Minor role for direct adrenoceptor-mediated calcium entry in rat mesenteric small arteries

Oxidative Regulation of Vascular Cav1.2 Channels Triggers Vascular Dysfunction in Hypertension-Related Disorders

Blood pressure is determined by cardiac output and peripheral vascular resistance. The L-type voltage-gated Ca²⁺ (Ca_v1.2) channel in small arteries and arterioles plays an essential role in regulating Ca²⁺ influx, vascular resistance, and blood pressure. Hypertension and preeclampsia are characterized by high blood pressure. In addition, diabetes has a high prevalence of hypertension. The etiology of these disorders remains elusive, involving the complex interplay of environmental and genetic factors. Common to these disorders are oxidative stress and vascular dysfunction. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) and mitochondria are primary sources of vascular oxidative stress, whereas dysfunction of the Ca_v1.2 channel confers increased vascular resistance in hypertension. This review will discuss the importance of ROS derived from NOXs and mitochondria in regulating vascular Ca_v1.2 and potential roles of ROS-mediated Ca_v1.2 dysfunction in aberrant vascular function in hypertension, diabetes, and preeclampsia.

Potassium-Channel-Independent Relaxing Influence of Adipose Tissue on Mouse Carotid Artery

Since the cardiovascular consequences of obesity reportedly vary in different types of obesity, we investigated the influence of adipose tissue from different locales on the phenylephrine-induced tone of the mouse carotid artery. Vessels were mounted in a Mulvany-Halpern-type wire myograph, and adipose tissue, from the back (brown) or mesenteric or inguinal subcutaneous (white), was placed around the artery. Contractile responses to phenylephrine were not affected by brown adipose tissue but were reduced (p < 0.001) by either type of white adipose tissue, with no difference between the 2 locales. The relaxing effect persisted in the presence of the Kv7 channel inhibitor XE991 (10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone), the KATP channel inhibitor glibenclamide (1 µM), or the KV channel inhibitor 4-amino pyridine (1 mM), as well as after elevation of the extracellular potassium concentration to 30 mM. Contractions of rat carotid artery were equally reduced by mouse and rat subcutaneous adipose tissue. Thus, white, but not brown, adipose tissue reduces the adrenergic contractions of the carotid artery with no differences between the locales of origin, and the effect appears largely independent of potassium channels.

Na+-independent, nifedipine-resistant rat afferent arteriolar Ca2+ responses to noradrenaline: possible role of TRPC channels

AIM

In rat afferent arterioles we investigated the role of Na(+) entry in noradrenaline (NA)-induced depolarization and voltage-dependent Ca(2+) entry together with the importance of the transient receptor potential channel (TRPC) subfamily for non-voltage-dependent Ca(2+) entry.

METHODS

R (340/380) Fura-2 fluorescence was used as an index for intracellular free Ca(2+) concentration ([Ca(2+)](i)). Immunofluorescence detected the expression of TRPC channels.

RESULTS

TRPC 1, 3 and 6 were expressed in afferent arteriolar vascular smooth muscle cells. Under extracellular Na(+)-free (0 Na) conditions, the plateau response to NA was 115% of the baseline R(340/380) (control response 123%). However, as the R(340/380) baseline increased (7%) after 0 Na the plateau reached the same level as during control conditions. Similar responses were obtained after blockade of the Na(+)/Ca(2+) exchanger. The L-type blocker nifedipine reduced the plateau response to NA both under control (from 134% to 116% of baseline) and 0 Na conditions (from 112% to 103% of baseline). In the presence of nifedipine, the putative TRPC channel blockers SKF 96365 (30 μm) and Gd(3+) (100 μm) further reduced the plateau Ca(2+) responses to NA (from 117% to 102% and from 117% to 110% respectively).

CONCLUSION

We found that Na(+) is not crucial for the NA-induced depolarization that mediates Ca(2+) entry via L-type channels. In addition, the results are consistent with the idea that TRPC1/3/6 Ca(2+) -permeable cation channels expressed in afferent arteriolar smooth muscle cells mediate Ca(2+) entry during NA stimulation.

A mathematical model of Ca2+ dynamics in rat mesenteric smooth muscle cell: agonist and NO stimulation

A mathematical model of calcium dynamics in vascular smooth muscle cell (SMC) was developed based on data mostly from rat mesenteric arterioles. The model focuses on (a) the plasma membrane electrophysiology; (b) Ca2+ uptake and release from the sarcoplasmic reticulum (SR); (c) cytosolic balance of Ca2+, Na+, K+, and Cl ions; and (d) IP3 and cGMP formation in response to norepinephrine(NE) and nitric oxide (NO) stimulation. Stimulation with NE induced membrane depolarization and an intracellular Ca2+ ([Ca2+]i) transient followed by a plateau. The plateau concentrations were mostly determined by the activation of voltage-operated Ca2+ channels. NE causes a greater increase in [Ca2+]i than stimulation with KCl to equivalent depolarization. Model simulations suggest that the effect of[Na+]i accumulation on the Na+/Ca2+ exchanger (NCX) can potentially account for this difference.Elevation of [Ca2+]i within a concentration window (150-300 nM) by NE or KCl initiated [Ca2+]i oscillations with a concentration-dependent period. The oscillations were generated by the nonlinear dynamics of Ca2+ release and refilling in the SR. NO repolarized the NE-stimulated SMC and restored low [Ca2+]i mainly through its effect on Ca2+-activated K+ channels. Under certain conditions, Na+-K+-ATPase inhibition can result in the elevation of [Na+]i and the reversal of NCX, increasing resting cytosolic and SR Ca2+ content, as well as reactivity to NE. Blockade of the NCX's reverse mode could eliminate these effects. We conclude that the integration of the selected cellular components yields a mathematical model that reproduces, satisfactorily, some of the established features of SMC physiology. Simulations suggest a potential role of intracellular Na+ in modulating Ca2+ dynamics and provide insights into the mechanisms of SMC constriction, relaxation, and the phenomenon of vasomotion. The model will provide the basis for the development of multi-cellular mathematical models that will investigate microcirculatory function in health and disease.

Ca2+ sensitisation of force production by noradrenaline in femoral conductance and resistance arteries from rats with postinfarction congestive heart failure

In this study we tested the hypothesis that arterial myofilament Ca(2+) sensitivity and/or the Ca(2+) sensitising effect of noradrenaline (NA) is enhanced in post-infarction congestive heart failure (CHF), which could contribute to the high peripheral vascular resistance in this condition. Femoral skeletal muscle resistance and conductance arteries (mean lumen diameters of 159 and 519 microm) from rats with CHF and sham-operated control rats were used. Isometric tension development and intracellular free calcium concentration ([Ca(2+)](i)) were measured simultaneously in isolated vessel segments using wire myography and the FURA-2 fluorescence technique. In conductance and resistance arteries, the resting levels of [Ca(2+)](i) and tension in physiological saline solution (PSS) and active tension in response to single doses of 125 mM K(+) (KPSS) were unaffected by CHF. During cumulative application of extracellular Ca(2+) to arteries depolarised with 125 mM K(+) or activated with 30 microM NA, [Ca(2+)](i) and vessel wall tension were similar in CHF and control rats. However, the conductance arteries showed significantly higher calcium sensitivity than resistance arteries in these experiments. We conclude that an abnormality in the sensitivity of the contractile apparatus to Ca(2+), or in NA-induced Ca(2+) sensitisation in arterial vascular smooth muscle cells is unlikely to contribute to the ubiquitously elevated vascular resistance associated with CHF. However, our data demonstrate significant differences in vascular Ca(2+) handling, myofilament Ca(2+) sensitivity and tension development between resistance and conductance arteries, regardless of CHF.

