Angiotensin II-activated Ca2+ entry-induced release of Ca2+ from intracellular stores in rat portal vein myocytes

Characterization of a Family of Scorpion Toxins Modulating Ca2+-Activated Cl- Current in Vascular Myocytes

The pharmacology of calcium-activated chloride current is not well developed. Peptides from scorpion venom present potent pharmacological actions on ionic conductance used to characterize the function of channels but can also be helpful to develop organic pharmacological tools. Using electrophysiological recording coupled with calcium measurement, we tested the potent effect of peptides extracted from Leuirus quinquestratus quinquestratus venom on the calcium-activated chloride current expressed in smooth muscle cells freshly dissociated from rat portal veins. We identified one peptide which selectively inhibited the chloride conductance without effects on either calcium signaling or calcium and potassium currents expressed in this cell type. The synthetic peptide had the same affinity, but the chemical modification of the amino acid sequence altered the efficiency to inhibit the calcium-activated chloride conductance.

Pd(II)-catalyzed, controllable C-H mono-/diarylation of aryl tetrazoles: concise synthesis of Losartan

A palladium(II)-catalyzed C-H arylation directed by tetrazole, a metabolically stable surrogate for the carboxylic acid group in drug design, has been developed. Excellent mono-/di-selectivity was achieved through adjustment of the protecting site on the tetrazole ring. The synthetic utility of this new transformation was demonstrated in the concise total synthesis of Losartan.

Blood brain barrier precludes the cerebral arteries to intravenously-injected antisense oligonucleotide

Alternative splicing of the ryanodine receptor subtype 3 (RyR3) produces a short isoform (RyR3S) able to negatively regulate the ryanodine receptor subtype 2 (RyR2), as shown in cultured smooth muscle cells from mice. The RyR2 subtype has a crucial role in the control of vascular reactivity via the fine tuning of Ca(2+) signaling to regulate cerebral vascular tone. In this study, we have shown that the inhibition of RyR3S expression by a specific antisense oligonucleotide (asRyR3S) was able to increase the Ca(2+) signals implicating RyR2 in cerebral arteries ex vivo. Moreover, we tried to inhibit the expression of RyR3S in vivo. The asRyR3S was complexed with JetPEI and injected intravenously coupled with several methods known to induce a blood brain barrier disruption. We tested solutions to induce osmotic choc (mannitol), inflammation (bacteria lipopolysaccharide and pertussis toxin), vasoconstriction or dilatation (sumatriptan, phenylephrine, histamine), CD73 activation (NECA) and lipid instability (Tween80). All tested technics failed to target asRyR3 in the cerebral arteries wall, whereas the molecule was included in hepatocytes or cardiomyocytes. Our results showed that the RyR3 alternative splicing could have a function in cerebral arteries ex vivo; however, the disruption of the blood brain barrier could not induce the internalization of antisense oligonucleotides in the cerebral arteries, in order to prove the function of RYR3 short isoform in vivo.

Spaceflight regulates ryanodine receptor subtype 1 in portal vein myocytes in the opposite way of hypertension

Gravity has a structural role for living systems. Tissue development, architecture, and organization are modified when the gravity vector is changed. In particular, microgravity induces a redistribution of blood volume and thus pressure in the astronaut body, abolishing an upright blood pressure gradient, inducing orthostatic hypotension. The present study was designed to investigate whether isolated vascular smooth muscle cells are directly sensitive to altered gravitational forces and, second, whether sustained blood pressure changes act on the same molecular target. Exposure to microgravity during 8 days in the International Space Station induced the decrease of ryanodine receptor subtype 1 expression in primary cultured myocytes from rat hepatic portal vein. Identical results were found in portal vein from mice exposed to microgravity during an 8-day shuttle spaceflight. To evaluate the functional consequences of this physiological adaptation, we have compared evoked calcium signals obtained in myocytes from hindlimb unloaded rats, in which the shift of blood pressure mimics the one produced by the microgravity, with those obtained in myocytes from rats injected with antisense oligonucleotide directed against ryanodine receptor subtype 1. In both conditions, calcium signals implicating calcium-induced calcium release were significantly decreased. In contrast, in spontaneous hypertensive rat, an increase in ryanodine receptor subtype 1 expression was observed as well as the calcium-induced calcium release mechanism. Taken together, our results shown that myocytes were directly sensitive to gravity level and that they adapt their calcium signaling pathways to pressure by the regulation of the ryanodine receptor subtype 1 expression.

Acetylcholine evokes an InsP3R1-dependent transient Ca2+ signal in rat duodenum myocytes

In smooth muscle myocytes, agonist-activated release of calcium ions (Ca2+) stored in the sarcoplasmic reticulum (SR) occurs via different but overlapping transduction pathways. Hence, to fully study how SR Ca2+ channels are activated, the simultaneous activation of different Ca2+ signals should be separated. In rat duodenum myocytes, we have previously characterized that acetylcholine (ACh) induces Ca2+ oscillations by binding to its M2 muscarinic receptor and activating the ryanodine receptor subtype 2. Here, we show that ACh simultaneously evokes a Ca2+ signal dependent on activation of inositol 1,4,5-trisphosphate (InsP3) receptor subtype 1. A pharmacologic approach, the use of antisense oligonucleotides directed against InsP3R1, and the expression of a specific biosensor derived from green-fluorescent protein coupled to the pleckstrin homology domain of phospholipase C, suggested that the InsP3R1-dependent Ca2+ signal is transient and due to a transient synthesis of InsP3 via M3 muscarinic receptor. Moreover, we suggest that both M2 and M3 signalling pathways are modulating phosphatidylinositol 4,5-bisphosphate and InsP3 concentration, thus describing closely interacting pathways activated by ACh in duodenum myocytes.

