L-type calcium channels in vascular smooth muscle cells from spontaneously hypertensive rats: effects of calcium agonist and antagonist

Vascular Smooth Muscle Remodeling in Conductive and Resistance Arteries in Hypertension

Cardiovascular disease is a leading cause of death worldwide and accounts for >17.3 million deaths per year, with an estimated increase in incidence to 23.6 million by 2030. ¹ Cardiovascular death represents 31% of all global deaths ² -with stroke, heart attack, and ruptured aneurysms predominantly contributing to these high mortality rates. A key risk factor for cardiovascular disease is hypertension. Although treatment or reduction in hypertension can prevent the onset of cardiovascular events, existing therapies are only partially effective. A key pathological hallmark of hypertension is increased peripheral vascular resistance because of structural and functional changes in large (conductive) and small (resistance) arteries. In this review, we discuss the clinical implications of vascular remodeling, compare the differences between vascular smooth muscle cell remodeling in conductive and resistance arteries, discuss the genetic factors associated with vascular smooth muscle cell function in hypertensive patients, and provide a prospective assessment of current and future research and pharmacological targets for the treatment of hypertension.

Contribution of electromechanical coupling between Kv and Ca v1.2 channels to coronary dysfunction in obesity

Previous investigations indicate that diminished functional expression of voltage-dependent K(+) (KV) channels impairs control of coronary blood flow in obesity/metabolic syndrome. The goal of this investigation was to test the hypothesis that KV channels are electromechanically coupled to CaV1.2 channels and that coronary microvascular dysfunction in obesity is related to subsequent increases in CaV1.2 channel activity. Initial studies revealed that inhibition of KV channels with 4-aminopyridine (4AP, 0.3 mM) increased intracellular [Ca(2+)], contracted isolated coronary arterioles and decreased coronary reactive hyperemia. These effects were reversed by blockade of CaV1.2 channels. Further studies in chronically instrumented Ossabaw swine showed that inhibition of CaV1.2 channels with nifedipine (10 μg/kg, iv) had no effect on coronary blood flow at rest or during exercise in lean swine. However, inhibition of CaV1.2 channels significantly increased coronary blood flow, conductance, and the balance between coronary flow and metabolism in obese swine (P < 0.05). These changes were associated with a ~50 % increase in inward CaV1.2 current and elevations in expression of the pore-forming subunit (α1c) of CaV1.2 channels in coronary smooth muscle cells from obese swine. Taken together, these findings indicate that electromechanical coupling between KV and CaV1.2 channels is involved in the regulation of coronary vasomotor tone and that increases in CaV1.2 channel activity contribute to coronary microvascular dysfunction in the setting of obesity.

Nifedipine-sensitive noradrenergic vasoconstriction is enhanced in spontaneously hypertensive rats: the influence of chronic captopril treatment

AIM

The relationship between increased sympathetic tone and enhanced activity of L-type voltage-dependent Ca2+ channels (L-VDCC) in spontaneously hypertensive rats (SHR) was studied using in vivo and in vitro approaches.

METHODS

The effects of acute L-VDCC blockade on sympathetic vasoconstriction or blood pressure (BP) and the contribution of calcium influx to norepinephrine (NE)-induced arterial contraction were investigated in 10-week-old SHR and in age-matched SHR made normotensive by chronic captopril treatment from weaning.

RESULTS

Blood pressure fall occurring after acute ganglionic or L-VDCC blockade was enhanced in SHR. Ganglionic blockade eliminated strain differences in BP response to acute L-VDCC blockade and vice versa, suggesting that enhanced contribution of L-VDCC is responsible for augmented sympathetic vasoconstriction in SHR. Both phasic (dependent on internal calcium stores) and tonic (dependent on calcium influx) contractions to NE were augmented in SHR femoral arteries in vitro. Nifedipine attenuated only tonic contractions but to a larger extent in SHR than in WKY arteries. Nifedipine effect was greater after endothelium removal, which augmented tonic but not phasic contractions after NE. Chronic captopril treatment of SHR prevented hypertension development by suppression of their sympathetic vasoconstriction including its nifedipine-sensitive component, but failed to influence enhanced NE-induced arterial contractions or increased relaxation to nifedipine in vitro.