Calcium handling in afferent arterioles

The cytosolic intracellular calcium concentration ([Ca(2+)](i)) is a major determining factor in the vascular smooth muscle tone. In the afferent arteriole it has been shown that agonists utilizing G-protein coupled receptors recruit Ca(2+) via release from intracellular stores and entry via pathways in the plasma membrane. The relative importances of entry vs. mobilization seem to differ between different agonists, species and preparations. The entry pathway might include different types of voltage sensitive Ca(2+) channels located in the plasmalemma such as dihydropyridine sensitive L-type channels, T-type channels and P/Q channels. A role for non-voltage sensitive entry pathways has also been suggested. The importance of voltage sensitive Ca(2+) channels in the control of the tone of the afferent arteriole (and thus in the control of renal function and whole body control of extracellular fluid volume and blood pressure) sheds light on the control of the membrane potential of afferent arteriolar smooth muscle cells. Thus, K(+) and Cl(-) channels are of importance in their role as major determinants of membrane potential. Some studies suggest a role for calcium-activated chloride (Cl(Ca)) channels in the renal vasoconstriction elicited by agonists. Other investigators have found evidence for several types of K(+) channels in the regulation of the afferent arteriolar tone. The available literature in this field regarding afferent arterioles is, however, relatively sparse and not conclusive. This review is an attempt to summarize the results obtained by others and ourselves in the field of agonist induced afferent arteriolar Ca(2+) recruitment, with special emphasis on the control of voltage sensitive Ca(2+) entry. Outline of the Manuscript: This manuscript is structured as follows: it begins with an introduction where the general role for [Ca(2+)](i) as a key factor in the regulation of the tone of vascular smooth muscles (VSMC) is detailed. In this section there is an emphasis is on observations that could be attributed to afferent arteriolar function. We then investigate the literature and describe our results regarding the relative roles for Ca(2+) entry and intracellular release in afferent arterioles in response to vasoactive agents, with the focus on noradrenalin (NA) and angiotensin II (Ang II). Finally, we examine the role of ion channels (i.e. K(+) and Cl(-) channels) for the membrane potential, and thus activation of voltage sensitive Ca(2+) channels.

Preservation of vascular contraction during ageing: dual effect on calcium handling and sensitization

(1) The present study was aimed to characterize the effects of ageing on vascular contraction by noradrenaline in rat isolated arteries. The existence of vascular bed heterogeneity was investigated in endothelium-denuded conductance (aorta) and resistance (small mesenteric artery, SMA) arteries, with respect to Ca(2+) handling, Ca(2+) sensitization or Ca(2+)-independent mechanisms. (2) In both arteries, contractions to noradrenaline were not different between adult and aged rats. (3) In Ca(2+)- free medium, noradrenaline elicited a transient increase in tension that was reduced by the Ca(2+) mobilizing agents, ryanodine and thapsigargin, in arteries from adult rats. A loss of the thapsigargin- but not the ryanodine-sensitive component of noradrenaline-induced contraction was observed in the two arteries from aged rats. (4) After depletion of Ca(2+) stores with noradrenaline, addition of exogenous CaCl(2) produced a sustained contraction that was decreased to the same extent by the protein kinase C inhibitor, GF 109203X and the tyrosine kinase inhibitor, tyrphostin A-23, in arteries from adult and aged rats. The Rho-associated protein kinase inhibitor, Y-27632, caused identical relaxation of noradrenaline pre-contracted arteries from both age groups. (5) Basal intracellular calcium ([Ca(2+)](i)) was higher in SMA from aged than from adult rats. In addition, the noradrenaline [Ca(2+)](i)-force relationship was significantly shifted to the right in the SMA from aged rats. (6) Altogether, these data indicate that responsiveness to noradrenaline is preserved both in conductance and resistance arteries with ageing. The latter results from the association of increased basal [Ca(2+)](i), changes in Ca(2+) handling at the level of thapsigargin-sensitive sarcoplasmic reticulum Ca(2+)-ATPases and decreased myofilament sensitivity to Ca(2+).

The effects of nifedipine and thapsigargin on the responses of pressurized rat mesenteric artery to 5-hydroxytryptamine and norepinephrine

The responses of isolated pressurized second order mesenteric resistance arteries of Wistar rats, superfused with physiological salt solution (PSS) were determined to 5-hydroxytryptamine (5-HT) and norepinephrine (NE). The contractility of the vessel was enhanced in response to 5-HT compared to NE (P<.001, ANOVA). The L-type calcium ion channel blocker, nifedipine (10(-6) M) abolished the response to either 5-HT or NE. In vessels with intact endothelium, thapsigargin (TG, 10(-6) M), which inhibits uptake of calcium ions into intracellular stores, significantly reduced the contractile response to 5-HT (P<.02) but had little or no effect on the response to NE (P=.2). However, in vessels denuded of the endothelium, there was no significant difference in the response of the mesenteric artery, after TG, to either 5-HT or NE. The results indicate that, in the rat mesenteric resistance vessel, both 5-HT and NE use calcium ions from extracellular sources for contraction, while NE relies mainly on extracellular ion influx with little or no contribution from intracellular sources. The reduced response of the de-endothelized vessel to 5-HT after TG suggests that the utilization of intracellular stores by this agonist is endothelium-dependent. These observations may explain the enhanced responsiveness of the mesenteric artery to 5-HT when compared with NE.

Comparison of catecholamine release in the isolated adrenal glands of SHR and WKY rats

1 The present study was designed to investigate the secretion of catecholamines (CA) evoked by stimulation of cholinergic receptors and membrane depolarization from the isolated perfused adrenal gland of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKYR) at adult age. 2 The wet weight of adrenal gland in SHR was greater than that in WKYR. The CA releasing responses evoked by acetylcholine (5.32 x 10-3 m), and high potassium (5.6 x 10-2 m), a membrane depolarizer, were significantly lower in WKYR than in SHR. 3 The secretory responses of CA evoked by DMPP (10-4 m for 2 min), a selective agonist of neuronal nicotinic receptors, and McN-A-343 (10-4 m for 2 min), a selective agonist of neuronal muscarinic receptors, were also significantly lower in WKYR than in SHR. 4 The CA release evoked by Bay-K-8644 (10-5 m), a dihydropyridine-sensitive Ca2+ channel activator, and cyclopiazonic acid (10-5 m), a selective inhibitor of Ca2+-ATPase in the endoplasmic reticulum, were also significantly greater in SHR than WKYR. 5 Taken together, these experimental results demonstrate that the CA secretion evoked by stimulation of cholinergic (nicotinic and muscarinic) receptors as well as membrane depolarization is enhanced more greatly in the perfused adrenal glands of SHR than in those of WKYR. It is suggested that the augmented CA release in SHR compared with WKYR was involved in essential hypertensive pathogenesis.

