Plasmalemmal dihydropyridine receptors modify the function of subplasmalemmal inositol 1,4,5-trisphosphate receptors: a hypothesis

Mechanisms of the sarcoplasmic reticulum Ca2+ release induced by P2X receptor activation in mesenteric artery myocytes

BACKGROUND

ATP is one of the principal sympathetic neurotransmitters which contracts vascular smooth muscle cells (SMCs) via activation of ionotropic P2X receptors (P2XRs). We have recently demonstrated that contraction of the guinea pig small mesenteric arteries evoked by stimulation of P2XRs is sensitive to inhibitors of IP3 receptors (IP3Rs). Here we analyzed contribution of IP3Rs and ryanodine receptors (RyRs) to [Ca(2+)]i transients induced by P2XR agonist αβ-meATP (10 μM) in single SMCs from these vessels.

METHODS

The effects of inhibition of L-type Ca(2+) channels (VGCCs), RyRs and IP3Rs (5 μM nicardipine, 100 μM tetracaine and 30 μM 2-APB, respectively) on αβ-meATP-induced [Ca(2+)]i transients were analyzed using fast x-y confocal Ca(2+) imaging.

RESULTS

The effect of IP3R inhibition on the [Ca(2+)]i transient was significantly stronger (67 ± 7%) than that of RyR inhibition (40 ± 5%) and was attenuated by block of VGCCs. The latter indicates that activation of VGCCs is linked to IP3R-mediated Ca(2+) release. Immunostaining of RyRs and IP3Rs revealed that RyRs are located mainly in deeper sarcoplasmic reticulum (SR) while sub-plasma membrane (PM) SR elements are enriched with type 1 IP3Rs. This structural peculiarity makes IP3Rs more accessible to Ca(2+) entering the cell via VGCCs. Thus, IP3Rs may serve as an "intermediate amplifier" between voltage-gated Ca(2+) entry and RyR-mediated Ca(2+) release.

CONCLUSIONS

P2X receptor activation in mesenteric artery SMCs recruits IP3Rs-mediated Ca(2+) release from sub-PM SR, which is facilitated by activation of VGCCs. Sensitivity of IP3R-mediated release to VGCC antagonists in vascular SMCs makes this mechanism of special therapeutic significance.

Reactive oxygen species, Ca2+ signaling and mitochondrial NAD(P)H level in adrenal glomerulosa cells

The acute effects of ultraviolet light, the superoxide-generating xanthine-xanthine oxidase system and H(2)O(2) to on calcium signaling and mitochondrial pyridine nucleotide metabolism were investigated in rat glomerulosa cells. UV light induced the formation of superoxide, that, similar to exogenously applied superoxide and H(2)O(2), decreased the level of mitochondrial NAD(P)H. Free radical scavengers antagonized this effect of UV light. Extracellularly generated superoxide elicited Ca(2+) transients and inhibited angiotensin II-induced cytoplasmic Ca(2+) signaling. Low intensity UV light did not affect basal [Ca(2+)] and failed to influence Ca(2+) signaling induced by depolarization or store depletion. UV light of the same low power reduced both cytoplasmic and mitochondrial Ca(2+) signals induced by angiotensin II. The lack of UV effect on inositol phosphate formation indicates that the inhibition of cytoplasmic Ca(2+) signaling is due to reduced Ca(2+) release from InsP(3)-sensitive stores. Decreased mitochondrial Ca(2+) uptake may be attributed to UV-induced perturbation of the perimitochondrial microdomain.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Selective inhibition of potassium-stimulated rat adrenal glomerulosa cells by ruthenium red

