensitive calcium channel kinetics

We study the models for calcium (Ca) dynamics developed in earlier studies, in each of which the key component is the kinetics of intracellular inositol-1,4,5-trisphosphate-sensitive Ca channels. After rapidly equilibrating steps are eliminated, the channel kinetics in these models are represented by a single differential equation that is linear in the state of the channel. In the reduced kinetic model, the graph of the steady-state fraction of conducting channels as a function of log10(Ca) is a bell-shaped curve. Dynamically, a step increase in inositol-1,4,5-trisphosphate induces an incremental increase in the fraction of conducting channels, whereas a step increase in Ca can either potentiate or inhibit channel activation, depending on the Ca level before and after the increase. The relationships among these models are discussed, and experimental tests to distinguish between them are given. Under certain conditions the models for intracellular calcium dynamics are reduced to the singular perturbed form epsilon dx/d tau = f(x, y, p), dy/d tau = g(x, y, p). Phase-plane analysis is applied to a generic form of these simplified models to show how different types of Ca response, such as excitability, oscillations, and a sustained elevation of Ca, can arise. The generic model can also be used to study frequency encoding of hormonal stimuli, to determine the conditions for stable traveling Ca waves, and to understand the effect of channel properties on the wave speed.

Calbindin-D28K facilitates cytosolic calcium diffusion without interfering with calcium signaling

The role of calbindin-D28K, in transcellular Ca2+ transport and Ca2+ signaling in rabbit cortical collecting system was investigated. Rabbit kidney connecting tubules and cortical collecting ducts, hereafter referred to as cortical collecting system, were isolated by immunodissection and cultured to confluence on permeable filters and glass coverslips. Calbindin-D28K was present in the cytosol of principal cells, but was absent from the intercalated cells. 1,25(OH)2D3 (48 h, 10(-7) M) significantly increased cellular calbindin-D28K levels (194 +/- 15%) and stimulated transcellular Ca2+ transport (41 +/- 3%). This stimulatory effect could be fully mimicked by the endogenous Ca2+ chelator, BAPTA (30 microM BAPTA/AM), which suggests that the presence of Ca2+ chelators alone is sufficient to enhance transcellular Ca2+ transport. Stimulation of Ca2+ transport was not accompanied by a rise in [Ca2+]i. Isosmotic replacement of extracellular Na+ ([Na+]o) for N-methylglucamine (NMG) generated oscillations in [Ca2+]i in individual cells of the monolayer. The functional parameters of these oscillations such as frequency of spiking, resting [Ca2+]i, increase in [Ca2+]i and percentage of responding cells, were not affected by the level of calbindin-D28K. In contrast, loading the cells with BAPTA abruptly stopped these [Ca2+]i oscillations. This suggests that the kinetics of Ca2+ binding by calbindin-D28K are slow relative to the initiation of the [Ca2+]i rise, so that calbindin-D28K, unlike BAPTA, is unable to reduce [Ca2+]i rapidly enough to prevent the initiation of Ca(2+)-induced Ca2+ release.