Role of calcium-dependent K+ channels in the regulation of arterial and venous tone by nitric oxide in pigs

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Requirement for functional BK channels in maintaining oscillation in venomotor tone revealed by species differences in expression of the β1 accessory subunits

We determined the possible role of large-conductance Ca2+-activated K (BK) channels in regulation of venous tone in small capacitance veins and blood pressure. In rat mesenteric venous smooth muscle cells (MV SMC), BK channel α- and β1-subunits were coexpressed, unitary BK currents were detected, and single-channel currents were sensitive to voltage and [Ca2+]i. Rat MV SMCs displayed Ca sparks and iberiotoxin-sensitive spontaneous transient outward currents. Under resting conditions in vitro, rat MV exhibited nifedipine-sensitive spontaneous oscillatory constrictions. Blockade of BK channels by paxilline and Ca2+ sparks by ryanodine constricted rat MV. Nifedipine caused venodilation and blocked paxilline-induced, KCl-induced (20 mM), and BayK8644-induced contraction. Acute inhibition of BK channels with iberiotoxin in vivo increased blood pressure and reduced venous capacitance, measured as an increase in mean circulatory filling pressure in conscious rats. BK channel α-subunits and L-type Ca2+ channel α1-C subunits are expressed in murine MV. However, these channels are not functional because murine MV lack nifedipine-sensitive basal tone and rhythmic constrictions. Murine MV were also insensitive to paxilline, ryanodine, KCl, and BayK8644, consistent with our previous studies showing that murine MV do not have BK β1-subunits. These data show that not only there are species-dependent properties in ion channel control of venomotor tone but also BK channels are required for rhythmic oscillations in venous tone.

Large-conductance Ca2+-activated K+ channel beta1-subunit knockout mice are not hypertensive

Large-conductance Ca2+-activated K+ (BK) channels are composed of pore-forming α-subunits and accessory β1-subunits that modulate Ca2+ sensitivity. BK channels regulate arterial myogenic tone and renal Na+ clearance/K+ reabsorption. Previous studies using indirect or short-term blood pressure measurements found that BK channel β1-subunit knockout (BK β1-KO) mice were hypertensive. We evaluated 24-h mean arterial pressure (MAP) and heart rate in BK β1-KO mice using radiotelemetry. BK β1-KO mice did not have a higher 24-h average MAP when compared with wild-type (WT) mice, although MAP was ∼10 mmHg higher at night. The dose-dependent peak declines in MAP by nifedipine were only slightly larger in BK β1-KO mice. In BK β1-KO mice, giving 1% NaCl to mice to drink for 7 days caused a transient (5 days) elevation of MAP (∼5 mmHg); MAP returned to pre-saline levels by day 6. BK β1-KO mesenteric arteries in vitro demonstrated diminished contractile responses to paxilline, increased reactivity to Bay K 8644 and norepinephrine (NE), and maintained relaxation to isoproterenol. Paxilline and Bay K 8644 did not constrict WT or BK β1-KO mesenteric veins (MV). BK β1-subunits are not expressed in MV. The results indicate that BK β1-KO mice are not hypertensive on normal or high-salt intake. BK channel deficiency increases arterial reactivity to NE and L-type Ca2+ channel function in vitro, but the L-type Ca2+ channel modulation of MAP is not altered in BK β1-KO mice. BK and L-type Ca(2+) channels do not modulate murine venous tone. It appears that selective loss of BK channel function in arteries only is not sufficient to cause sustained hypertension.

Renovascular BKCachannels are not activated in vivo under resting conditions and during agonist stimulation