Full length ryanodine receptor subtype 3 encodes spontaneous calcium oscillations in native duodenal smooth muscle cells

Two isoforms of the ryanodine receptor subtype 3 (RYR3) have been described in smooth muscle. The RYR3 short isoform (RYR3S) negatively regulates the calcium-induced calcium release mechanism encoded by the RYR2, whereas the role of the full length isoform of RYR3 (RYR3L) was still unclear. Here, we describe RYR-dependent spontaneous Ca(2+) oscillations measured in 10% of native duodenum myocytes. We investigated the role of RYR3 isoforms in these spontaneous Ca(2+) signals. Inhibition of RYR3S expression by antisense oligonucleotides revealed that both RYR2 and RYR3L were able to propagate spontaneous Ca(2+) waves that were distinguishable by frequency analysis. When RYR3L expression was inhibited, the spontaneous Ca(2+) oscillations were never observed, indicating that RYR3S inhibited the function of RYR2. RYR2 expression inhibition led to Ca(2+) oscillations identical to those observed in control cells suggesting that RYR3S did not functionally interact with RYR3L. The presence and frequency of RYR3L-dependent Ca(2+) oscillations were dependent on sarcoplasmic reticulum Ca(2+) content as revealed by long-term changes of the extracellular Ca(2+) concentration. Our study shows that, in native duodenal myocytes, the spontaneous Ca(2+) waves are encoded by the RYR3L alone, which activity is regulated by sarcoplasmic reticulum Ca(2+) loading.

Acetylcholine-induced Ca2+ oscillations are modulated by a Ca2+ regulation of InsP3R2 in rat portal vein myocytes

Oscillations of cytosolic Ca2+ levels are believed to have important roles in various metabolic and signalling processes in many cell types. Previously, we have demonstrated that acetylcholine (ACh) evokes Ca2+ oscillations in vascular myocytes expressing InsP3R1 and InsP3R2, whereas transient responses are activated in vascular myocytes expressing InsP3R1 alone. The molecular mechanisms underlying oscillations remain to be described in these native smooth muscle cells. Two major hypotheses are proposed to explain this crucial signalling activity: (1) Ca2+ oscillations are activated by InsP3 oscillations; and (2) Ca2+ oscillations depend on the regulation of the InsP3R by both InsP3 and Ca2+. In the present study, we used a fluorescent InsP3 biosensor and revealed that ACh induced a transient InsP3 production in all myocytes. Moreover, steady concentrations of 3F-InsP3, a poorly hydrolysable analogue of InsP3, and pharmacological activation of PLC evoked Ca2+ oscillations. Increasing cytosolic Ca2+ inhibited the ACh-induced calcium oscillations but not the transient responses and strongly reduced the 3F-InsP3-evoked Ca2+ response in oscillating cells but not in non-oscillating cells. These results suggest that, in native vascular myocytes, ACh-induced InsP3 production is transient and Ca2+ oscillations depend on a Ca2+ modulation of InsP3R2.

Angiotensin II-induced delayed stimulation of phospholipase C gamma1 requires activation of both phosphatidylinositol 3-kinase gamma and tyrosine kinase in vascular myocytes

In vascular smooth muscles, angiotensin II (AII) has been reported to activate phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI3K). We investigated the time-dependent effects of AII on both phosphatidylinositol 3,4,5-trisphosphate (PtdInsP3) and inositol phosphates (InsPs) accumulation in permeabilized microsomes from rat portal vein smooth muscle in comparison with those of noradrenaline (NA). AII stimulated an early production of PtdInsP3 (within 30 s) followed by a delayed production of InsPs (within 3-5 min), in contrast to NA which activated only a fast production of InsPs. The use of pharmacological inhibitors and antibodies raised against the PI3K and PLC isoforms expressed in portal vein smooth muscle showed that AII specifically activated PI3Kδ and that this isoform was involved in the AII-induced stimulation of InsPs accumulation. NA-induced InsPs accumulation depended on PLCβ1 activation whereas AII-induced InsPs accumulation depended on PLCγ1 activation. AII-induced PLCδ1 activation required both tyrosine kinase and PI3Kδ since genistein and tyrphostin B48 (inhibitors of tyrosine kinase), LY294002 and wortmannin (inhibitors of PI3K) and anti-PI3Kδ antibody abolished AII-induced stimulation of InsPs accumulation. Increased tyrosine phosphorylation of PLCβ1 was only detected for long-lasting applications of AII and was suppressed by genistein. These data indicate that activation of both PI3Kβ and tyrosine kinase is a prerequisite for AII-induced stimulation of PLCβ1 in vascular smooth muscle and suggest that the sequential activation of the three enzymes may be responsible for the slow and long-lasting contraction induced by AII.

Modulation of calcium signalling by dominant negative splice variant of ryanodine receptor subtype 3 in native smooth muscle cells

The ryanodine receptor subtype 3 (RYR3) is expressed ubiquitously but its physiological function varies from cell to cell. Here, we investigated the role of a dominant negative RYR3 isoform in Ca2+ signalling in native smooth muscle cells. We used intranuclear injection of antisense oligonucleotides to specifically inhibit endogenous RYR3 isoform expression. In mouse duodenum myocytes expressing RYR2 subtype and both spliced and non-spliced RYR3 isoforms, RYR2 and non-spliced RYR3 were activated by caffeine whereas the spliced RYR3 was not. Only RYR2 was responsible for the Ca2+-induced Ca2+ release mechanism that amplified Ca2+ influx- or inositol 1,4,5-trisphosphate-induced Ca2+ signals. However, the spliced RYR3 negatively regulated RYR2 leading to the decrease of amplitude and upstroke velocity of Ca2+ signals. Immunostaining in injected cells showed that the spliced RYR3 was principally expressed near the plasma membrane whilst the non-spliced isoform was revealed around the nucleus. This study shows for the first time that the short isoform of RYR3 controls Ca2+ release through RYR2 in native smooth muscle cells.

