Potassium channels modulate cerebral autoregulation during acute hypertension

Impact of impaired cerebral blood flow autoregulation on cognitive impairment

Although the causes of cognitive impairment are multifactorial, emerging evidence indicates that cerebrovascular dysfunction plays an essential role in dementia. One of the most critical aspects of cerebrovascular dysfunction is autoregulation of cerebral blood flow (CBF), mainly mediated by the myogenic response, which is often impaired in dementia individuals with comorbidities, such as diabetes and hypertension. However, many unsolved questions remain. How do cerebrovascular networks coordinately modulate CBF autoregulation in health and disease? Does poor CBF autoregulation have an impact on cognitive impairment, and what are the underlying mechanisms? This review summarizes the cerebral vascular structure and myogenic (a three-phase model), metabolic (O2, CO2, adenosine, and H+), and endothelial (shear stress) factors in the regulation of CBF; and the consequences of CBF dysautoregulation. Other factors contributing to cerebrovascular dysfunction, such as impaired functional hyperemia and capillary abnormalities, are included as well. Moreover, this review highlights recent studies from our lab in terms of novel mechanisms involved in CBF autoregulation and addresses a hypothesis that there is a three-line of defense for CBF autoregulation in the cerebral vasculature.

Steady-state cerebral autoregulation in older adults with amnestic mild cognitive impairment: linear mixed model analysis

We examined whether the efficacy of steady-state cerebral autoregulation (CA) is reduced in older adults with amnestic mild cognitive impairment (aMCI), a prodromal stage of clinical Alzheimer disease (AD). Forty-two patients with aMCI and 24 cognitively normal older adults (NC) of similar age, sex, and education underwent stepwise decreases and increases in mean arterial pressure (MAP) induced by intravenous infusion of sodium nitroprusside and phenylephrine, respectively. Changes in cerebral blood flow (CBF) were measured repeatedly in the internal carotid and vertebral artery. Linear mixed modeling, including random effects of both individual intercept and regression slope, was used to quantify the MAP-CBF relationship accounting for nonindependent, repeated CBF measures. Changes in end-tidal CO₂ (EtCO₂) associated with changes in MAP were also included in the model to account for their effects on CBF. Marginal mean values of MAP were reduced by 13-14 mmHg during sodium nitroprusside and increased by 20-24 mmHg during phenylephrine infusion in both groups with similar doses of drug infusion. A steeper slope of changes in CBF in response to changes in MAP was observed in aMCI relative to NC, indicating reduced efficacy of CA (MAP × Group, P = 0.040). These findings suggest that cerebrovascular dysfunction may occur early in the development of AD.NEW & NOTEWORTHY Cerebral autoregulation is a fundamental regulatory mechanism to protect brain perfusion against changes in blood pressure that, if impaired, may contribute to the development of Alzheimer's disease. Using a linear mixed model, we demonstrated that the efficacy of cerebral autoregulation, assessed during stepwise changes in arterial pressure, was reduced in individuals with amnestic mild cognitive impairment, a prodromal stage of Alzheimer's disease. These findings support the hypothesis that cerebrovascular dysfunction may be an important underlying pathophysiological mechanism for the development of clinical Alzheimer's disease.

Traumatic Brain Injury Impairs Myogenic Constriction of Cerebral Arteries: Role of Mitochondria-Derived H2O2 and TRPV4-Dependent Activation of BKca Channels

Traumatic brain injury (TBI) impairs autoregulation of cerebral blood flow, which contributes to the development of secondary brain injury, increasing mortality of patients. Impairment of pressure-induced myogenic constriction of cerebral arteries plays a critical role in autoregulatory dysfunction; however, the underlying cellular and molecular mechanisms are not well understood. To determine the role of mitochondria-derived H₂O₂ and large-conductance calcium-activated potassium channels (BK_Ca) in myogenic autoregulatory dysfunction, middle cerebral arteries (MCAs) were isolated from rats with severe weight drop-impact acceleration brain injury. We found that 24 h post-TBI MCAs exhibited impaired myogenic constriction, which was restored by treatment with a mitochondria-targeted antioxidant (mitoTEMPO), by scavenging of H₂O₂ (polyethylene glycol [PEG]-catalase) and by blocking both BK_Ca channels (paxilline) and transient receptor potential cation channel subfamily V member 4 (TRPV4) channels (HC 067047). Further, exogenous administration of H₂O₂ elicited significant dilation of MCAs, which was inhibited by blocking either BK_Ca or TRPV4 channels. Vasodilation induced by the TRPV4 agonist GSK1016790A was inhibited by paxilline. In cultured vascular smooth muscle cells H₂O₂ activated BK_Ca currents, which were inhibited by blockade of TRPV4 channels. Collectively, our results suggest that after TBI, excessive mitochondria-derived H₂O₂ activates BK_Ca channels via a TRPV4-dependent pathway in the vascular smooth muscle cells, which impairs pressure-induced constriction of cerebral arteries. Future studies should elucidate the therapeutic potential of pharmacological targeting of this pathway in TBI, to restore autoregulatory function in order to prevent secondary brain damage and decrease mortality.

Relationship between cognitive function and regulation of cerebral blood flow

Ageing is the primary risk factor for cognitive deterioration. Given that the cerebral blood flow (CBF) or regulation of cerebral circulation is attenuated in the elderly, it could be expected that ageing-induced cognitive deterioration may be affected by a decrease in CBF as a result of brain ischemia and energy depletion. CBF regulation associated with cerebral metabolism thus likely plays an important role in the preservation of cognitive function. However, in some specific conditions (e.g. during exercise), change in CBF does not synchronize with that of cerebral metabolism. Our recent study demonstrated that cognitive function was more strongly affected by changes in cerebral metabolism than by changes in CBF during exercise. Therefore, it remains unclear how an alteration in CBF or its regulation affects cognitive function. In this review, I summarize current knowledge on previous investigations providing the possibility of an interaction between regulation of CBF or cerebral metabolism and cognitive function.