We investigated the role of large-conductance Ca2+-activated K+(BKCa) channels for the basal renal vascular tone in vivo. Furthermore, the possible buffering by BKCaof the vasoconstriction elicited by angiotensin II (ANG II) or norepinephrine (NE) was investigated. The possible activation of renal vascular BKCachannels by cAMP was investigated by infusing forskolin. Renal blood flow (RBF) was measured in vivo using electromagnetic flowmetry or ultrasonic Doppler. Renal preinfusion of tetraethylammonium (TEA; 3.0 μmol/min) caused a small reduction of baseline RBF, but iberiotoxin (IBT; 0.3 nmol/min) did not have any effect. Renal injection of ANG II (1–4 ng) or NE (10–40 ng) produced a transient decrease in RBF. These responses were not affected by preinfusion of TEA or IBT. Renal infusion of the BKCaopener NS-1619 (90.0 nmol/min) did not affect basal RBF or the response to NE, but it attenuated the response to ANG II. Coadministration of NS-1619 with TEA or IBT abolished this effect. Forskolin caused renal vasodilation that was not inhibited by IBT. The presence of BKCachannels in the preglomerular vessels was confirmed by immunohistochemistry. Despite their presence, there is no indication for a major role for BKCachannels in the control of basal renal tone in vivo. Furthermore, BKCachannels do not have a buffering effect on the rat renal vascular responses to ANG II and NE. The fact that NS-1619 attenuates the ANG II response indicates that the renal vascular BKCachannels can be activated under certain conditions.

Nitrite-derived nitric oxide: a possible mediator of 'acidic-metabolic' vasodilation

The fundamental, yet poorly understood, physiological mechanism known as 'acidic-metabolic' vasodilation, contributes to local blood flow regulation during hypoxia/ischaemia and increased metabolic activity. The vasodilator nitric oxide (NO) has been suggested to be involved in this event. Besides enzymatic production by NO synthases, a novel mechanism for generation of this gas in vivo was recently described. This involves non-enzymatic reduction of inorganic nitrite to NO, a reaction that takes place predominantly during acidic/reducing conditions. We have studied the effects of physiological amounts of nitrite on NO generation and relaxation of rat aorta in vitro in a situation where environmental pH was reduced to levels seen in tissues during hypoxia/ischaemia. The relaxatory effect of nitrite was increased in an acidic buffer solution (pH 6.6) compared with neutral pH; EC50 for nitrite was reduced from 200 to 40 microM. Nitrite-evoked relaxation was effectively prevented by coadministration of an inhibitor of soluble guanylyl cyclase. The relaxation was further potentiated by the addition of ascorbic acid. In parallel, NO was generated from nitrite in a pH dependent manner with even larger amounts seen after addition of ascorbic acid. NO generation from nitrite correlated to the the degree of relaxation of rat aorta. These results illustrate non-enzymatic release of NO from nitrite at physiological concentrations. This may be an important auto-regulated physiological mechanism involved in the regulation of vascular tone during hypoxia/ischaemia.

A catalytically inactive mutant of type I cGMP-dependent protein kinase prevents enhancement of large conductance, calcium-sensitive K+ channels by sodium nitroprusside and cGMP

The activation of large conductance, calcium-sensitive K(+) (BK(Ca)) channels by the nitric oxide (NO)/cyclic GMP (cGMP) signaling pathway appears to be an important cellular mechanism contributing to the relaxation of smooth muscle. In HEK 293 cells transiently transfected with BK(Ca) channels, we observed that the NO donor sodium nitroprusside and the membrane-permeable analog of cGMP, dibutyryl cGMP, were both able to enhance BK(Ca) channel activity 4-5-fold in cell-attached membrane patches. This enhancement correlated with an endogenous cGMP-dependent protein kinase activity and the presence of the alpha isoform of type I cGMP-dependent protein kinase (cGKI). We observed that co-transfection of cells with BK(Ca) channels and a catalytically inactive ("dead") mutant of human cGKIalpha prevented enhancement of BK(Ca) channel in response to either sodium nitroprusside or dibutyryl cGMP in a dominant negative fashion. In contrast, expression of wild-type cGKIalpha supported enhancement of channel activity by these two agents. Importantly, both endogenous and expressed forms of cGKIalpha were found to associate with BK(Ca) channel protein, as demonstrated by a reciprocal co-immunoprecipitation strategy. In vitro, cGKIalpha was able to directly phosphorylate immunoprecipitated BK(Ca) channels, suggesting that cGKIalpha-dependent phosphorylation of BK(Ca) channels in situ may be responsible for the observed enhancement of channel activity. In summary, our data demonstrate that cGKIalpha alone is sufficient to promote the enhancement of BK(Ca) channels in situ after activation of the NO/cGMP signaling pathway.