CONCLUSION

The contribution of nifedipine-sensitive component to noradrenergic vasoconstriction is enhanced during excessive NE stimulation (increased sympathetic tone of SHR in vivo or supramaximal NE stimulation in vitro). It seems that captopril-induced reduction of central sympathetic tone is able to normalize augmented nifedipine-sensitive vasoconstriction in SHR.

Role of voltage-sensitive calcium-channels in nitric oxide-mediated vasodilation in spontaneously hypertensive rats

This study demonstrates that the vasodilator potencies of nitric oxide (NO) donors such as sodium nitroprusside are increased in conscious Spontaneously Hypertensive (SH) as compared to Wistar Kyoto (WKY) rats. For example, the NO donors do not dilate hindlimb resistance arteries in WKY rats whereas they elicit pronounced vasodilator responses in SH rats. This study also demonstrates that the NO-mediated vasodilator responses in WKY and SH rats were markedly diminished after blockade of voltage-sensitive Ca2+-channels (CaVS2+-channels) with nifedipine, diltiazem or verapamil. These findings suggest that NO dilates resistance arteries in vivo via direct and/or hyperpolarization-induced closure of CaVS2+-channels and that the increased potency of NO in SH rats may be due to the augmented CaVS2+-channel activity reported in this strain.

Vascular smooth muscle cell membrane depolarization after NOS inhibition hypertension

Nitric oxide (NO) synthase (NOS) inhibition with N(omega)-nitro-L-arginine (L-NNA) produces L-NNA hypertensive rats (LHR), which exhibit increased sensitivity to voltage-dependent Ca(2+) channel-mediated vasoconstriction. We hypothesized that enhanced contractile responsiveness after NOS inhibition is mediated by depolarization of membrane potential (E(m)) through attenuated K(+) channel conductance. E(m) measurements demonstrated that LHR vascular smooth muscle cells (VSMCs) are depolarized in open, nonpressurized (-44.5 +/- 1.0 mV in control vs. -36.8 +/- 0.8 mV in LHR) and pressurized mesenteric artery segments (-41.8 +/- 1.0 mV in control vs. -32.6 +/- 1.4 mV in LHR). Endothelium removal or exogenous L-NNA depolarized control VSMCs but not LHR VSMCs. Superfused L-arginine hyperpolarized VSMCs from both the control and LHR groups and reversed L-NNA-induced depolarization (-44.5 +/- 1.0 vs. -45.8 +/- 2.1 mV). A Ca(2+)-activated K(+) channel agonist, NS-1619 (10 microM), hyperpolarized both groups of arteries to a similar extent (from -50.8 +/- 1.0 to -62.5 +/- 1.2 mV in control and from -43.7 +/- 1.1 to -55.6 +/- 1.2 mV in LHR), although E(m) was still different in the presence of NS-1619. In addition, superfused iberiotoxin (50 nM) depolarized both groups similarly. Increasing the extracellular K(+) concentration from 1.2 to 45 mM depolarized E(m), as predicted by the Goldman-Hodgkin-Katz equation. These data support the hypothesis that loss of NO activation of K(+) channels contributes to VSMC depolarization in L-NNA-induced hypertension without a change in the number of functional large conductance Ca(2+)-activated K(+) channels.

Contribution of sarcoplasmic reticulum Ca2+ to the activation of Ca2+ -activated K+ channels in the resting state of arteries from spontaneously hypertensive rats

OBJECTIVE

Localized release of Ca2+ from the sarcoplasmic reticulum (SR) toward the plasmalemma, sometimes visualized as Ca2+ sparks, can activate Ca2+-activated K+ (KCa) channels. We have already reported that the addition of charybdotoxin (ChTX), a blocker of KCa channels, to the resting state of arteries from spontaneously hypertensive rats (SHR) caused a powerful contraction, suggesting that KCa channels were active in the resting state. This study aimed to determine whether the Ca2+ responsible for activity of KCa channels was derived from SR.

METHODS

Possible mechanisms underlying the ChTX-induced contractions were examined in endothelium-denuded strips of femoral, mesenteric, small mesenteric and carotid arteries from 13-week-old SHR and normotensive Wistar-Kyoto (WKY) rats by using selective inhibitors of the Ca2+ spark process.