The effect of the cationic dye, ruthenium red (RR), on ionic fluxes, Ca2+ signal generation, and stimulation of aldosterone production was studied in isolated rat adrenal glomerulosa cells. In these cells, increased extracellular [K+] as well as angiotensin II (Ang II) elevate cytoplasmic Ca2+ concentration and thereupon activate steroidogenesis. However, the mode of action of the two stimuli are different: while a dihidropyridine-sensitive mechanism contributes to the response to both agonists, Ang II induces Ca2+ release from intracellular stores and causes capacitative Ca2+ influx, whereas K+ was recently shown to activate a plasma membrane Ca2+ current (Igl) independently of membrane depolarization. The difference is reflected in the sensitivity of the response of the cells to RR. The Ang II-induced Ca2+ signal and aldosterone production were not inhibited, but rather slightly potentiated by the dye. This potentiation was probably the consequence of the membrane-depolarizing effect of RR, due to the observed inhibition of the resting K+ conductance. Conversely, Ca2+ signal and aldosterone production were significantly reduced by RR when the cells were stimulated by moderately elevated [K+] (6-8 mM). Our patch clamp studies suggest that this effect was related to the inhibition of different voltage-dependent and -independent inward Ca2+ currents and indicates the functional importance of the latter in the signal transduction of the potassium-stimulated glomerulosa cell.

Interactions between ouabain, atrial natriuretic peptide, angiotensin-II and potassium: effects on rat zona glomerulosa aldosterone production

The effects of ouabain, atrial natriuretic peptide, angiotensin-II and potassium on aldosterone production by collagenase dispersed rat zona glomerulosa cells were studied. A-II and 10(-4) M ouabain-induced increases in aldosterone production was inhibited by 10(-9) M ANP at all potassium concentrations examined. 10(-4) M ouabain inhibited the A-II induced increase in aldosterone production at all potassium concentrations. The degree of this inhibition was smaller at higher potassium levels. Ouabain enhanced the inhibitory effect of ANP on A-II-induced aldosterone synthesis at all potassium concentrations. Interactions between A-II, ANP, ouabain and potassium may be of physiological significance in the regulation of aldosterone secretion.

Agonist-induced down-regulation of type 1 and type 3 inositol 1,4,5-trisphosphate receptors in A7r5 and DDT1 MF-2 smooth muscle cells

Prolonged stimulation of rat A7r5 aortic smooth muscle cells with 3 microM vasopressin, or of hamster DDT1 MF-2 smooth muscle cells with 10 microM bradykinin or 100 microM histamine led within 4 h to a 40-50% down-regulation of the type 1 InsP3 receptor (InsP3R-1) and of the type 3 InsP3 receptor (InsP3R-3). InsP3R down-regulation was a cell- and agonist-specific process, since several other agonists acting on PLC-coupled receptors did not change the expression level of the InsP3R isoforms in these cell types and since no agonist-induced down-regulation of InsP3Rs was observed in HeLa cells. Down-regulation of InsP3Rs was prevented by an inhibitor of proteasomal protease activity, N-acetyl-Leu-Leu-norleucinal (ALLN). The Ca2+ channel blocker verapamil (2 microM) also induced InsP3R-1 down-regulation (43%) in A7r5 cells, which was inhibited by ALLN. In A7r5 cells transiently transfected with a cDNA construct, bearing a luciferase coding sequence under control of the rat InsP3R-1 promoter, reduced luciferase activity could be demonstrated upon stimulation of cells with vasopressin or verapamil. Thus, besides enhanced protein degradation, a reduction of InsP3R promoter activity might contribute to the down-regulation of InsP3Rs in A7r5 cells. We next investigated the effect of InsP3R down-regulation on Ca2+ responses in A7r5 cells. A rightward shift in the dose-response curve for InsP3-induced Ca2+ release was observed in permeabilized monolayers of vasopressin-pretreated A7r5 cells (EC50 630 nM and 400 nM for pretreated and non-pretreated cells, respectively). The Ca2+ responses to threshold doses of vasopressin were markedly reduced in intact vasopressin-pretreated cells. We conclude that prolonged agonist-exposure leads to down-regulation of InsP3Rs in A7r5 and DDT, MF-2 smooth muscle cells. The mechanism of down-regulation likely involves proteasomal degradation and reduction of InsP3R promoter activity. Moreover, down-regulation of InsP3Rs resulted in desensitization of Ca2+ release from InsP3 sensitive stores.

