Induction of Ca2+ oscillations by vasopressin in the presence of tetraethylammonium chloride in cultured vascular smooth muscle cells

The activation by estrogen receptor agonists of the BKCa-channel in human cardiac fibroblasts

The agonists selective for estrogen receptor (ER)-alpha (4,4',4''-(4-propyl-[(1)H]-pyrazole-1,3,5-triyl) tris-phenol, PPT) and ER-beta (2,3-bis(4-hydroxyphenyl)-propionitrile, DPN) are known to stimulate ER-alpha and ER-beta receptors, respectively. It remains unknown whether these two agents regulate the activity of ion channels via a direct stimulation. In this study, we tested the hypothesis that DPN or PPT stimulates the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in cultured human cardiac fibroblasts (HCFs). In whole-cell configuration, depolarizing pulses evoked K(+) outward currents in an outward rectification in HCFs, the amplitude of which was increased in the presence of DPN or PPT. In inside-out patches, the activity of BK(Ca)-channel with a conductance of 167+/-8 pS was observed in these cells. PPT or DPN applied to the intracellular face of the membrane enhanced the activity of BK(Ca) channels with no change in single-channel conductance. DPN and PPT increased BK(Ca)-channel activity with an EC(50) value of 2.3 and 2.6 microM, respectively. The mean closed time of these channels during the exposure to these compounds was reduced with no change in the gating charge of the channels. Intracellular Ca(2+) was not altered by these two compounds. RT-PCR analysis revealed that no change in the transcriptional level of the BK(Ca)-channel alpha-subunit was observed in chronic treatment with these two compounds. PPT- and DPN-stimulated increase in BK(Ca) channels reveal novel pharmacological properties attributable to the activity of these channels, and their increase in BK(Ca) channels activity in HCFs may contribute to cell function.

Localised calcium release events in cells from the muscle of guinea-pig gastric fundus

After enzymatic dispersion of the muscle of the guinea-pig gastric fundus, single elongated cells were observed which differed from archetypal smooth muscle cells due to their knurled, tuberose or otherwise irregular surface morphology. These, but not archetypal smooth muscle cells, consistently displayed spontaneous localized (i.e. non-propagating) intracellular calcium ([Ca(2+)](i)) release events. Such calcium events were novel in their magnitude and kinetic profiles. They included short transient events, plateau events and events which coalesced spatially or temporally (compound events). Quantitative analysis of the events with an automatic detection programme showed that their spatio-temporal characteristics (full width and full duration at half-maximum amplitude) were approximately exponentially distributed. Their amplitude distribution suggested the presence of two release modes. Carbachol application caused an initial cell-wide calcium transient followed by an increase in localized calcium release events. Pharmacological analysis suggested that localized calcium release was largely dependent on external calcium entry acting on both inositol trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) to release stored calcium. Nominally calcium-free external solution immediately and reversibly abolished all localized calcium release without blocking the initial transient calcium release response to carbachol. This was inhibited by 2-APB (100 microm), ryanodine (10 or 50 microm) or U-73122 (1 microm). 2-APB (100 microm), xestospongin C (XeC, 10 microm) or U-73122 (1 microm) blocked both spontaneous localized calcium release and localized release stimulated by 10 microm carbachol. Ryanodine (50 microm) also inhibited spontaneous release, but enhanced localized release in response to carbachol. This study represents the first characterization of localized calcium release events in cells from the gastric fundus.

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.

Alpha-adrenoceptor agonists produce Ca2+ oscillations in isolated rat aorta: role of protein kinase C

We investigated the relationship between tension development and the cytosolic free Ca2+ level ([Ca2+]i) in responses to norepinephrine (NE) and selective alpha2-adrenoceptor agonist, UK14,304 of the endothelium-denuded rat aorta loaded with fura PE-3. NE (3 x 10(-8) M) evoked a rapid increase in [Ca2+]i followed by slight decreasing to a steady state level and produced a contraction. After the NE-induced increase in [Ca2+]i had reached a maximum, the [Ca2+]i showed persistent oscillations. The Ca2+ oscillations were superimposed on the sustained increase in [Ca2+]i. UK14,304 (3 x 10(-6) M) also evoked an increase in [Ca2+]i and produced a contraction. However, the UK14,304-induced effect on [Ca2+]i was characterized by pronounced oscillations, and the amplitude of the sustained increase in [Ca2+]i was less than that seen with NE. Protein kinase C inhibitor, Ro31-8220 (3 x 10(-6) M) and verapamil (10(-5) M) abolished both NE and UK14,304-evoked Ca2+ oscillations. UK14,304-induced contractions were also strongly inhibited by Ro31-8220 and verapamil. However, NE induced contractions were partly inhibited by these inhibitors. The sustained increases in [Ca2+]i evoked NE and UK14,304 were not significantly inhibited by Ro31-8220 and verapamil. These results suggest that NE and UK14,304 produce Ca2+ oscillations during sustained contractions in rat aorta. The alpha2 adrenoceptor agonist, UK14,304-induced sustained contraction and Ca2+ oscillations may be due to PKC activation and opening of voltage-dependent L type Ca2+ channels.