Effect of hypercholesterolemia on Ca(2+)-dependent K(+) channel-mediated vasodilatation in vivo

Nitric oxide (NO)-mediated and NO-independent mechanisms of endothelium-dependent vasodilatation involve Ca(2+)-dependent K(+) (K(Ca)) channels. We examined the role in vivo of K(Ca) channels in NO-independent vasodilatation in hypercholesterolemia. Hindlimb vascular conductance was measured at rest and after aortic injection of ACh, bradykinin (BK), and sodium nitroprusside in anesthetized control and cholesterol-fed rabbits. Conductances were measured before and after treatment with the NO synthase antagonist N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) or K(Ca) blockers tetraethylammonium (30 mg/kg), charybdotoxin (10 microgram/kg), and apamin (50 microgram/kg). The contribution of NO to basal conductance was greater in control than in cholesterol-fed rabbits [2.2 +/- 0.4 vs. 1.1 +/- 0.3 (SE) ml. min(-1). kg(-1). 100 mmHg(-1), P < 0.05], but the NO-independent K(Ca) channel-mediated component was greater in the cholesterol-fed than in the control group (1.1 + 0.4 vs. 0.3 +/- 0.1 ml. min(-1). kg(-1). 100 mmHg(-1), P < 0.05). Maximum conductance response to ACh and BK was less in cholesterol-fed than in control rabbits, and the difference persisted after L-NAME (ACh: 7.7 +/- 0.7 vs. 10.1 +/- 0.5 ml. min(-1). kg(-1). 100 mmHg(-1), P < 0.005). Blockade of K(Ca) channels with tetraethylammonium or charybdotoxin + apamin almost completely abolished L-NAME-resistant vasodilatation after ACh or BK. The magnitude of K(Ca)-mediated vasodilatation after ACh or BK was impaired in hypercholesterolemic rabbits. Vasodilator responses to nitroprusside did not differ between groups. In vivo, hypercholesterolemia is associated with an altered balance between NO-mediated and NO-independent K(Ca) channel contributions to resting vasomotor tone and impairment of both mechanisms of endothelium-dependent vasodilatation.

Inverse relation between aldosterone and venous capacitance in chronically treated congestive heart failure

The purpose of this study was to examine if there is a relation between the aldosterone escape phenomenon and venous capacitance of the upper and lower limbs in patients with long-term congestive heart failure (CHF) receiving chronic treatment with angiotensin-converting enzyme (ACE) inhibitors. The study group consisted of 16 subjects with ischemic CHF in New York Heart Association functional class II (age 59 +/-2 years, ejection fraction 24+/-4%), stabilized under a constant drug regimen comprising furosemide, captopril 50 mg 3 times daily, and digoxin for at least 3 months. Thirteen apparently healthy volunteers, aged 50+/-4 years acted as controls. Forearm and calf venous capacitances were measured simultaneously by venous occlusion plethysmography using mercury-in-silastic strain gauges. The equilibration technique was used to derive venous capacitance from the recorded pressure-volume curves. Active renin, angiotensin II, and aldosterone levels were determined on venous blood samples obtained in the supine position. Angiotensin II (p<0.05) and aldosterone (p<0.01) were statistically significantly higher in patients with CHF under long-term ACE inhibition than in controls (aldosterone escape phenomenon). In CHF, forearm venous capacitance was 2.19+/-0.18 ml/100 ml; calf venous capacitance was 2.83+/-0.27 ml/100 ml. Aldosterone significantly and inversely correlated with venous capacitance in both upper (r = -0.586; p = 0.017) and lower (r = -0.625; p = 0.01) limbs. No correlations were found between forearm or calf venous capacitance and renin or angiotensin II. In patients with heart failure chronically treated with diuretics and full ACE inhibition, venous capacitance is inversely correlated with aldosterone through the mechanism of aldosterone escape, creating the potential for further deterioration of the CHF process.