Ryanodine receptor subtype 2 encodes Ca2+ oscillations activated by acetylcholine via the M2 muscarinic receptor/cADP-ribose signalling pathway in duodenum myocytes

In this study, we characterized the signalling pathway activated by acetylcholine that encodes Ca2+ oscillations in rat duodenum myocytes. These oscillations were observed in intact myocytes after removal of external Ca2+, in permeabilized cells after abolition of the membrane potential and in the presence of heparin (an inhibitor of inositol 1,4,5-trisphosphate receptors) but were inhibited by ryanodine, indicating that they are dependent on Ca2+ release from intracellular stores through ryanodine receptors. Ca2+ oscillations were selectively inhibited by methoctramine (a M2 muscarinic receptor antagonist). The M2 muscarinic receptor-activated Ca2+ oscillations were inhibited by 8-bromo cyclic adenosine diphosphoribose and inhibitors of adenosine diphosphoribosyl cyclase (ZnCl2 and anti-CD38 antibody). Stimulation of ADP-ribosyl cyclase activity by acetylcholine was evaluated in permeabilized cells by measuring the production of cyclic guanosine diphosphoribose (a fluorescent compound), which resulted from the cyclization of nicotinamide guanine dinucleotide. As duodenum myocytes expressed the three subtypes of ryanodine receptors, an antisense strategy revealed that the ryanodine receptor subtype 2 alone was required to initiate the Ca2+ oscillations induced by acetylcholine and also by cyclic adenosine diphosphoribose and rapamycin (a compound that induced uncoupling between 12/12.6 kDa FK506-binding proteins and ryanodine receptors). Inhibition of cyclic adenosine diphosphoribose-induced Ca2+ oscillations, after rapamycin treatment, confirmed that both compounds interacted with the ryanodine receptor subtype 2. Our findings show for the first time that the M2 muscarinic receptor activation triggered Ca2+ oscillations in duodenum myocytes by activation of the cyclic adenosine diphosphoribose/FK506-binding protein/ryanodine receptor subtype 2 signalling pathway.

NFATc3 Regulates Kv2.1 Expression in Arterial Smooth Muscle

Voltage-gated K+ (Kv) channels control the excitability of arterial smooth muscle. However, the molecular mechanisms regulating Kv channel function in smooth muscle remain unclear. We examined the hypothesis that the vasoactive peptide angiotensin II (Ang II) regulates arterial smooth muscle Kv channel function via calcineurin-dependent activation of the transcription factor NFAT. We found that sustained administration of Ang II decreased Kv currents (IKv) by reducing the expression of Kv2.1 K+ channel subunits. This effect of Ang II was independent of pressure but required Ca2+ influx through L-type Ca2+ channels. Consistent with our hypothesis, we found that calcineurin and NFATc3 are obligatory components of the signaling cascade mediating reduced IKv by Ang II. We conclude that sustained Ang II exposure increases smooth muscle Ca2+, which leads to activation of calcineurin and NFATc3, culminating in decreased Kv2.1 expression and reduced IKv function. These results support the novel concept that NFATc3 controls the excitability of arterial smooth muscle by regulating Kv2.1 expression.

Crucial role of type 2 inositol 1,4,5-trisphosphate receptors for acetylcholine-induced Ca2+ oscillations in vascular myocytes

OBJECTIVE

The aim of this study was to correlate the expression of InsP3R subtypes in native vascular and visceral myocytes with specific Ca2+-signaling patterns.

METHODS AND RESULTS

By Western blot and immunostaining, we showed that rat portal vein expressed InsP3R1 and InsP3R2 but not InsP3R3, whereas rat ureter expressed InsP3R1 and InsP3R3 but not InsP3R2. Acetylcholine induced single Ca2+ responses in all ureteric myocytes but only in 50% of vascular myocytes. In the remaining vascular myocytes, the first transient peak was followed by Ca2+ oscillations. By correlating Ca2+ signals and immunostaining, we revealed that oscillating vascular cells expressed both InsP3R1 and InsP3R2 whereas nonoscillating vascular cells expressed only InsP3R1. Acetylcholine-induced oscillations were not affected by inhibitors of ryanodine receptors, Ca2+-ATPases, Ca2+ influx, and mitochondrial Ca2+ uniporter but were inhibited by intracellular infusion of heparin. Using specific antibodies against InsP3R subtypes, we showed that acetylcholine-induced Ca2+ oscillations were specifically blocked by the anti-InsP3R antibody. These data were supported by antisense oligonucleotides targeting InsP3R2, which selectively inhibited Ca2+ oscillations.

CONCLUSIONS

Our results suggest that in native smooth muscle cells, a differential expression of InsP3R subtypes encodes specific InsP3-mediated Ca2+ responses and that the presence of the InsP3R2 subtype is required for acetylcholine-induced Ca2+ oscillations in vascular myocytes.

Stimulation of L-type Ca2+ channels by inositol pentakis- and hexakisphosphates in rat vascular smooth muscle cells

The electrophysiological effects of D-myo-inositol 1,3,4,5,6-pentakisphosphate (InsP5) and D-myo-inositol hexakisphosphate (InsP6), which represent the main cellular inositol polyphosphates, were studied on L-type Ca2+ channels in single myocytes of rat portal vein. Intracellular infusion of InsP5 (up to 50 micro M) or 10 micro M InsP6 had no action on Ba2+ current, whereas 50 micro M InsP6 or 10 micro M InsP5 plus 10 micro M InsP6 (InsP5,6) stimulated the inward current. The stimulatory effect of InsP5,6 was also obtained in external Ca2+-containing solution. The stimulated Ba2+ current retained the properties of L-type Ba2+ current and was oxodipine sensitive. PKC inhibitors Ro 32-0432 (up to 500 nM), GF109203X (5 micro M) or calphostin C (100 nM) abolished the InsP5,6-induced stimulation. Neither the PKA inhibitor H89 (1 micro M) nor the protein phosphatase inhibitors okadaic acid (500 nM) or cypermethrin (1 micro M) prevented or mimicked the InsP5,6-induced stimulation of Ba2+ current. However, InsP5 or InsP6 could mimic some effects of protein phosphatase inhibitor so as to extend after washing-out forskolin the stimulatory effects of the adenylyl cyclase activator on Ba2+ current. These results indicate that InsP5 and InsP6 may act as intracellular messengers in modulating L-type Ca2+ channel activity and so could be implicated in mediator-induced contractions of vascular smooth muscle cells.