Reversible impairment of endothelium-dependent relaxation in golden hamster carotid arteries during hibernation

The effects of hibernation on endothelium-dependent vasodilatation were investigated in the golden hamster carotid artery, paying special attention to hibernating body temperature (10 degrees C). To record mechanical and electrical membrane responses, we applied pharmacological (organ bath) and electrophysiological (microelectrode) techniques, using acetylcholine (ACh; 0.001-100 microM) and ATP (0.01-1000 microM) for endothelium-dependent vasodilatation and sodium nitroprusside (SNP; 0.05-10 microM) for endothelium-independent vasodilatation. At 34 degrees C, ACh, ATP and SNP each induced a relaxation or a hyperpolarization, and these responses were similar in all the preparations from control and hibernated animals. At 10 degrees C, on the other hand, ACh-induced relaxations and hyperpolarizations were reduced to approximately 35 % and 50 % of the euthermic level in controls and 1 % and 4 % of the euthermic level in hibernated animals, respectively. In contrast, at 10 degrees C, ATP induced only a contraction or depolarization in all preparations with no significant difference between control and hibernated animals. SNP-induced relaxations and hyperpolarizations obtained at 34 degrees C were not attenuated by cooling to 10 degrees C. In the presence of a P2X receptor blocker, pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 5 microM), at 34 degrees C ATP-induced relaxations and hyperpolarizations were significantly enhanced whereas no responses were induced by ATP at 10 degrees C. After endothelium removal, on the other hand, ATP induced only a contraction or depolarization at both 34 degrees C and 10 degrees C. These results suggest that depression of endothelium-dependent vasodilator responses to ACh and ATP may occur in the hibernating golden hamster carotid artery.

Norepinephrine-induced calcium signaling pathways in afferent arterioles of genetically hypertensive rats

This study provides new information about the relative importance of calcium mobilization and entry in the renal vascular response to adrenoceptor activation in afferent arterioles isolated from 7- to 8-wk-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Intracellular free calcium concentration ([Ca(2+)](i)) was measured in microdissected arterioles utilizing ratiometric photometry of fura 2 fluorescence. There was no significant strain difference in baseline [Ca(2+)](i). Norepinephrine (NE; 10(-6) and 10(-7) M) elicited immediate, sustained increases in [Ca(2+)](i). The general temporal pattern of response to 10(-6) M NE consisted of an initial peak and a maintained plateau phase. The response to NE was partially blocked by nifedipine (10(-6) M) or 8-(N,N-diethylamino) octyl-3,4,5-trimetoxybenzoate (TMB-8; 10(-5) M). A calcium-free external solution abolished the sustained [Ca(2+)](i) plateau response to NE, with less influence on the peak response. In the absence of calcium entry, TMB-8 (10(-5) M) completely blocked the calcium response to NE in WKY but not SHR, suggesting strain differences in mobilization. A higher concentration of TMB-8 (10(-4) M), however, blocked all discernible mobilization in both strains. We conclude that there are differences in Ca(2+) handling in renal resistance vessels between young WKY and SHR with respect to mobilization stimulated by alpha-adrenoceptors. Afferent arterioles of young SHR appear to have a larger inositol-1,4,5-trisphosphate-sensitive pool or release from a site less accessible to TMB-8.

Graded alpha1-adrenoceptor activation of arteries involves recruitment of smooth muscle cells to produce 'all or none' Ca(2+) signals

Confocal laser scanning microscopy and Fluo-4 were used to visualize Ca(2+) transients within individual smooth muscle cells (SMC) of rat resistance arteries during alpha(1)-adrenoceptor activation. The typical spatio-temporal pattern of [Ca(2+)] in an artery after exposure to a maximally effective concentration of phenylephrine (PE, 10.0 microM) was a large, brief, relatively homogeneous Ca(2+) transient, followed by Ca(2+) waves, which then declined in frequency over the course of 5 min and which were asynchronous in different SMC. Concentration-Effect (CE) curves relating the concentration of PE (range: 0.1 microM to 10.0 microM) to the effects (fraction of cells producing at least one Ca(2+) wave, and number of Ca(2+) waves during 5 min) had EC(50) values of approximately 0.5 microM and approximately 1.0 microM respectively. The initial Ca(2+) transient and the subsequent Ca(2+) waves were abolished in the presence of caffeine (10.0 mM). A repeated exposure to PE, 1.5 min after the first had ended, elicited fewer Ca(2+) waves in fewer cells than did the initial exposure. Caffeine-sensitive Ca(2+) stores were not depleted at this time, however, as caffeine alone was capable of inducing a large release of Ca(2+)1.5 min after PE. In summary, the mechanism of a graded response to graded alpha(1)-adrenoceptor activation is the progressive 'recruitment' of individual SMC, which then respond in 'all or none' fashion (viz. asynchronous Ca(2+) waves). Ca(2+) signaling continues in the arterial wall throughout the time-course (at least 5 min) of activation of alpha(1)-adrenoceptors. The fact that the Ca(2+) waves are asynchronous accounts for the previously reported fall in 'arterial wall [Ca(2+)]' (i.e. spatial average [Ca(2+)] over all cells).

Neuropeptide Y regulates intracellular calcium through different signalling pathways linked to a Y(1)-receptor in rat mesenteric small arteries

Simultaneous measurements of intracellular calcium concentration ([Ca(2+)](i)) and tension were performed to clarify whether the mechanisms which cause the neuropeptide Y (NPY)-elicited contraction and potentiation of noradrenaline contractions, and the NPY inhibition of forskolin responses are linked to a single or different NPY receptor(s) in rat mesenteric small arteries. In resting arteries, NPY moderately elevated [Ca(2+)](i) and tension. These effects were antagonized by the selective Y(1) receptor antagonist, (R)-N(2)-(diphenacetyl)-N-[(4-hydroxyphenyl)methyl]-D-argininea mide (BIBP 3226) (apparent pK(B) values of 8.54+/-0.25 and 8.27+/-0.17, respectively). NPY (0.1 microM) caused a near 3 fold increase in sensitivity to noradrenaline but did not significantly modify the tension-[Ca(2+)](i) relationship for this agonist. BIBP 3226 competitively antagonized the contractile response to NPY in arteries submaximally preconstricted with noradrenaline (pA(2) 7.87+/-0.20). In arteries activated by vasopressin, the adenylyl cyclase activator forskolin (3 microM) induced a maximum relaxation and a return of [Ca(2+)](i) to resting levels. NPY completely inhibited these effects. The contractile responses to NPY in arteries maximally relaxed with either sodium nitroprusside (SNP) or nifedipine were not significantly higher than those evoked by the peptide at resting tension, in contrast to the contractions to NPY in forskolin-relaxed arteries. BIBP 3226 competitively antagonized the contraction to NPY in forskolin-relaxed arteries with a pA(2) of 7.92+/-0.29. Electrical field stimulation (EFS) at 8-32 Hz caused large contractions in arteries relaxed with either forskolin or noradrenaline in the presence of phentolamine. These responses to EFS were inhibited by BIBP 3226. Similar EFS in resting, non-activated arteries did not produce any response. The present results suggest that different intracellular pathways are linked to a single NPY Y(1) receptor in intact rat mesenteric small arteries, and provide little support for involvement of other postjunctional NPY receptors in the contractile responses to NPY. Neurally released NPY also seems to act through Y(1) receptors, and may serve primarily as an inhibitor of vasodilatation.