Enhanced ET(A)-receptor-mediated inhibition of K(v) channels in hypoxic hypertensive rat pulmonary artery myocytes

Endothelin (ET)-1 has been implicated as a critical mediator in the pathogenesis of hypoxic pulmonary hypertension. We questioned whether, during exposure to chronic hypobaric hypoxia, rat pulmonary artery smooth muscle cells (PASMC) became sensitized to ET-1. Two effects of ET-1, inhibition of voltage-gated K(+) (K(v)) channels and release of intracellular Ca(2+), were studied using whole cell patch clamp and single cell indo 1 fluorescence, respectively. In both normotensive and chronically hypoxic-hypertensive PASMC, ET-1 caused concentration-dependent inhibition of voltage-gated K(+) current [I(K(v))], with maximum inhibition of 12-18% seen at a concentration of 0.1-1 nM. Although the chronically hypoxic-hypertensive PASMC was no more susceptible to ET-1-mediated I(K(v)) inhibition, a switch in coupling between ET-1 and I(K(v)) from ET(B) to ET(A) receptors occurred. This switch in receptor coupling, combined with reduced I(K(v)) density and increased ET-1 production in the hypoxic rat lung, may help explain the ability of ET(A)-receptor blockers to attenuate the development of hypoxic pulmonary hypertension in vivo.

Nicotinamide-inhibited vasoconstriction: lack of dependence on agonist signalling pathways

Previously, we have shown that nicotinamide inhibits both high [K+]- and phenylephrine-induced constrictions in a dose-dependent manner in rat tail arteries. We have now investigated the effect of nicotinamide on intracellular signalling pathways in vascular smooth muscle. Nicotinamide (8.2 mM) reduced the response to phenylephrine- and [Arg8]vasopressin-induced constrictions by means of 72.9+/-6.9 and 51.8+/-5.7%, respectively. It also blocked phenylephrine-induced constrictions in the absence of a functional endothelium (P < 0.0136). In addition, pre-treatment of the artery with nifedipine (10 mM) also failed to inhibit nicotinamide's activity (P < 0.0178). Moreover, nicotinamide significantly reduced the sensitivity to phenylephrine in Ca2+-free Krebs' solution (P < 0.0152). Continuous perfusion of maximal concentrations of ryanodine or thapsigargin significantly inhibited the response to phenylephrine; the addition of nicotinamide (8.2 mM) caused a significant additional inhibition when compared to the effect of ryanodine (P < 0.0006) or thapsigargin (P<0.037) alone. In addition, beta-escin (0.02%) permeabilisation and Ca2+ (2.5 mM)-mediated constriction was also significantly attenuated by nicotinamide (P < 0.0001). However, phorbol ester-induced constriction was not attenuated by nicotinamide. This would suggest that nicotinamide directly inhibits vascular smooth muscle cell contraction and is unlikely to act via blockage of external Ca2+ entry or release of Ca2+ from intracellular stores.

Differential regulation of intracellular Ca2+ signalling induced by high K+ and endothelin-1 in single smooth muscle cells of intact canine basilar artery: detection by means of confocal laser microscopy

Changes in intracellular calcium concentration ([Ca2+]i) in smooth muscle cells play the key role in regulation of vascular smooth muscle tone and pathogenesis of cerebral vasospasm. In this study, we adopted the confocal laser microscopy to detect the fluorescence signals arising from the individual smooth muscle cells of canine basilar artery. Ring preparations were made, loaded with fluo-3 and changes in fluorescence induced by high K+ and endothelin-1 (ET-1) were measured by confocal laser microscopy. In some unstimulated smooth muscle cells Ca2+ waves arising from discrete region of the cell propagated to the whole cell with a velocity of approximately 10 microm/s. High K+ (80 mmol/L) induced a rapid rise in [Ca2+]i, the peak level being consistently reached approximately 10 s after stimulation. In contrast, the time to peak level of [Ca2+]i induced by ET-1 (0.3 micromol/L) varied widely between 13 and 26 s among individual cells, an indication that the extent of nonuniform coordination of increases in [Ca2+]i in individual cells may be partly responsible for the different time courses of tension development of vascular smooth muscle in response to the vasoactive stimulants. The increase in [Ca2+]i induced by ET-1 was transient but a pronounced and sustained contraction developed further in response to ET-1. Thus ET-1 has a biological property as a potential candidate to elicit cerebral vasospasm. Confocal laser microscopy could be a useful tool to measure the changes in [Ca2+]i in individual smooth muscle cells of cerebral artery.