Mechanism of action of angiotensin II in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were mounted in a myograph and isometric tension measured. In some experiments, intracellular calcium [Ca(2+)]i was also measured using fura-2. 2. Angiotensin II (100 pM - 1 microM) increased [Ca(2+)]i and tone in a concentration-dependent manner. The effects of angiotensin II (100 nM) were inhibited by an AT1-receptor antagonist, candesartan (100 pM). 3. Ryanodine (10 microM), had no effect on angiotensin II-induced responses, but removal of extracellular Ca(2+) abolished angiotensin II-induced rise in [Ca(2+)]i and tone. Inhibition of Ca(2+) entry by Ni(2+) (2 mM), also inhibited angiotensin II responses. The dihydropyridine, L-type calcium channel antagonist, amlodipine (10 microM), only partially attenuated angiotensin II responses. 4. Inhibition of protein kinase C (PKC) by chelerythrine (1 microM), or by overnight exposure to a phorbol ester (PDBu; 500 nM) had no effect on angiotensin II-induced contraction. 5. Genistein (10 microM), a tyrosine kinase inhibitor, inhibited angiotensin II-induced contraction, but did not inhibit the rise in [Ca(2+)]i, suggesting that at this concentration it affected the calcium sensitivity of the contractile apparatus. Genistein did not affect responses to norepinephrine (NE) or high potassium (KPSS). 6. A selective MEK inhibitor, PD98059 (30 microM), inhibited both the angiotensin II-induced contraction and rise in [Ca(2+)]i, but had no effect on responses to NE or KPSS. 7. AT1 activation causes Ca(2+) influx via L-type calcium channels and a dihydropyridine-insensitive route, but does not release Ca(2+) from intracellular sites. Activation of tyrosine kinase(s) and the ERK 1/2 pathway, but not classical or novel PKC, also play a role in angiotensin II-induced contraction in human subcutaneous resistance arteries.

Phosphoinositide 3-kinase gamma mediates angiotensin II-induced stimulation of L-type calcium channels in vascular myocytes

Previous results have shown that in rat portal vein myocytes the betagamma dimer of the G(13) protein transduces the angiotensin II-induced stimulation of calcium channels and increase in intracellular Ca(2+) concentration through activation of phosphoinositide 3-kinase (PI3K). In the present work we determined which class I PI3K isoforms were involved in this regulation. Western blot analysis indicated that rat portal vein myocytes expressed only PI3Kalpha and PI3Kgamma and no other class I PI3K isoforms. In the intracellular presence of an anti-p110gamma antibody infused by the patch clamp pipette, both angiotensin II- and Gbetagamma-mediated stimulation of Ca(2+) channel current were inhibited, whereas intracellular application of an anti-p110alpha antibody had no effect. The anti-PI3Kgamma antibody also inhibited the angiotensin II- and Gbetagamma-induced production of phosphatidylinositol 3,4,5-trisphosphate. In Indo-1 loaded cells, the angiotensin II-induced increase in [Ca(2+)](i) was inhibited by intracellular application of the anti-PI3Kgamma antibody, whereas the anti-PI3Kalpha antibody had no effect. The specificity of the anti-PI3Kgamma antibody used in functional experiments was ascertained by showing that this antibody did not recognize recombinant PI3Kalpha in Western blot experiments. Moreover, anti-PI3Kgamma antibody inhibited the stimulatory effect of intracellularly infused recombinant PI3Kgamma on Ca(2+) channel current without altering the effect of recombinant PI3Kalpha. Our results show that, although both PI3Kgamma and PI3Kalpha are expressed in vascular myocytes, the angiotensin II-induced stimulation of vascular L-type calcium channel and increase of [Ca(2+)](i) involves only the PI3Kgamma isoform.

Contribution of ryanodine receptor subtype 3 to ca2+ responses in Ca2+-overloaded cultured rat portal vein myocytes

Using an antisense strategy, we have previously shown that in vascular myocytes, subtypes 1 and 2 of ryanodine receptors (RYRs) are required for Ca(2+) release during Ca(2+) sparks and global Ca(2+) responses, evoked by activation of voltage-gated Ca(2+) channels, whereas RYR subtype 3 (RYR3) has no contribution. Here, we investigated the effects of increased Ca(2+) loading of the sarcoplasmic reticulum (SR) on the RYR-mediated Ca(2+) responses and the role of the RYR3 by injecting antisense oligonucleotides targeting the RYR subtypes. RYR3 expression was demonstrated by immunodetection in both freshly dissociated and cultured rat portal vein myocytes. Confocal Ca(2+) measurements revealed that the number of cells showing spontaneous Ca(2+) sparks was strongly increased by superfusing the vascular myocytes in 10 mm Ca(2+)-containing solution. These Ca(2+) sparks were blocked after inhibition of RYR1 or RYR2 by treatment with antisense oligolucleotides but not after inhibition of RYR3. In contrast, inhibition of RYR3 reduced the global Ca(2+) responses induced by caffeine and phenylephrine, indicating that RYR3 participated together with RYR1 and RYR2 to these Ca(2+) responses in Ca(2+)-overloaded myocytes. Ca(2+) transients evoked by photolysis of caged Ca(2+) with increasing flash intensities were also reduced after inhibition of RYR3 and revealed that the [Ca(2+)](i) sensitivity of RYR3 would be similar to that of RYR1 and RYR2. Our results show that, under conditions of increased SR Ca(2+) loading, the RYR3 becomes activable by caffeine and local increases in [Ca(2+)](i).