Local and cellular Ca2+ transients in smooth muscle of pressurized rat resistance arteries during myogenic and agonist stimulation

1. Confocal laser scanning microscopy was used to visualize Ca2+ transients in the vascular smooth muscle cells (VSMC) of intact, pressurized rat mesenteric resistance arteries loaded with fluorescent calcium indicators. Vasoconstriction was assessed by measuring inner arterial diameter. All arteries were studied at 70 mmHg intralumenal pressure and 37 C. 2. In the control condition of myogenic tone the arteries were constricted to 62 % (n = 10) of their passive diameter (p.d.). The [Ca2+]i in most VSMC of these arteries was constant over time. In a small percentage (< 10 %) of cells in each artery, [Ca2+]i oscillated regularly. Local calcium transients (Ca2+ sparks) were observed in five arteries studied with confocal linescan imaging. 3. Activation of alpha-adrenoceptors by phenylephrine (PE, 1.0 microM) induced further vasoconstriction of pressurized arteries (to 27 % of p.d.). In this condition, [Ca2+]i oscillations were prominent in a large percentage (83 %) of the VSMC. The Ca2+ oscillations ranged in frequency from 4 to 22 min-1, and were usually asynchronous between cells. 4. High [KCl]o (65 mM) induced nearly comparable vasoconstriction to PE (37 % of p.d.) but [Ca2+]i oscillated in only about 13 % of cells in each artery. 5. Block of L-type Ca2+ channels (with nifedipine) in arteries activated by PE caused nearly full vasodilatation, but did not abolish the Ca2+ oscillations. Subsequent block of the sarcoplasmic reticulum Ca2+ pump (with cyclopiazonic acid) abolished Ca2+ oscillations in all cells. 6. We conclude that Ca2+ entering VSMC via L-type Ca2+ channels has an obligatory role in force development, both in myogenic tone and during alpha1-adrenoceptor activation. The oscillatory pattern of [Ca2+]i that persists in the absence of Ca2+ entry via L-type Ca2+ channels is ineffective in activating contraction.

Ca(2+) influx inhibits voltage-dependent and augments Ca(2+)-dependent K(+) currents in arterial myocytes

These experiments were performed to determine the effects of reducing Ca(2+) influx (Ca(in)) on K(+) currents (I(K)) in myocytes from rat small mesenteric arteries by 1) adding external Cd(2+) or 2) lowering external Ca(2+) to 0.2 mM. When measured from a holding potential (HP) of -20 mV (I(K20)), decreasing Ca(in) decreased I(K) at voltages where it was active (>0 mV). When measured from a HP of -60 mV (I(K60)), decreasing Ca(in) increased I(K) at voltages between -30 and +20 mV but decreased I(K) at voltages above +40 mV. Difference currents (DeltaI(K)) were determined by digital subtraction of currents recorded under control conditions from those obtained when Ca(in) was decreased. At test voltages up to 0 mV, DeltaI(K60) exhibited kinetics similar to control I(K60), with rapid activation to a peak followed by slow inactivation. At 0 mV, peak DeltaI(K60) averaged 75 +/- 13 pA (n = 8) with Cd(2+) and 120 +/- 20 pA (n = 9) with low Ca(2+) concentration. At test voltages from 0 to +60 mV, DeltaI(K60) always had an early positive peak phase, but its apparent "inactivation" increased with voltage and its steady value became negative above +20 mV. At +60 mV, the initial peak DeltaI(K60) averaged 115 +/- 18 pA with Cd(2+) and 187 +/- 34 pA with low Ca(2+). With 10 mM pipette BAPTA, Cd(2+) produced a small inhibition of I(K20) but still increased I(K60) between -30 and +10 mV. In Ca(2+)-free external solution, Cd(2+) only decreased both I(K20) and I(K60). In the presence of iberiotoxin (100 nM) to inhibit Ca(2+)-activated K(+) channels (K(Ca)), Cd(2+) increased I(K60) at all voltages positive to -30 mV while BAY K 8644 (1 microM) decreased I(K60). These results suggest that Ca(in), through L-type Ca(2+) channels and perhaps other pathways, increases K(Ca) (i.e., I(K20)) and decreases voltage-dependent K(+) currents in this tissue. This effect could contribute to membrane depolarization and force maintenance.

Calcium recruitment in renal vasculature: NE effects on blood flow and cytosolic calcium concentration

This study provides new information about the relative importance of Ca2+ mobilization and entry in the renal vascular response to adrenoceptor activation. We measured renal blood flow (RBF) in Sprague-Dawley rats in vivo using electromagnetic flowmetry. We measured intracellular free Ca2+ concentration ([Ca2+]i) in isolated afferent arterioles utilizing ratiometric photometry of fura-2 fluorescence. Renal arterial injection of NE produced a transient decrease in RBF. The response was attenuated, in a dose-dependent manner, up to approximately 50% by nifedipine, an antagonist of L-type Ca2+ entry channels. Inhibition of Ca2+ mobilization by 3,4, 5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8) inhibited the renal vascular effects of NE in a dose-dependent manner, with maximal blockade of approximately 80%. No additional attenuation was observed when nifedipine and TMB-8 were administered together. In microdissected afferent arterioles, norepinephrine (NE; 10(-6) M) elicited an immediate square-shaped increase in [Ca2+]i, from 110 to 240 nM. This in vitro response was blocked by nifedipine (10(-6) M) and TMB-8 (10(-5) M) to a degree similar to that of the in vivo experiments. A nominally calcium-free solution blocked 80-90% of the [Ca2+]i response to NE. The increased [Ca2+]i elicited by depolarization with medium containing 50 mM KCl was totally blocked by nifedipine. In contrast, TMB-8 had no effect. Our results indicate that both Ca2+ entry and mobilization play important roles in the renal vascular Ca2+ and contractile response to adrenoceptor activation. The entry and mobilization mechanisms activated by NE may interact. That a calcium-free solution caused a larger inhibition of the NE effects on afferent arterioles than nifedipine suggests more than one Ca2+ entry pathway.

Effects of IBMX on norepinephrine-induced vasoconstriction in small mesenteric arteries

The present study assesses the effects of the phosphodiesterase inhibitor IBMX on norepinephrine (NE)-induced constriction of small mesenteric arteries. Arteries ( approximately 150 micrometer) were dissected from rats and mounted on a wire myograph for isometric force measurement. NE concentration effect curves were generated after exposure to 500 microM IBMX for 60 min. IBMX significantly reduced NE-induced tension development. Studies were also conducted following sarcoplasmic reticulum (SR) depletion (ryanodine, 10 microM) or L-type Ca2+ channel blockade [(+)-BAY K 8644, 10 microM] in the presence and absence of IBMX. Both SR depletion and L-channel blockade reduced NE-induced tension generation, consistent with incomplete Ca2+ mobilization. IBMX significantly attenuated NE responses in ryanodine and (+)-BAY K 8644-treated vessels. Finally, treatment of NE-stimulated vessels with IBMX (500 microM) caused a reduction in vascular tension that was greater than the concomitant reduction in cytosolic Ca2+ concentration ([Ca2+]i), indicating that a portion of the IBMX-mediated relaxation is Ca2+-independent. These data suggest that IBMX attenuation of NE responsiveness not only involves a reduction in [Ca2+]i but also a significant decrease in Ca2+ sensitivity.