Increased vascular responsiveness to alpha 2-adrenergic stimulation during NOS inhibition-induced hypertension

Increased vascular resistance during systemic nitric oxide synthase (NOS) inhibition is dependent on adrenergic vasoconstriction. This study tested the hypothesis that increased vascular sensitivity to adrenergic agonists contributes to this vasoconstriction. Superior mesenteric arteries and thoracic aortae from male Sprague-Dawley rats drinking water containing N omega-nitro-L-arginine (L-NNA; 14 days, 60 mg.kg-1.day-1) and control rats were-cut into helical strips, and endothelium was removed for contractile experiments. L-NNA arteries were more sensitive to UK-14304 (alpha 2-adrenergic agonist) and norepinephrine (NE), whereas responses to phenylephrine (PE) were not different concentration causing 50% maximal response (EC50), L-NNA vs. control: UK-14304, 0.071 vs. 0.71 mumol/l; NE, 1.15 vs. 9.95 nmol/l]. Yohimbine, an alpha 2-selective antagonist, caused a concentration-dependent inhibition of contraction to NE only in L-NNA arteries (EC50 = 6.3 vs. 1.6 nmol/l at 1 nmol/l yohimbine), whereas prazosin shifted NE curves similarly in arteries from both groups. Yohimbine (10 nmol/l) inhibited contractions to UK-14304 (EC50 = 59 mumol/l vs. 17 mumol/l) but not contractions to PE, whereas prazosin inhibited both. These data indicate that L-NNA-induced hypertension leads to increased sensitivity of prazosin-sensitive alpha 2-adrenoceptors, an upregulation that could cause the increased vasoconstrictor response to NE in this model of hypertension.

The involvement of ATP-sensitive potassium channels in beta 2-adrenoceptor agonist-induced vasodilatation on rat diaphragmatic microcirculation

1. The effects of glibenclamide (GLB), a blocker of ATP-sensitive potassium (KATP) channels, on diaphragmatic microcirculation in male Sprague-Dawley rats were assessed under basal conditions and after beta 2-adrenoceptor-agonist stimulation. In addition, forskolin was used to bypass beta-adrenoceptors and GTP-binding proteins (G-protein) to explore the possible mechanism of GLB effects. For comparison, the relationships between KATP channel activity and cyclic GMP-mediated vasodilator responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were also assessed. 2. Male Sprague-Dawley rats were anaesthetized with urethane and mechanically ventilated. The left hemi-diaphragm of each rat was prepared and microvascular blood flow (QLDF) was recorded with laser-Doppler flowmetry during continuous superfusion with bicarbonate-buffered, prewarmed Ringer solution. The drugs were topically applied to the surface of the hemi-diaphragm. 3. Salbutamol (0.32-32 microM), terbutaline (0.32 microM-0.32 microM) and forskolin (0.32-10 microM) each elicited a concentration-dependent increase in QLDF. 4. Baseline microvascular blood flow was unaffected by a 30 min suffusion of 1 microM GLB (295 +/- 51 mV vs 325 +/- 62 mV. P = 0.738). 5. The vasodilator response elicited by salbutamol (0.32 microM, 1 microM and 3.2 microM and 10 microM), was significantly attenuated by a 30 min superfusion with 1 microM GLB; this salbutamol-induced vasodilatation was mediated via an interaction with beta-adrenoceptor receptors, as in other experiments it was greatly inhibited by 30-min superfusion with propranolol (10 microM). 6. Similarly, following 30-min superfusion with GLB (1 microM), the terbutaline (1 microM, 3.2 microM and 10 microM)-induced vasodilator response was almost abolished and the vasodilator responses induced by incremental concentrations of forskolin (0.32 microM, 1 microM and 3.2 microM) were also significantly attenuated. 7. Cromakalim (1.5 microM, 3 microM and 3.2 microM) produced an increase of QLDF in a dose-dependent manner, which was virtually abolished by GLB (1 microM). In contrast, the vasodilator responses induced by acetylcholine (32 microM, 0.1 mM, and 0.32 mM) or sodium nitroprusside (3.2 microM, 10 microM and 20 microM) were independent of GLB (1 microM). 8. In conclusion, KATP channels may be functional, but tonically inactive in the resting diaphragmatic microcirculation and the vasodilator effect of beta 2-adrenoceptor agonists may be partly mediated by KATP channels; the activation of KATP channels may involve the accumulation of cyclic AMP in vascular smooth muscle cells.