Calcium signaling mechanisms in renal vascular responses to vasopressin in genetic hypertension

Previous blood flow studies demonstrated that arginine vasopressin (AVP) produces exaggerated renal vasoconstriction in young spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto control rats (WKY). The purpose of the present study was to determine the role of postreceptor calcium signaling pathways in AVP-induced renal vasoconstriction in vivo. Renal blood flow (RBF) was measured by electromagnetic flowmetry in anesthetized, water-loaded, 8-week-old WKY and SHR pretreated with indomethacin to avoid interactions with prostaglandins. AVP was injected into the renal artery to produce a transient 25% to 30% decrease in RBF without affecting arterial pressure. To achieve similar control levels of vasoconstriction, SHR received a lower dose (2 versus 5 ng). Coadministration of nifedipine with AVP produced dose-dependent inhibition of the AVP-induced renal vasoconstriction. Nifedipine exerted maximum inhibition by blocking 30% to 35% of the peak AVP response, indicating the involvement of dihydropyridine-sensitive voltage-dependent calcium channels. To evaluate intracellular calcium mobilization, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) or heparin was coadministered with AVP. Each agent produced a dose-dependent inhibition of up to 65% of the maximum blood flow change produced by AVP. The degrees of inhibition produced by maximum effective doses of nifedipine and TMB-8 were additive; the combination blocked up to 85% of the response to AVP. These observations indicate that about one third of the AVP-induced constriction of renal resistance vessels is mediated by voltage-dependent L-type calcium channels responsive to the dihydropyridine nifedipine. Approximately two thirds of the change in vascular tone is due to inositol 1,4,5-trisphosphate-mediated calcium mobilization from intracellular sources sensitive to TMB-8 and heparin. The results suggest that the exaggerated renal vascular reactivity to AVP challenge in SHR is probably not due to a strain difference in postreceptor calcium signal transduction. After AVP receptor stimulation, calcium mobilization and calcium entry signaling pathways participate to similar degrees in WKY and SHR.

Ionic mechanisms of tetrandrine in cultured rat aortic smooth muscle cells

The ionic mechanism of tetrandrine, an alkaloid extracted from the Chinese medicinal herb Radix stephania tetrandrae, was investigated in A7r5 vascular smooth muscle cells. The nystatin-perforated whole-cell voltage-clamp technique was performed to examine the effects of tetrandrine on ionic currents. Tetrandrine (1-100 microM) reversibly caused an inhibition of L-type voltage-dependent Ca2+ current (I(Ca,L)) in a concentration-dependent manner. Tetrandrine did not cause any change in the overall shape of the current-voltage relationship of I(Ca,L). The IC50 value of tetrandrine-induced inhibition of I(Ca,L) was 5 microM. In the presence of Bay K 8644 (3 microM) or cyclopiazonic acid (30 microM), tetrandrine still produced a significant inhibition of I(Ca,L). The inhibitory effects of tetrandrine on I(Ca,L) exhibited tonic and use-dependent characteristics. Moreover. tetrandrine (3 microM) shifted the steady-state inactivation curve of I(Ca,L) to more negative membrane potentials by approximately -15 mV. These results indicate that tetrandrine directly inhibits the voltage-dependent L-type Ca2+ current in vascular smooth muscle cells, which may predominantly contribute to the vasodilatory actions of tetrandrine.

The characteristics in the inhibitory effects of capsaicin on voltage-dependent K(+) currents in rat atrial myocytes

The electrophysiological effects of capsaicin in rat atrial myocytes were examined. Measurement of contractile force was done in rat left atria. Whole-cell patch-clamp technique was primarily used to study the change in membrane potential and ionic currents. Capsaicin produced an initial rise and a sustained increase in contractile force in rat left atria. Capsaicin (10 μM) caused a significant prolongation of atrial action potential. In voltage-clamp experiments, capsaicin (1-100 μM) caused the reversible reduction in the amplitude of transient outward (I(TO)) and late outward (I(L)) K(+) currents in concentration- and voltage-dependent manners. The time course for inactivation of I(TO) was changed to the biexponential process after the application of capsaicin. Capsaicin failed to cause any significant shift in quasi-steady-state inactivation curve of I(TO). The EC(50) values for the inhibitory effects of capsaicin on I(TO) and I(L) were 5 and 20 μM, respectively. Capsaicin also suppressed the amplitude of acetylcholine- or adenosine-induced K(+) current, i.e., I(K(ACh,Ado)). The EC(50) value for capsaicin-mediated inhibition of I(K(ACh,Ado)) is 50 μM. The present findings suggest that in isolated rat atria, during capsaicin exposure, the capsaicin-mediated inhibition of these K(+) channels is one of the ionic mechanisms underlying the positive inotropic and chronotropic actions.