Mechanisms of melatonin-induced vasoconstriction in the rat tail artery: a paradigm of weak vasoconstriction

1. Vasoconstrictor effects of melatonin were examined in isolated rat tail arteries mounted either in an isometric myograph or as cannulated pressurized segments. Melatonin failed by itself to mediate observable responses but preactivation of the arteries with vasopressin (AVP) reliably uncovered vasoconstriction responses to melatonin with maxima about 50% of maximum contraction. Further experiments were conducted with AVP preactivation to 5-10% of the maximum contraction. 2. Responses to melatonin consisted of steady contractions with superimposed oscillations which were large and irregular in isometric but small in isobaric preparations. Nifedipine (0.3 microM) reduced the responses and abolished the oscillations. Charybdotoxin (30 nM) increased the magnitude of the oscillations with no change in the maximum response. 3. Forskolin (0.6 microM) pretreatment increased the responses to melatonin compared to control and sodium nitroprusside (1 microM) treated tissues. The AVP concentration required for preactivation was 10 fold higher than control in both the forskolin and nitroprusside treated groups. 4. In isometrically-mounted arteries treated with nifedipine, melatonin receptor agonists had the potency order 2-iodomelatonin > melatonin > S20098 > GR196429, and the MT2-selective antagonist luzindole antagonized the effects of melatonin with a low pK(B) of 6.1+/-0.1. 5. It is concluded that melatonin elicits contraction of the rat tail artery via an mt1 or mt1-like receptor that couples via inhibition of adenylate cyclase and opening of L-type calcium channels. Calcium channels and charybdotoxin-sensitive K channels may be recruited into the responses via myogenic activation rather than being coupled directly to the melatonin receptors. 6. It is proposed that the requirement of preactivation for overt vasoconstrictor responses to melatonin results from the low effector reserve of the melatonin receptors together with the tail artery having threshold inertia. Potentiative interactions between melatonin and other vasoconstrictor stimuli probably also result from the threshold inertia. A simple model is presented and a general framework for consideration of interactions between weak vasoconstrictor agonists and other vasoconstrictor stimuli is discussed.

Mechanism of Ca2+ release and entry during contraction elicited by norepinephrine in rat resistance arteries

The intracellular Ca2+ stores and the mechanisms of Ca2+ entry produced by norepinephrine (NE) were investigated in small mesenteric resistance arteries of the rat. In Ca2+-free medium, NE (10 microM) elicited a transient increase in both intracellular free Ca2+ concentration ([Ca2+]i) and tension that were both drastically reduced by caffeine and only partially reduced by the two sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) blockers thapsigargin and cyclopiazonic acid, despite the presence of SERCA2a and SERCA2b isoforms in the medial smooth muscle layer of the artery. After depletion of intracellular Ca2+ stores with 10 microM NE, addition of exogenous CaCl2 (2.5 mM) produced large and sustained increases in both [Ca2+]i and contraction of the arteries provided that the agonist was continuously present. In these conditions, the responses to CaCl2 were inhibited by the voltage-dependent Ca2+ entry blocker nitrendipine (1 microM), the putative inhibitor of receptor-operated Ca2+ entry SKF-96365 (30 microM), and NiCl2 (1 mM). The inhibition produced by SKF-96365 and NiCl2 was greater than that of nitrendipine. Also, the responses to CaCl2 were greatly reduced or abolished in the presence of the Na+/Ca2+ exchanger inhibitors 1,3-dimethyl-2-thiourea, 3',4'-dichlorobenzamil, MgCl2, and amiloride or after omission of NaCl in the medium. Also, protein kinase C inhibitors, calphostin C and staurosporine, and tyrosine kinase inhibitors, genistein and tyrphostin 23, both reduced the responses to CaCl2. The inhibitory effect of protein kinase C inhibitor and tyrosine kinase were additive. These results suggest that NE releases Ca2+ from intracellular stores that are caffeine sensitive and partially sensitive to SERCA inhibitors. They indicate that in addition to Ca2+ influx via nitrendipine-sensitive and SKF-96365-sensitive channels, Na+/Ca2+ exchanger participates in the CaCl2-induced contraction produced in NE-exposed vessels. The pathway leading to Ca2+ entry probably involves tyrosine kinase and protein kinase C. All the above mechanisms require ongoing receptor stimulation.

Interactions between membrane potential and intracellular calcium concentration in vascular smooth muscle

The intracellular calcium concentration is a major determinant of vascular tone. In the steady state it is regulated mainly by membrane potential. At the same time, several mechanisms regulating the calcium concentration, including the membrane potential, are influenced by the intracellular calcium concentration itself. There are thus multiple possible positive and negative feedback loops involved in calcium regulation. This review gives a brief overview of the different mechanisms involved, including calcium-dependent ion channels, exchangers, and ATPases, and discusses their role in agonist-mediated responses, in relation primarily to studies on the portal vein and mesenteric small arteries.

Role of tyrosine phosphorylation in excitation-contraction coupling in vascular smooth muscle

Increasingly it is recognized that tyrosine phosphorylation plays an important part in the regulation of function in differentiated contractile vascular smooth muscle. Tyrosine kinases and phosphatases are present in large amounts in vascular smooth muscle and have been reported to influence a number of processes crucial to contraction, including ion channel gating, calcium homeostasis and sensitization of the contractile process to [Ca2+]i. This review summarizes current understanding regarding the role of tyrosine phosphorylation in excitation-contraction coupling in blood vessels.

Ca2+ channel-blocking activity of propranolol and betaxolol in isolated bovine retinal microartery

The relaxant action of the standard beta-blocker propranolol was compared with betaxolol, a beta-blocker with established vasorelaxant properties. Ring segments of bovine retinal microartery (n=36, theta=237 microm), which lacks adrenergic nerves and beta-adrenoceptors, were mounted in an organ bath for isometric force recording. l-, d-, dl-Propranolol and betaxolol were equally effective in relaxing tonic K+-induced contractions. The median effective dose (ED50) value was approximately 10(-5) M for both beta-blockers. The relaxation by both beta-blockers was unaffected by endothelium removal. Like verapamil, both beta-blockers induced smaller relaxation of tonic prostaglandin F2alpha (PGF2alpha)-induced force, which depended less on Ca2+ influx than did K+-induced force: K+-, but not PGF2alpha-induced contractions were abolished in Ca2+-free medium. The minor betaxolol-induced relaxation of tonic PGF2alpha-induced force was blocked in Ca2+-free medium. With repeated exposures to PGF2alpha in Ca2+-free medium, initial phasic PGF2alpha-induced force declined less with every exposure than did subsequent tonic force. When the preparations were briefly equilibrated with K+- and Ca2+-rich solution before every exposure to PGF2alpha phasic force did not decline, indicating that phasic force primarily depended on Ca2+ released from intracellular stores. Both beta-blockers failed to relax phasic PGF2alpha-induced force. Thus propranolol and betaxolol are equipotent vasorelaxant drugs in retinal microartery, both probably acting via Ca2+ channel blockade. This activity (that shows no stereospecificity) thus appears to be a more general property of beta-blockers. Microarteries might be more sensitive to this activity than are conductance arteries.