Lack of involvement of endothelin-1 in angiotensin II-induced contraction of the isolated rat tail artery

1. The contribution of endothelin-1 (ET-1) to angiotensin II (Ang II)-mediated contraction of the isolated rat tail artery was assessed with measurements of tension, and cytosolic calcium ([Ca(2+)](i)). The distribution of the AT(1) receptor was studied with RT - PCR and immunohistochemistry. 2. Ang II induced an endothelium-independent contraction (pEC(50) 7.95+/-0.06 and E(max): 0.46 g+/-0.05 with endothelium vs 7.81+/-0.02 and 0.41 g+/-0.07 without endothelium; P>0.05). Ang II (0.003 - 0.3 microM)-induced a non-sustained contraction of endothelium-intact preparations which was not antagonized by BQ-123 (1 microM), but was inhibited by losartan (10 nM). In addition, the maximal contraction induced by ET-1 (0.1 microM) could be further increased by the addition of 0.1 microM Ang II. 3. Ang II (0.001 - 0.3 microM) elevated [Ca(2+)](i) in single vascular smooth muscle cells (VSMCs) in a dose-dependent manner (pEC(50) 9.12+/-0.26) and the Ang II-induced increases in [Ca(2+)](i) were not affected by a Ca(2+)-free solution, but were abolished by pretreatment with caffeine (5 mM). Ang II did not increase [Ca(2+)](i) in endothelial cells. ET-1 (0.1 microM) increased [Ca(2+)](i) in single VSMCs in a normal Ca(2+) containing physiological saline solution (PSS), but not in a Ca(2+)-free solution. 4. Ang II-induced contraction was insensitive to inhibition by nifedipine (0.1 microM), an antagonist of L-type voltage-gated Ca(2+) channels, and SK&F96365 (10 microM), which blocks non-selective cation channels, whereas that to ET-1 was inhibited by SK&F69365. 5. RT - PCR data indicate the expression of AT(1A) and AT(1B) on both VSMCs and endothelial cells, but immunohistochemical evidence illustrates that the AT(1) is located primarily on VSMCs. 6. These results indicate that endothelium-derived ET-1 is not involved in the Ang II-mediated vasoconstriction of the rat tail artery and that Ang II- and ET-1-mediated VSM contractions utilize distinct pathways.

Ca(2+) signals mediated by Ins(1,4,5)P(3)-gated channels in rat ureteric myocytes

Localized Ca(2+)-release signals (puffs) and propagated Ca(2+) waves were characterized in rat ureteric myocytes by confocal microscopy. Ca(2+) puffs were evoked by photorelease of low concentrations of Ins(1,4,5)P(3) from a caged precursor and by low concentrations of acetylcholine; they were also observed spontaneously in Ca(2+)-overloaded myocytes. Ca(2+) puffs showed some variability in amplitude, time course and spatial spread, suggesting that Ins(1,4,5)P(3)-gated channels exist in clusters containing variable numbers of channels and that within these clusters a variable number of channels can be recruited. Immunodetection of Ins(1,4,5)P(3) receptors revealed the existence of several spots of fluorescence in the confocal cell sections, supporting the existence of clusters of Ins(1,4,5)P(3) receptors. Strong Ins(1,4,5)P(3) photorelease and high concentrations of acetylcholine induced Ca(2+) waves that originated from an initiation site and propagated in the whole cell by spatial recruitment of neighbouring Ca(2+)-release sites. Both Ca(2+) puffs and Ca(2+) waves were blocked selectively by intracellular applications of heparin and an anti-Ins(1,4,5)P(3)-receptor antibody, but were unaffected by ryanodine and intracellular application of an anti-ryanodine receptor antibody. mRNAs encoding for the three subtypes of Ins(1,4,5)P(3) receptor and subtype 3 of ryanodine receptor were detected in these myocytes, and the maximal binding capacity of [(3)H]Ins(1,4,5)P(3) was 10- to 12-fold higher than that of [(3)H]ryanodine. These results suggest that Ins(1,4,5)P(3)-gated channels mediate a continuum of Ca(2+) signalling in smooth-muscle cells expressing a high level of Ins(1,4,5)P(3) receptors and no subtypes 1 and 2 of ryanodine receptors.

Store-operated Ca(2+) channels in human glomerular mesangial cells

Experiments were performed to identify the biophysical properties of store-operated Ca(2+) channels (SOC) in cultured human glomerular mesangial cells (MC). A fluorometric technique (fura 2) was utilized to monitor the change in intracellular calcium concentration ([Ca(2+)](i)) evoked by elevating external [Ca(2+)] from 10 nM to 1 mM (Delta[Ca(2+)]). Under control conditions, Delta[Ca(2+)] averaged 6 nM and was unaffected by elevating bath [K(+)]. After treatment with 1 microM thapsigargin to deplete the intracellular Ca(2+) store, the change in [Ca(2+)](i) (Delta[Ca(2+)](th)) averaged 147 +/- 16 nM. In thapsigargin-treated MC studied under depolarizing conditions (75 mM bath K(+)), Delta[Ca(2+)](th) was 45 +/- 7 nM. The Delta[Ca(2+)](th) response of thapsigargin-treated cells was inhibited by La(3+) (IC(50) = 335 nM) but was unaffected by 5 microM Cd(2+). In patch clamp studies, inward currents were observed in cell-attached patches with either 90 mM Ba(2+) or Ca(2+) in the pipette and 140 mM KCl in the bathing solution. The single-channel conductance was 2.1 pS with Ba(2+) and 0.7 pS with Ca(2+). The estimated selectivities were Ca(2+) > Ba(2+) >> K(+). These channels were sensitive to 2 microM La(3+), insensitive to 5 microM Cd(2+), and voltage independent, with an average channel activity (NP(o)) of 1.02 at command potential (-V(p)) ranging from 0 to -80 mV. In summary, MC exhibited an electrogenic Ca(2+) influx pathway that is suggestive of Ca(2+) entry through SOC, as well as a small-conductance divalent-selective channel displaying biophysical properties consistent with SOC. Based on estimates of whole cell Ca(2+) influx derived from our data, we conclude that SOC with low single-channel conductance must be highly abundant in MC to allow significant capacitative Ca(2+) entry in response to depletion of the intracellular store.