RESULTS

ChTX (100 nmol/l) induced a contraction in the SHR arteries. The ChTX-induced contractions were increased by a moderate membrane depolarization by 15.9 mmol/l K+ and were abolished by nifedipine (100 nmol/l). When SR Ca2+ was depleted by treatment of the strips with ryanodine (10 mumol/l) plus caffeine (20 mmol/l) or with thapsigargin (100 nmol/l), the ChTX-induced contraction was decreased in femoral, mesenteric and small mesenteric arteries and was almost abolished in the carotid artery. A similar phenomenon can be observed in arteries from WKY rats after a moderate membrane depolarization. In both SHR and WKY rats, SR Ca2+-dependent ChTX-induced contraction always represents 20-30% of the maximal K+-induced contraction.

CONCLUSIONS

We conclude that activation of KCa channels depended upon influx of Ca2+ through L-type Ca2+ channels and release of Ca2+ from the SR, suggesting that recycling of entering Ca2+ from the superficial SR toward the plasmalemma sufficiently elevated Ca2+ near these channels to activate them.

Calcium buffering of resting, voltage-dependent Ca2+ influx by sarcoplasmic reticulum in femoral arteries from spontaneously hypertensive rats at prehypertensive stage

We examined the Ca2+-buffering function of the sarcoplasmic reticulum (SR) in the resting state of arteries from spontaneously hypertensive rats (SHR) at a prehypertensive stage. Differences in the effects of cyclopiazonic acid (CPA) and thapsigargin, agents that inhibit SR Ca2+-ATPase, and of ryanodine, which depletes SR Ca2+, on tension and cellular Ca2+ level were assessed in endothelium-denuded strips of femoral arteries from 4-week-old SHR and normotensive Wistar-Kyoto rats (WKY). Addition of CPA, thapsigargin or ryanodine to the resting state of the strips caused an elevation of cytosolic Ca2+ level and a contraction in both WKY and SHR. These responses were larger in SHR than in WKY. The contractions were inhibited strongly by 100 nM nifedipine or 3 microM verapamil and were abolished by Ca2+-free solution. Nifedipine, verapamil or Ca2+-free solution itself caused a relaxation from the resting state of SHR strips, but not from that of WKY strips. The resting Ca2+ influx in arteries measured by a 5-min incubation with 45Ca was significantly larger in SHR than in WKY. This influx was decreased by 10 microM CPA or 10 microM ryanodine in both WKY and SHR. These results suggest that in the resting state of the femoral artery from 4-week-old SHR, the greater part of the increased Ca2+ influx via L-type Ca2+ channels is buffered by Ca2+ uptake into the SR, while some Ca2+ reaches the myofilaments, resulting in the maintenance of resting tone.

Gender-specific reduction in contractility and [Ca(2+)](i) in vascular smooth muscle cells of female rat

The hypothesis that vascular protection in females and its absence in males reflects gender differences in [Ca(2+)](i) and Ca(2+) mobilization mechanisms of vascular smooth muscle contraction was tested in fura 2-loaded aortic smooth muscle cells isolated from intact and gonadectomized male and female Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In WKY cells incubated in Hanks' solution (1 mM Ca(2+)), the resting length and [Ca(2+)](i) were significantly different in intact males (64.5 +/- 1.2 microm and 83 +/- 3 nM) than in intact females (76.5 +/- 1.5 microm and 64 +/- 7 nM). In intact male WKY, phenylephrine (Phe, 10(-5) M) caused transient increase in [Ca(2+)](i) to 428 +/- 13 nM followed by maintained increase to 201 +/- 8 nM and 32% cell contraction. In intact female WKY, the Phe-induced [Ca(2+)](i) transient was not significantly different, but the maintained [Ca(2+)](i) (159 +/- 7 nM) and cell contraction (26%) were significantly less than in intact male WKY. In Ca(2+)-free (2 mM EGTA) Hanks', Phe and caffeine (10 mM) caused transient increases in [Ca(2+)](i) and contraction that were not significantly different between males and females. Membrane depolarization by 51 mM KCl caused 31% cell contraction and increased [Ca(2+)](i) to 259 +/- 9 nM in intact male WKY, which were significantly greater than a 24% contraction and 214 +/- 8 nM [Ca(2+)](i) in intact female WKY. Maintained Phe- and KCl-stimulated cell contraction and [Ca(2+)](i) were significantly greater in SHR than WKY in all groups of rats. Reduction in cell contraction and [Ca(2+)](i) in intact females compared with intact males was significantly greater in SHR ( approximately 30%) than WKY ( approximately 20%). No significant differences in cell contraction or [Ca(2+)](i) were observed between castrated males, ovariectomized (OVX) females, and intact males, or between OVX females with 17beta-estradiol implants and intact females. Exogenous application of 17beta-estradiol (10(-8) M) to cells from OVX females caused greater reduction in Phe- and KCl-induced contraction and [Ca(2+)](i) in SHR than WKY. Thus the basal, maintained Phe- and depolarization-induced [Ca(2+)](i) and contraction of vascular smooth muscle triggered by Ca(2+) entry from the extracellular space exhibit differences depending on gender and the presence or absence of female gonads. Cell contraction and [Ca(2+)](i) due to Ca(2+) release from the intracellular stores are not affected by gender or gonadectomy. Gender-specific reduction in contractility and [Ca(2+)](i) in vascular smooth muscle of female rats is greater in SHR than WKY rats.