Mechanisms of Ca2+ sensitization of force production by noradrenaline in rat mesenteric small arteries

1. Mechanisms of Ca2+ sensitization of force production by noradrenaline were investigated by measuring contractile responses, intracellular Ca2+ concentration ([Ca2+]i) and phosphorylation of the myosin light chain (MLC) in intact and alpha-toxin-permeabilized rat mesenteric small arteries. 2. The effects of noradrenaline were investigated at constant membrane potential by comparing fully depolarized intact arteries in the absence and presence of noradrenaline. Contractile responses to K-PSS (125 mM K+) and NA-K-PSS (K-PSS + 10 microM noradrenaline) were titrated to 30 and 75%, respectively, of control force, by adjusting extracellular Ca2+ ([Ca2+]o). At both force levels, [Ca2+]i was substantially lower with NA-K-PSS than with K-PSS. With K-PSS, the proportion of MLC phosphorylated (approximately 30%) was similar at 30 and 75% of control force; with NA-K-PSS, MLC phosphorylation was greater at the higher force level (40 vs. 34%). 3. In alpha-toxin-permeabilized arteries, the force response to 1 microM Ca2+ was increased by 10 microM noradrenaline, and MLC phosphorylation was increased from 35 to 45%. The protein kinase C (PKC) inhibitor calphostin C (100 nM) abolished the noradrenaline-induced increase in MLC phosphorylation and contractile response, without affecting the contraction in response to Ca2+. Treatment with ATP gamma S in the presence of the MLC kinase inhibitor ML-9 increased the sensitivity to Ca2+ and abolished the response to noradrenaline. 4. The present results show that that in rat mesenteric small arteries noradrenaline-induced Ca2+ sensitization is associated with an increased proportion of phosphorylated MLC. The results are consistent with a decreased MLC phosphatase activity mediated through PKC. Furthermore, while MLC phosphorylation is a requirement for force production, the results show that other factors are also involved in force regulation.

Regulation of arterial diameter and wall [Ca2+] in cerebral arteries of rat by membrane potential and intravascular pressure

1. The regulation of intracellular [Ca2+] in the smooth muscle cells in the wall of small pressurized cerebral arteries (100-200 micron) of rat was studied using simultaneous digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Elevation of intravascular pressure (from 10 to 100 mmHg) caused a membrane depolarization from -63 +/- 1 to -36 +/- 2 mV, increased arterial wall [Ca2+] from 119 +/- 10 to 245 +/- 9 nM, and constricted the arteries from 208 +/- 10 micron (fully dilated, Ca2+ free) to 116 +/- 7 micron or by 45 % ('myogenic tone'). 3. Pressure-induced increases in arterial wall [Ca2+] and vasoconstriction were blocked by inhibitors of voltage-dependent Ca2+ channels (diltiazem and nisoldipine) or to the same extent by removal of external Ca2+. 4. At a steady pressure (i.e. under isobaric conditions at 60 mmHg), the membrane potential was stable at -45 +/- 1 mV, intracellular [Ca2+] was 190 +/- 10 nM, and arteries were constricted by 41 % (to 115 +/- 7 micron from 196 +/- 8 micron fully dilated). Under this condition of -45 +/- 5 mV at 60 mmHg, the voltage sensitivity of wall [Ca2+] and diameter were 7.5 nM mV-1 and 7.5 micron mV-1, respectively, resulting in a Ca2+ sensitivity of diameter of 1 mum nM-1. 5. Membrane potential depolarization from -58 to -23 mV caused pressurized arteries (to 60 mmHg) to constrict over their entire working range, i.e. from maximally dilated to constricted. This depolarization was associated with an elevation of arterial wall [Ca2+] from 124 +/- 7 to 347 +/- 12 nM. These increases in arterial wall [Ca2+] and vasoconstriction were blocked by L-type voltage-dependent Ca2+ channel inhibitors. 6. The relationship between arterial wall [Ca2+] and membrane potential was not significantly different under isobaric (60 mmHg) and non-isobaric conditions (10-100 mmHg), suggesting that intravascular pressure regulates arterial wall [Ca2+] through changes in membrane potential. 7. The results are consistent with the idea that intravascular pressure causes membrane potential depolarization, which opens voltage-dependent Ca2+ channels, acting as 'voltage sensors', thus increasing Ca2+ entry and arterial wall [Ca2+], which leads to vasoconstriction.

Action of ryanodine on neurogenic responses in rat isolated mesenteric small arteries

1. Rat mesenteric (approximately 250 microns) were set up in a single-channel isometric myograph designed to allow with 6 microM fura-2AM for 2 h and simultaneous recordings of neurogenic contraction (force) and intracellular calcium [Ca2+]i were obtained. In other experiments, arteries were loaded with 1 microCi ml-1 [3H]-noradrenaline (NA) for 30 min in order to measure release of [3H]-NA in response to field stimulation to examine whether ryanodine directly inhibited neuronal release of NA. 2. Arteries were activated by single intermittent field stimulation or continuously to excite intrinsic sympathetic nerves, or by cumulative addition of noradrenaline (1 nM-10 microM) to the bathing solution. 3. Pre-incubation with ryanodine markedly inhibited the contraction and [Ca2+]i release in response to single-pulse nerve stimulation. Ryanodine also inhibited an early phasic component of the response to continuous field stimulation and reduced the rate of rise in force in response to continuous field stimulation. However, stable maximal contraction and [Ca2+]i in response to continuous field stimulation as well as maximal responses to exogenous NA were unaffected. Release of [3H]-NA in response to single intermittent field stimulation was not affected by ryanodine when compared to vehicle. 4. Our results suggest that brief intermittent activation of intramural sympathetic nerves increases [Ca2+]i and contracts small arteries primarily by releasing Ca2+ from a ryanodine-sensitive intracellular store. In contrast, the stable rise in tone and [Ca2+]i resulting from continuous nerve stimulation may largely depend on sources of Ca2+ other than the ryanodine-sensitive intracellular store.

Interactions between neuropeptide Y and the adenylate cyclase pathway in rat mesenteric small arteries: role of membrane potential

1. Simultaneous measurements of membrane potential and tension were performed to investigate the intracellular mechanisms of neuropeptide Y (NPY) in rat mesenteric small arteries. 2. NPY (0.1 microM) depolarized arterial smooth muscle cells from -55 to -47 mV and increased wall tension by 0.22 N m-1, representing 11% of the contraction elicited by a high-potassium solution. Isoprenaline (1 microM) and acetylcholine (1 microM) evoked hyperpolarizations of 11 and 17 mV, respectively. NPY inhibited the isoprenaline-induced effects on membrane potential without affecting those of acetylcholine. 3. Forskolin evoked sustained concentration-dependent hyperpolarizations of small mesenteric arteries. NPY (0.1 microM) inhibited the responses to 1 microM forskolin, but did not alter the stable hyperpolarization elicited by the specific activator of protein kinase A (PKA) SP-5,6-DCl-cBIMPS (0.1 mM). Forskolin increased the cyclic AMP (cAMP) content of the arteries 21-fold, and NPY inhibited the forskolin-evoked increase in cAMP levels by 91%. 4. The hyperpolarization produced by 1 microM forskolin was not affected by either charybdotoxin (0.1 microM) or 4-aminopyridine (0.5 mM), but glibenclamide (5 microM) inhibited the hyperpolarization by 70%. Glibenclamide also inhibited the hyperpolarization evoked by SP-5,6-DCl-cBIMPS by 59%. 5. Neither depolarization nor contraction caused by NPY were significantly affected by either glibenclamide (5 microM) or nifedipine (1 microM), but they were reduced by gadolinium (10 microM). However, the blocking effect of NPY on forskolin-elicited hyperpolarization was not affected by gadolinium. 6. Charybdotoxin (0.1 microM) and 4-aminopyridine (0.5 mM) strongly enhanced the depolarization and contraction caused by NPY (0.1 microM), and nifedipine (1 microM) prevented the enhanced responses to NPY in the presence of charybdotoxin. 7. These findings suggest that NPY acts through at least two different intracellular mechanisms in mesenteric small arteries: a depolarization of arterial smooth muscle which is probably due to activation of non-selective cation channels, and a marked inhibition of adenylate cyclase activity, which in turn inhibits the hyperpolarization produced by cAMP accumulation in these arteries.