Requirement of ryanodine receptor subtypes 1 and 2 for Ca(2+)-induced Ca(2+) release in vascular myocytes

While the roles of subtypes 1 and 2 of the ryanodine receptors (RYRs) have been studied in cellular systems expressing specifically one or the other of these subtypes (i.e. skeletal and cardiac muscle), the function of these receptors has not been evaluated in smooth muscles. We have previously reported RYR-mediated elementary (Ca(2+) sparks) and global Ca(2+) responses in rat portal vein myocytes. Here, we investigated the respective roles of all three RYR subtypes expressed in these cells as revealed by reverse transcriptase-polymerase chain reaction. Antisense oligonucleotides targeting each one of the three RYR subtypes were shown to specifically inhibit the expression of the corresponding mRNA and protein without affecting the other RYR subtypes. Confocal Ca(2+) measurements revealed that depolarization-induced Ca(2+) sparks and global Ca(2+) responses were blocked when either RYR1 or RYR2 expression was suppressed. Caffeine-induced Ca(2+) responses were partly inhibited by the same antisense oligonucleotides. Neither the corresponding scrambled oligonucleotides nor the antisense oligonucleotides targeting RYR3 affected depolarization- or caffeine-induced Ca(2+) responses. Our results show that, in vascular myocytes, the two RYR1 and RYR2 subtypes are required for Ca(2+) release during Ca(2+) sparks and global Ca(2+) responses, evoked by activation of voltage-gated Ca(2+) channels.

Angiotensin II type 1 receptors stimulate protein synthesis in human cardiac fibroblasts via a Ca2+-sensitive PKC-dependent tyrosine kinase pathway

The aim of the present study was to investigate the proliferative effects of Ang II in human cardiac fibroblasts. The effects of Ang II in human cardiac fibroblasts on the 3H-thymidine incorporation, the cell number, the 3H-leucine incorporation and the total protein content were measured. The expression of receptor mRNA was performed by reverse transcription-polymerase chain reaction (RT-PCR). Ang II increased 3H-leucine incorporation in a concentration-dependent manner but not 3H-thymidine incorporation in primary cultures of human cardiac fibroblasts. The maximum effect (24 +/- 3% over control) was obtained at a concentration of 10 nM. There were no significant alterations of cell number or total protein content, suggesting that Ang II stimulated protein synthesis but did not induce hypertrophy. The accumulation of 3H-leucine was blocked by the AT1 receptor antagonist candesartan but not by the AT2 receptor antagonist PD123319. By using RT-PCR, both AT1 and AT2 receptors mRNA were found to be expressed in human cardiac fibroblasts. The selective MAPKK inhibitor PD098059, the protein kinase C inhibitor K252a or the phospholipase C inhibitor U73122 did not significantly inhibit Ang II augmented 3H-leucine incorporation. However, this was significantly blocked by the Ca2+-dependent protein kinase C inhibitor GO6976, the non-selective protein kinase inhibitor staurosporine and the tyrosine kinase inhibitor tyrphostin 25. The effects of Ang II were unaffected by the Gi-protein blocker pertussis toxin, indicating a Gi-protein-independent pathway. Ang II was synergistic with insulin but showed no significant increase on 3H-leucine incorporation when combined with PDGF or EGF. In summary, Ang II stimulates protein synthesis through AT1 receptors in human cardiac fibroblasts, but has no hypertrophic or hyperplastic effect. The response is mediated by a MAPKK-independent and Ca2+-sensitive PKC-dependent pathway.

5-hydroxytryptamine induces transient Ca2+ influx through Ni2+-insensitive Ca2+ channels in rat vascular smooth muscle cells

The effects of Ni2+, a non-selective cation channel inhibitor, on 5-hydroxytryptamine (5-HT)- and angiotensin II (Ang II)-induced intracellular Ca2+ dynamics in rat aortic smooth muscle cells were investigated. Ni2+ (1 mM) significantly inhibited the transient increase in intracellular Ca2+ concentration ([Ca2+]i) induced by Ang II (100 nM) in aortic smooth muscle cells, as measured using fura-2. However, Ni2+ did not suppress the transient increase in Ca2+ influx induced by 5-HT (10 microM), while significantly suppressed the sustained increase. Ca2+ influx evoked by high KCl (80 mM), thapsigargin (TG) (1 microM) or depletion of intracellular Ca2+ store was almost completely suppressed by Ni2+. Ni2+ had no effect on 5-HT-induced inositol triphosphate production and Ca2+ release from the intracellular store(s). These results suggest that 5-HT, but not Ang II, induces transient Ca2+ influx through Ni2+-insensitive Ca2+ channels, which are distinguishable from the voltage-dependent or store-operated Ca2+ channels.

Effects of hindlimb suspension on cytosolic Ca2+ and [3H]ryanodine binding in cardiac myocytes

Effects of a 14-day hindlimb suspension were examined on [3H]ryanodine binding to rat ventricular microsomes and on cytosolic Ca2+ concentration ([Ca2+]i) and voltage-dependent Ca2+ channels in isolated ventricular myocytes. In suspended rats, the amplitude of the twitch [Ca2+]i transient was increased without significant modifications of the basal [Ca2+]i and sarcoplasmic reticulum content. Because cell capacitance, L-type Ca2+-current density, and Ca2+-channel gating were not significantly modified after suspension, the increase in [Ca2+]i was expected to reside in a change in ryanodine receptors. Scatchard analysis of [3H]ryanodine binding revealed that suspension enhanced binding by increasing the affinity of the receptors for [3H]ryanodine without affecting the maximal binding capacity. Both Ca2+-release channel activity and [3H]ryanodine binding are modulated by Ca2+. However, the Ca2+ sensitivity of [3H]ryanodine binding remained unchanged after suspension. Taken together, these results suggest that the increase in twitch [Ca2+]i transients after suspension may result from a change in the intrinsic properties of the ryanodine receptors but not from a change in the expression level of these receptors.

Intracellular and extracellular blood magnesium fractions in hemodialysis patients; is the ionized fraction a measure of magnesium excess?