Calcium-containing organelles display unique reactivity to chemical stimulation in cultured hippocampal neurons

Cultured rat hippocampal neurons grown on glass coverslips for 1-3 weeks were loaded with the calcium-sensitive fluorescent dye Fluo-3 and viewed with a confocal laser scanning microscope. Large pyramidal-shaped neurons were found to contain dye-accumulating organelles in their somata, primarily around nuclei and near the base of their primary dendrites. These organelles varied in size and increased in density over weeks in culture, and were not colocalized with the endoplasmic reticulum or with mitochondria. The Fluo-3 fluorescence in these calcium-containing organelles (CCOs) was transiently quenched by exposure to Mn2+, indicating that the dye is a genuine [Ca2+] reporter and is not just a site of accumulating Fluo-3 dye. Recovery of fluorescence in the CCOs after washout of Mn2+ involved activation of a thapsigargin-sensitive process. CCOs responded to stimuli that evoke a rise of cytosolic [Ca2+] ([Ca]i) in a unique manner; perfusion of caffeine caused a prolonged rise of [Ca] in the CCOs ([Ca]C), whereas it caused only a transient rise of [Ca]i. Pulse application of caffeine also caused a faster effect on [Ca]C than on [Ca]i. Glutamate caused a transient rise of both [Ca]i and [Ca]C, followed by a prolonged fall of only [Ca]C to below rest level. This fall was blocked by preincubation with thapsigargin. Ryanodine blocked the cytosolic effects of caffeine but not its effect on [C]C. A clear distinction between CCOs and the known calcium stores was seen in digitonin-permeabilized cells; in these, remaining Fluo-3 reported changes in store calcium, i.e., caffeine caused a reduction in Fluo-3 fluorescence in permeabilized cells, whereas it still caused an increase in [Ca]C. A possible role of CCOs in regulation of release of calcium from ryanodine-sensitive stores was indicated by the observation that CCO-containing cells exhibited a larger and faster response to caffeine than cells that did not have them. We propose that CCOs constitute a unique functional compartment involved in release of calcium from calcium-sensitive stores.

The role of voltage-dependent calcium channels in angiotensin-stimulated glomerulosa cells

The concept that voltage-dependent Ca2+ influx is essential in the aldosterone stimulating action of angiotensin II (AII) has been recently challenged by the demonstration of the dihydropyridine (DHP) insensitive 'capacitative' Ca2+ uptake mechanism. The DHP-sensitivity of AII-induced aldosterone secretion is still to be explained. In rat glomerulosa cells the lag phase of AII-induced depolarization is more than 30 s, and there is no enhanced Ca2+ influx within the first min of stimulation. Yet we observed that DHPs as well as diltiazem influenced also the peak of cytoplasmic Ca2+ signal, although the peak (approximately 12 s) is attributed to Ca2+ release alone. Nifedipine reduced the Ca2+ transient induced by AII even after complete inhibition of Ca2+ channel activity. Recalling the loose attachment of InsP3 receptors (IP3R) to the plasma membrane, and the homology between the cytosolic domain of IP3R and the Ca2+ release channel (ryanodine receptor) of skeletal muscle, we proposed that DHP-sensitive L-type Ca2+ channels (DHP receptors) influence InsP3-induced Ca2+ release rather than Ca2+ influx in AII-stimulated cells. Although the dominant isoform is the neuroendocrine (D) one, the skeletal muscle isoform of L-type voltage-dependent Ca2+ channel is also expressed in rat glomerulosa cells. This isoform may be a candidate for protein-protein interaction between DHPR and subplasmalemmal IP3R, similarly to that occurring between DHP receptors and ryanodine receptors in skeletal muscle.