Contribution of alpha-adrenoceptors to depolarization and contraction evoked by continuous asynchronous sympathetic nerve activity in rat tail artery

1. The effects of continuous but asynchronous nerve activity induced by ciguatoxin (CTX-1) on the membrane potential and contraction of smooth muscle cells have been investigated in rat proximal tail arteries isolated in vitro. These effects have been compared with those produced by the continuous application of phenylephrine (PE). 2. CTX-1 (0.4 nM) and PE (10 microM) produced a maintained depolarization of the arterial smooth muscle that was almost completely blocked by alpha-adrenoceptor blockade. In both cases, the depolarization was more sensitive to the selective alpha-adrenoceptor antagonist, idazoxan (0.1 microM), than to the selective alpha 1-adrenoceptor antagonist, prazosin (0.01 microM). 3. In contrast, the maintained contraction of the tail artery induced by CTX-1 (0.2 nM) and PE (2 and 10 microM) was more sensitive to prazosin (0.01) microM, than to idazoxan (0.01 microM). In combination, these antagonists almost completely inhibited contraction to both agents. 4. Application of the calcium channel antagonist, nifedipine (1 microM), had no effect on the depolarization induced by either CTX-1 or PE but maximally reduced the force of the maintained contraction to both agents by about 50%. 5. We conclude that the constriction of the tail artery induced by CTX-1, which mimics the natural discharge of postganglionic perivascular axons, is due almost entirely to alpha-adrenoceptor activation. The results indicate that neuronally released noradrenaline activates more than one alpha-adrenoceptor subtype. The depolarization is dependent primarily on alpha 2-adrenoceptor activation whereas the contraction is dependent primarily on alpha 1-adrenoceptor activation. The links between alpha-adrenoceptor activation and the voltage-dependent and voltage-independent mechanisms that deliver Ca2+ to the contractile apparatus appear to be complex.

Inhibitory action of niflumic acid on noradrenaline- and 5-hydroxytryptamine-induced pressor responses in the isolated mesenteric vascular bed of the rat

1. The effects of niflumic acid, an inhibitor of calcium-activated chloride currents, were compared with the actions of the calcium channel blocker nifedipine on noradrenaline- and 5-hydroxytryptamine (5-HT)-induced pressor responses of the rat perfused isolated mesenteric vascular bed. 2. Bolus injections of noradrenaline (1 and 10 nmol) increased the perfusion pressure in a dose-dependent manner. Nifedipine (1 microM) inhibited the increase in pressure produced by 1 nmol noradrenaline by 31 +/- 5%. Niflumic acid (10 and 30 microM) also inhibited the noradrenaline-induced increase in perfusion pressure and 30 microM niflumic acid reduced the pressor response to 1 nmol noradrenaline by 34 +/- 6%. 3. The increases in perfusion elicited by 5-HT (0.3 and 3 nmol) were reduced by niflumic acid (10 and 30 microM) in a concentration-dependent manner and 30 microM niflumic acid inhibited responses to 0.3 and 3 nmol 5-HT by, respectively, 49 +/- 8% and 50 +/- 7%. Nifedipine (1 microM) decreased the pressor response to 3 nmol 5-HT by 44 +/- 9%. 4. In the presence of a combination of 30 microM niflumic acid and 1 microM nifedipine the inhibition of the pressor effects of noradrenaline (10 nmol) and 5-HT (3 nmol) was not significantly greater than with niflumic acid (30 microM) alone. Thus the effects of niflumic acid and nifedipine were not additive. 5. In Ca-free conditions the transient contractions induced by 5-HT (3 nmol) were not reduced by 30 microM niflumic acid, suggesting that this agent does not inhibit calcium release from the intracellular store or the binding of 5-HT to its receptor. 6. Niflumic acid 30 microM did not inhibit the pressor responses induced by KCl (20 and 60 mumol) which were markedly reduced by 1 microM nifedipine. In addition, 1 microM levcromakalim decreased pressor responses produced by 20 mumol KCl. These data suggest that niflumic acid does not block directly calcium channels or activate potassium channels. 7. It is concluded that niflumic acid selectively reduces a component of noradrenaline- and 5-HT-induced pressor responses by inhibiting a mechanism which leads to the opening of voltage-gated calcium channels. Our data suggest that the Ca(2+)-activated chloride conductance may play a pivotal role in the activation of voltage-gated calcium channels in agonist-induced constriction of resistance blood vessels.

Gap junctions in vascular tissues. Evaluating the role of intercellular communication in the modulation of vasomotor tone

Integration and coordination of responses among vascular wall cells are critical to the local modulation of vasomotor tone and to the maintenance of circulatory homeostasis. This article reviews the vast literature concerning the principles that govern the initiation and propagation of vasoactive stimuli among vascular smooth muscle cells, which are nominally the final effectors of vasomotor tone. In light of the abundance of new information concerning the distribution and function of gap junctions between vascular wall cells throughout the vascular tree, particular attention is paid to this integral aspect of vascular physiology. Evidence is provided for the important contribution of intercellular communication to vascular function at all levels of the circulation, from the largest elastic artery to the terminal arterioles. The thesis of this review is that the presence of gap junctions, in concert with the autonomic nervous system, pacemaker cells, myogenic mechanisms, and/or electrotonic current spread (both hyperpolarizing and depolarizing waves through gap junctions), confers a plasticity, adaptability, and flexibility to vasculature that may well account for the observed diversity in regulation and function of vascular tissues throughout the vascular tree. It is hoped that the summary information provided here will serve as a launching pad for a new discourse on the mechanistic basis of the integrative regulation and function of vasculature, which painstakingly accounts for the undoubtedly complex and manifold role of gap junctions in vascular physiology/dysfunction.

Tyrosine kinase signaling pathways modulate angiotensin II-induced calcium ([Ca2+]i) transients in vascular smooth muscle cells

Tyrosine kinases have been implicated in vascular smooth muscle cell proliferation and contraction. Underlying mechanisms may involve C(a2+) -dependent pathways. This study assesses relationships between angiotensin II (Ang II)-stimulated phospholipase C-mediated Ca2+ transients and tyrosine kinase-dependent pathways in vascular smooth muscle cells. Intracellular free Ca2+ concentration ([Ca2+]i) was measured in primary cultured unpassaged vascular smooth muscle cells derived from mesenteric resistance vessels of Wistar-Kyoto rats with the use of fura 2 methodology. [Ca2+]i effects of Ang II (1 nmol/L) were determined in vascular smooth muscle cells in which tyrosine kinase pathways were stimulated by insulin (70 muU/mL; 0.5 nmol/L), insulin-like growth factor-I (1 ng/mL; 0.13 nmol/L), or platelet-derived growth factor-BB (1 ng/mL; 0.04 nmol/L) and in cells in which tyrosine kinase was inhibited by specific inhibitors (1 mumol/L tyrphostin A-23 and genistein). Ang II elicited a rapid and transient [Ca2+]i response (from 94 +/- 8 to 239 +/- 5.8 nmol/L). Activation of the receptor tyrosine kinase by insulin, platelet-derived growth factor, and insulin-like growth factor-I significantly reduced (P < .01) Ang II-induced [Ca2+]i to 161 +/- 7, 189 +/- 3.7, and 183 +/- 5 nmol/L, respectively. In the presence of tyrphostin A-23 and genistein, Ang II-stimulated [Ca2+]i remained persistently elevated and failed to return to basal levels. Tyrphostin A-1, the inactive tyrphostin analogue, had not significant effect on Ang II-induced [Ca2+]i. This study demonstrates that activation of tyrosine kinase pathways reduces Ang II-elicited [Ca2+]i responses, whereas tyrosine kinase inhibition prevents [Ca2+]i recovery after agonist stimulation. Interaction between tyrosine kinase- and phospholipase C-dependent signaling pathways modulates vascular smooth muscle cell [Ca2+]i responses to Ang II.