To establish the best measure for determining magnesium overload, we measured ionized and total magnesium in serum and mononuclear blood cells and total magnesium in erythrocytes in blood of 23 hemodialysis patients, known for their disturbed magnesium homeostasis. When comparing the mean magnesium values obtained in the patient population with those of a control population, all of these magnesium markers, including the biologically active fractions, were significantly (P <0.05) increased. Because serum total magnesium was not increased in all dialysis patients studied, the population was divided into two groups, according to total serum magnesium >1.0 mmol/L or less than that. Results in these two populations showed that ionized serum magnesium and ionized magnesium in mononuclear blood cells might give a better indication about the magnesium status of the tested patients than the currently used total serum magnesium data. However, neither of the two markers, especially ionized serum magnesium, was able to discriminate fully between normal magnesium homeostasis and magnesium excess. We therefore conclude that the two ionized magnesium markers offer minimal advantage for this discrimination, and that the total magnesium concentration in serum remains the measurement of choice.

Role of calcium influx and intracellular calcium stores in angiotensin II-mediated calcium hyper-responsiveness in smooth muscle from spontaneously hypertensive rats

OBJECTIVE

To investigate post-receptor mechanisms that underlie enhanced angiotensin II (Ang II)-stimulated cytosolic free Ca2+ concentration ([Ca2+]i) responses in vascular smooth muscle cells from small arteries of SHR.

METHODS

To determine whether Ca2+ influx is altered in SHR, effects of Ca2+ channel antagonists (nitrendipine and diltiazem) and depletion of extracellular Ca2+ on Ang II-stimulated [Ca2+]i responses in primary cultured unpassaged vascular smooth muscle cells from mesenteric arteries of spontaneously hypertensive rats (SHR), Wistar and Wistar-Kyoto (WKY) rats aged 17 weeks were studied. To assess whether Ca2+ stores contribute to increases in Ang II-stimulated Ca2+ mobilization and [Ca2+]i in SHR, cells were exposed to thapsigargin, a selective reticular Ca2+-ATPase inhibitor. [Ca2+]i was measured by fura-2 methodology.

RESULTS

Basal and 1 nmol/l Ang II-stimulated [Ca2+]i were significantly greater in SHR cells (123 +/- 7.1 nmol/l basal; 268 +/- 7.0 nmol/l stimulated) than they were in those from WKY rats (88 +/- 4.8 nmol/l basal; 221 +/- 8.6 nmol/l stimulated) and Wistar rats (85 +/- 3.0 nmol/l basal; 216 +/- 8.3 nmol/l stimulated). In Ca2+-free medium, basal and Ang II-induced [Ca2+]i were reduced in all groups, but Ang II-stimulated [Ca2+]i responses were still significantly enhanced in SHR cells compared with those in Wistar and WKY rat cells (205 +/- 11.2 versus 173 +/- 8.0 and 161 +/- 2.6 nmol/l, respectively). Administrations of 10(-6) mol/l diltiazem and 10(-7) mol/l nitrendipine decreased Ang II-elicited [Ca2+]i responses and normalized basal [Ca2+]i in SHR cells. The inhibition induced by Ca2+ channel antagonists was greater (P < 0.05) in WKY and Wistar rat cells than it was in those from SHR. Administration of thapsigargin, in Ca2+-free buffer, induced a greater (P < 0.05) dose-dependent [Ca2+]i increase in SHR cells than it did in WKY rat cells. Administration of 1 nmol/l Ang II increased [Ca2+]i in thapsigargin-pretreated cells of SHR but not in those of WKY rats.

CONCLUSION

Different mechanisms contribute to increases in basal and Ang II-stimulated [Ca2+]i responses in vascular smooth muscle cells from small arteries of SHR, which contribute to elevated peripheral resistance in hypertension. Increases in basal [Ca2+]i may be partly due to augmentation of Ca2+ influx, whereas Ang II-induced [Ca2+]i hyper-responsiveness might depend primarily on Ca2+ mobilization rather than on influx of extracellular Ca2+.

Effect of a 14-day hindlimb suspension on cytosolic Ca2+ concentration in rat portal vein myocytes

Effects of a 14-day hindlimb suspension were examined on increases in cytosolic Ca2+ concentration ([Ca2+]i) evoked by vasoactive compounds and on Ca2+ channels in rat portal vein myocytes. The maximal increases in [Ca2+]i elicited by caffeine, norepinephrine, and angiotensin II were reduced by 30-50% in suspended rats, and complete recovery was obtained 4 days after suspension removal. In contrast, voltage-gated Ca2+ channels were unaffected by hindlimb suspension. Using both confocal microscopy and the patch-clamp technique, we measured local increases in [Ca2+]i which corresponded to activation of a small number of ryanodine-sensitive Ca(2+)-release channels (Ca2+ sparks) and D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-gated Ca2+ channels. After hindlimb suspension, these local Ca2+ events, as well as the Ca2+ sensitivity of ryanodine-sensitive Ca2+ release channels, remained unchanged. In contrast, the propagated Ca2+ responses (Ca2+ waves) were significantly reduced in parallel with a noticeable inhibition of [3H]ryanodine binding to vascular membranes. Taken together, these results suggest that inhibition of the vasoconstrictor-induced increases in [Ca2+]i during long-term suspension may be related to a reduction of the number of functional ryanodine-sensitive and Ins(1,4,5)P3-gated channels in the sarcoplasmic reticulum of rat portal vein myocytes.