Angiotensin II mediates pressure loading-induced mitogen-activated protein kinase activation in isolated rat aorta

Vascular hypertrophy occurs during chronic hypertension and contributes to the elevation of peripheral vascular resistance in hypertension. In this study, we examined whether acute pressure overloading of the vascular wall produces activation of mitogen-activated protein (MAP) kinases, enzymes believed to be involved in the pathway for cell proliferation, in isolated perfused rat aortae, and examined whether the mechanical overloading-induced MAP kinase activation is mediated via the vascular angiotensin system. Aortae were perfused with Tyrode solution. Increases in perfusion pressure caused a pressure-dependent increase in MAP kinase activity in endothelium-intact aortae and in endothelium-denuded aortae. The increase in MAP kinase activity induced by pressure loading was inhibited by the angiotensin receptor antagonist, losartan, the renin inhibitor, pepstatin A, and the angiotensin-converting enzyme inhibitor, captopril. Ca(2+) depletion and the Ca(2+) channel antagonist, nifedipine, did not affect the pressure loading-induced MAP kinase activation. The results of the present study suggest that pressure loading of the vascular wall per se can activate MAP kinases in the vasculature and that the MAP kinase activation is mediated at least partly via the vascular angiotensin system. It seems unlikely that the pressure loading-induced increase in MAP kinase activity is mainly mediated via increases in Ca(2+) influx in vascular cells.

Gender differences in Ca(2+) entry mechanisms of vasoconstriction in Wistar-Kyoto and spontaneously hypertensive rats

We investigated whether putative vascular protection against hypertension in females reflects differences in the Ca(2+) mobilization mechanisms of vasoconstriction depending on the gender and the status of the gonads. Active stress and (45)Ca(2+) influx were measured in aortic strips isolated from intact and gonadectomized male and female Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). In aortic strips of intact male WKY incubated in normal Krebs' solution (2.5 mmol/L Ca(2+)), both phenylephrine (10(-5) mol/L) and membrane depolarization by 96 mmol/L KCl caused significant increases in active stress and (45)Ca(2+) influx. In intact female WKY, the phenylephrine- and KCl-induced stress and (45)Ca(2+) influx were significantly reduced. In Ca(2+)-free (2 mmol/L EGTA) Krebs' solution, stimulation of aortic strips with phenylephrine or caffeine (25 mmol/L) to induce Ca(2+) release from intracellular stores caused a transient increase in stress that was not significantly different between males and females. In SHR, the phenylephrine- and KCl-induced stress and (45)Ca(2+) influx were significantly greater than those in WKY in all groups of rats. The reduction in stress and Ca(2+) entry in intact females compared with intact males was greater in SHR than in WKY. The contractile responses and Ca(2+) entry in castrated male and ovariectomized female WKY or SHR were not significantly different from the respective responses in intact males. The contractile responses and Ca(2+) entry in ovariectomized female WKY or SHR with 17beta-estradiol implant were not significantly different from the respective responses in intact females. Thus, the phenylephrine- and depolarization-induced vascular reactivity and Ca(2+) entry in vascular smooth muscle are dependent on gender and on the presence or absence of functional female gonads. Ca(2+) release from intracellular stores is not affected by gender or gonadectomy. The gender-specific changes in vascular reactivity and Ca(2+) entry are augmented in hypertension.