Augmented calcium currents in mesenteric artery branches of the spontaneously hypertensive rat

The greater efficacy of organic channel blockers in lowering peripheral resistance and blood pressure in hypertensive subjects has been suggested to be the result of augmented calcium influx through L-type calcium channels in arterial smooth muscle. These studies were performed to determine whether differences exist in voltage-gated calcium channels of mesenteric artery branches from 20-week-old spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto rats (WKY). Single myocytes were acutely isolated by collagenase and elastase treatment and studied at room temperature (approximately 20 degrees C) with the use of whole-cell, patch-clamp methods. Maximum values of calcium current measured at 0 mV from a holding potential of -90 mV were larger in SHR myocytes (105 +/- 11 versus 149 +/- 15 pA). Values of cell capacitance were smaller in SHR (29.5 +/- 1.3 pF) compared with WKY (35.0 +/- 1.5 pF) myocytes. Cell capacitance measures surface membrane area and, when used to normalize calcium currents, magnified the difference between WKY and SHR to approximately 47%. There was a larger percent reduction of maximum calcium current at holding potentials of -60 and -40 mV in SHR compared with WKY myocytes: for example, at -40 mV calcium current was reduced from values at -90 mV by -73 +/- 2% in SHR compared with -58 +/- 1% in WKY. When divided by the maximum current for each holding potential, the voltage dependence of normalized calcium currents for the two groups was completely superimposed. Difference currents were calculated by subtracting currents measured from holding potentials of -90 and -40 mV. The voltage dependence of difference currents was identical to that of the calcium currents measured from the two values of holding potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation of arterial smooth muscle by calcium sparks

Local increases in intracellular calcium ion concentration ([Ca2+]i) resulting from activation of the ryanodine-sensitive calcium-release channel in the sarcoplasmic reticulum (SR) of smooth muscle cause arterial dilation. Ryanodine-sensitive, spontaneous local increases in [Ca2+]i (Ca2+ sparks) from the SR were observed just under the surface membrane of single smooth muscle cells from myogenic cerebral arteries. Ryanodine and thapsigargin inhibited Ca2+ sparks and Ca(2+)-dependent potassium (KCa) currents, suggesting that Ca2+ sparks activate KCa channels. Furthermore, KCa channels activated by Ca2+ sparks appeared to hyperpolarize and dilate pressurized myogenic arteries because ryanodine and thapsigargin depolarized and constricted these arteries to an extent similar to that produced by blockers of KCa channels. Ca2+ sparks indirectly cause vasodilation through activation of KCa channels, but have little direct effect on spatially averaged [Ca2+]i, which regulates contraction.

Electrical activity in rat tail artery during asynchronous activation of postganglionic nerve terminals by ciguatoxin-1

1. The effects of ciguatoxin-1 (CTX-1) on the membrane potential of smooth muscle cells have been examined in rat proximal tail arteries isolated in vitro. 2. CTX-1 (> or = 10 pM) increased the frequency of spontaneous excitatory junction potentials (s.e.j.ps). At 100-400 pM, there was also a marked and maintained depolarization (19.7 +/- 1.4 mV, n = 14, at 400 pM). 3. In 20-400 pM CTX-1, perivascular stimuli evoked excitatory junction potentials (e.j.ps) which were prolonged in time course relative to control. 4. Although threshold and latency of the e.j.p. were not affected by CTX-1 (< or = 400 pM), propagated impulses were blocked at > or = 100 pM. 5. The spontaneous activity and the depolarization produced by CTX-1 were reduced in the presence of Ca2+ (0.1 mM)/Mg2+ (25 mM), omega-conotoxin (0.1 microM) or Cd2+ (50-100 microM). 6. All effects of CTX-1 were abolished by tetrodotoxin (0.3 microM). 7. Raised Ca2+ (6 mM) reduced the depolarization and spontaneous activity produced by CTX-1. 8. In 400 pM CTX-1, the membrane repolarized (17 +/- 3.2 mV, n = 4) following the addition of phentolamine (1 microM). S.e.j.ps and e.j.ps were selectively abolished by suramin (1 mM), and the membrane repolarized by 1.3 +/- 1.6 mV (n = 4). 9. We conclude that CTX-1 releases noradrenaline and ATP by initiating asynchronous discharge of postganglionic perivascular axons. In 100-400 pM CTX-1, the smooth muscle was depolarized to levels resembling those recorded in this artery during ongoing vasoconstrictor discharge in vivo.

Effects of tyrosine kinase inhibitors on the contractility of rat mesenteric resistance arteries

1. A pharmacological characterization of tyrosine kinase inhibitors (TKI) belonging to two distinct groups (competitors at the ATP-binding site and the substrate-binding site, respectively) was performed, based on their effects on the contractility of rat mesenteric arteries. 2. Both the ATP-site competitors (genistein and its inactive analogue, daidzein) and the substrate-site competitors (tyrphostins A-23, A-47 and the inactive analogue, A-1) reversibly inhibited noradrenaline (NA, (10 microM)) and KCl (125 mM) induced contractions, concentration-dependently. Genistein was slightly but significantly more potent than daidzein; the tyrphostins were all less potent than genistein, and there were no significant differences between the individual potencies. The tyrosine kinase substrate-site inhibitor bis-tyrphostin had no inhibitory effect. 3. Genistein, daidzein, A-23 and A-47 each suppressed the contraction induced by Ca2+ (1 microM) in alpha-toxin permeabilized arteries. A-1 and bis-tyrphostin had little or no effect on contraction of the permeabilized arteries. 4. Genistein was significantly more potent than daidzein with respect to inhibition of the contraction induced by 200 nM Ca2+ in the presence of NA (100 microM) and GTP (3 microM). The effect of A-23, A-47, A-1 and bis-tyrphostin was similar in permeabilized arteries activated with Ca2+ (200 nM) + NA (100 microM) + GTP (3 microM) and permeabilized arteries activated with 1 microM Ca2+. 5. Genistein (30 microM) reduced the fura-2 measured intracellular calcium activity ([Ca2+]j) in arteries stimulated with NA but had no effect on [Ca2+]i in arteries stimulated with KCl (125 mM).6. The potent effect of the TKIs in this study is consistent with a role for tyrosine kinases in the mechanisms which regulate both cytoplasmic Ca2+ levels and the effect of Ca2+ on the contractile apparatus in smooth muscle cells in resistance arteries. However, the results must be interpreted cautiously because the enzyme inhibitors may have a poor specificity in intact tissues and because the presumed inactive analogues had potent effects.