A betagamma dimer derived from G13 transduces the angiotensin AT1 receptor signal to stimulation of Ca2+ channels in rat portal vein myocytes

A G protein composed of alpha13, beta1, and gamma3 subunits selectively couples the angiotensin AT1A receptors to increase cytoplasmic Ca2+ concentration ([Ca2+]i) in rat portal vein myocytes (Macrez-Leprêtre, N., Kalkbrenner, F., Morel, J. L., Schultz, G., and Mironneau, J. (1997) J. Biol. Chem. 272, 10095-10102). We show here that Gbetagamma transduces the signal leading to stimulation of L-type Ca2+ channels. Intracellular dialysis through the patch pipette of a carboxyl-terminal anti-betacom antibody and a peptide corresponding to the Gbetagamma binding region of the beta-adrenergic receptor kinase 1 inhibited the stimulation of Ca2+ channels and the increase in [Ca2+]i evoked by angiotensin II. The Gbetagamma binding peptide did not prevent the dissociation of the heterotrimeric G protein into its subunits, as it did not block activation of phospholipase C-beta by Galphaq in response to stimulation of alpha1-adrenoreceptors. Transient overexpression of the beta-adrenergic receptor kinase 1 fragment and of Galpha subunits also inhibited the angiotensin II-induced increase in [Ca2+]i. Both anti-alpha13 antibody and carboxyl-terminal alpha13 peptide abrogated the angiotensin II-induced stimulation of Ca2+ channels. We conclude that activation of angiotensin AT1 receptors requires all three alpha, beta, and gamma subunits of G13 for receptor-G protein interaction, whereas the transduction of the signal to L-type Ca2+ channels is mediated by Gbetagamma.

Purification of a new dimeric protein from Cliona vastifica sponge, which specifically blocks a non-L-type calcium channel in mouse duodenal myocytes

Marine sponges are synthesizing a wide variety of peptidic and organic molecules with biological activities. Multiple-step purification of Cliona vastifica extract led to a new dimeric peptide (mapacalcine; M(r) = 19,064) that is composed of two homologous chains, each containing nine cysteins. This protein has been found to selectively block a new calcium conductance characterized in mouse duodenal myocytes with an IC50 value of approximately 0.2 microM. The mapacalcine-sensitive current was a non-L-type calcium current activated from a holding potential of -80 mV that persisted during stimulation of the cell at high frequencies (0.1-0.2 Hz) within 5-10 min. Time constants of inactivation were similar for both L-type and non-L-type calcium currents. The non-L-type calcium current of duodenal myocytes was not blocked by the pharmacological agents specific for N-, L-, P-, or Q-type calcium channels. Mapacalcine was unable to block T-type calcium current in portal vein myocytes as well as voltage-dependent potassium currents and calcium-activated chloride currents in duodenal and portal vein cells. Mapacalcine did not affect caffeine-induced calcium responses, indicating that it did not interfere with intracellular calcium stores. Competition experiments on mouse intestinal membranes showed that mapacalcine did not interact with dihydropyridines receptors. These data suggest that mapacalcine may be a specific inhibitor of a new type of calcium current, first identified in duodenal myocytes.

G protein heterotrimer Galpha13beta1gamma3 couples the angiotensin AT1A receptor to increases in cytoplasmic Ca2+ in rat portal vein myocytes

The subunit composition of angiotensin AT1 receptor-activated G protein was identified by using antisense oligonucleotide injection into the nucleus of rat portal vein myocytes. In these cells, we have previously shown that increases in the cytoplasmic calcium concentration ([Ca2+]i) induced by activation of angiotensin AT1 receptors were dependent on extracellular Ca2+ entry by L-type Ca2+ channels and subsequent Ca2+-induced Ca2+ release from the intracellular stores. The angiotensin AT1 receptor-activated increases in [Ca2+]i were selectively inhibited by injection of antisense oligonucleotides directed against the mRNAs coding for the alpha13, beta1, and gamma3 subunits. A correlating reduction in Galpha13, Gbeta1, and Ggamma3 protein expression was confirmed by immunocytochemistry. In addition, anti-alpha13 antibody and synthetic peptide corresponding to the carboxyl terminus of the Galpha13 subunit inhibited, in a concentration-dependent manner, the angiotensin AT1 receptor-mediated Ca2+ response. Reverse transcription-polymerase chain reaction analysis showed that only the angiotensin AT1A receptor was expressed in rat portal vein smooth muscle. Furthermore, injection of anti-AT1A oligonucleotides selectively inhibited the angiotensin II-induced increase in [Ca2+]i. We conclude that the receptor-activated signal leading to increases in [Ca2+]i is transduced by the heterotrimeric G13 protein composed of alpha13/beta1/gamma3 subunits and that the carboxyl terminus of the Galpha13 subunit interacts with the angiotensin AT1A receptor.

Distinct functions of Gq and G11 proteins in coupling alpha1-adrenoreceptors to Ca2+ release and Ca2+ entry in rat portal vein myocytes

In this study, we identified the subunit composition of Gq and G11 proteins coupling alpha1-adrenoreceptors to increase in cytoplasmic Ca2+ concentration ([Ca2+]i) in rat portal vein myocytes maintained in short-term primary culture. We used intranuclear antisense oligonucleotide injection to inhibit selectively the expression of subunits of G protein. Increases in [Ca2+]i were measured in response to activation of alpha1-adrenoreceptors, angiotensin AT1 receptors, and caffeine. Antisense oligonucleotides directed against the mRNAs coding for alphaq, alpha11, beta1, beta3, gamma2, and gamma3 subunits selectively inhibited the increase in [Ca2+]i activated by alpha1-adrenoreceptors. A corresponding reduction of the expression of these G protein subunits was immunochemically confirmed. In experiments performed in Ca2+-free solution only cells injected with anti-alphaq antisense oligonucleotides displayed a reduction of the alpha1-adrenoreceptor-induced Ca2+ release. In contrast, in Ca2+-containing solution, injection of anti-alpha11 antisense oligonucleotides suppressed the alpha1-adrenoreceptor-induced stimulation of the store-operated Ca2+ influx. Agents that specifically bound Gbetagamma subunits (anti-betacom antibody and overexpression of a beta-adrenergic receptor kinase carboxyl-terminal fragment) had no effect on the alpha1-adrenoreceptor-induced signal transduction. Taken together, these results suggest that alpha1-adrenoreceptors utilize two different Galpha subunits to increase [Ca2+]i. Galphaq may activate phosphatidylinositol 4,5-bisphosphate hydrolysis and induce release of Ca2+ from intracellular stores. Galpha11 may enhance the Ca2+-activated Ca2+ influx that replenishes intracellular Ca2+ stores.