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.

Functional vascular contributions to cognitive impairment and dementia: mechanisms and consequences of cerebral autoregulatory dysfunction, endothelial impairment, and neurovascular uncoupling in aging

Increasing evidence from epidemiological, clinical and experimental studies indicate that age-related cerebromicrovascular dysfunction and microcirculatory damage play critical roles in the pathogenesis of many types of dementia in the elderly, including Alzheimer's disease. Understanding and targeting the age-related pathophysiological mechanisms that underlie vascular contributions to cognitive impairment and dementia (VCID) are expected to have a major role in preserving brain health in older individuals. Maintenance of cerebral perfusion, protecting the microcirculation from high pressure-induced damage and moment-to-moment adjustment of regional oxygen and nutrient supply to changes in demand are prerequisites for the prevention of cerebral ischemia and neuronal dysfunction. This overview discusses age-related alterations in three main regulatory paradigms involved in the regulation of cerebral blood flow (CBF): cerebral autoregulation/myogenic constriction, endothelium-dependent vasomotor function, and neurovascular coupling responses responsible for functional hyperemia. The pathophysiological consequences of cerebral microvascular dysregulation in aging are explored, including blood-brain barrier disruption, neuroinflammation, exacerbation of neurodegeneration, development of cerebral microhemorrhages, microvascular rarefaction, and ischemic neuronal dysfunction and damage. Due to the widespread attention that VCID has captured in recent years, the evidence for the causal role of cerebral microvascular dysregulation in cognitive decline is critically examined.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

Role of 20-HETE, TRPC channels, and BKCa in dysregulation of pressure-induced Ca2+ signaling and myogenic constriction of cerebral arteries in aged hypertensive mice

Hypertension in the elderly substantially increases the risk of stroke and vascular cognitive impairment in part due to an impaired functional adaptation of aged cerebral arteries to high blood pressure. To elucidate the mechanisms underlying impaired autoregulatory protection in aging, hypertension was induced in young (3 mo) and aged (24 mo) C57BL/6 mice by chronic infusion of angiotensin II and pressure-induced changes in smooth muscle cell (SMC) intracellular Ca(2+) concentration ([Ca(2+)]i) and myogenic constriction of middle cerebral arteries (MCA) were assessed. In MCAs from young hypertensive mice, pressure-induced increases in vascular SMC [Ca(2+)]i and myogenic tone were increased, and these adaptive responses were inhibited by the cytochrome P-450 ω-hydroxylase inhibitor HET0016 and the transient receptor potential (TRP) channel blocker SKF96365. Administration of 20- hydroxyeicosatetraenoic acid (HETE) increased SMC [Ca(2+)]i and constricted MCAs, and these responses were inhibited by SKF96365. MCAs from aged hypertensive mice did not show adaptive increases in pressure-induced calcium signal and myogenic tone and responses to HET0016 and SKF96365 were blunted. Inhibition of large-conductance Ca(2+)-activated K(+) (BK) channels by iberiotoxin enhanced SMC [Ca(2+)]i and myogenic constriction in MCAs of young normotensive animals, whereas it was without effect in MCAs of young hypertensive mice. Iberiotoxin did not restore myogenic adaptation in MCAs of aged hypertensive mice. Thus functional maladaptation of aged cerebral arteries to hypertension is due to the dysregulation of pressure-induced 20-HETE and TRP channel-mediated SMC calcium signaling, whereas overactivation of BK channels is unlikely to play a role in this phenomenon.

Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging

Cognitive performances are tightly associated with the maximal aerobic exercise capacity, both of which decline with age. The benefits on mental health of regular exercise, which slows the age-dependent decline in maximal aerobic exercise capacity, have been established for centuries. In addition, the maintenance of an optimal cerebrovascular endothelial function through regular exercise, part of a healthy lifestyle, emerges as one of the key and primary elements of successful brain aging. Physical exercise requires the activation of specific brain areas that trigger a local increase in cerebral blood flow to match neuronal metabolic needs. In this review, we propose three ways by which exercise could maintain the cerebrovascular endothelial function, a premise to a healthy cerebrovascular function and an optimal regulation of cerebral blood flow. First, exercise increases blood flow locally and increases shear stress temporarily, a known stimulus for endothelial cell maintenance of Akt-dependent expression of endothelial nitric oxide synthase, nitric oxide generation, and the expression of antioxidant defenses. Second, the rise in circulating catecholamines during exercise not only facilitates adequate blood and nutrient delivery by stimulating heart function and mobilizing energy supplies but also enhances endothelial repair mechanisms and angiogenesis. Third, in the long term, regular exercise sustains a low resting heart rate that reduces the mechanical stress imposed to the endothelium of cerebral arteries by the cardiac cycle. Any chronic variation from a healthy environment will perturb metabolism and thus hasten endothelial damage, favoring hypoperfusion and neuronal stress.

Endothelial SK(Ca) and IK(Ca) channels regulate brain parenchymal arteriolar diameter and cortical cerebral blood flow

Calcium-sensitive potassium (K(Ca)) channels have been shown to modulate the diameter of cerebral pial arteries; however, little is known regarding their roles in controlling cerebral parenchymal arterioles (PAs). We explored the function and cellular distribution of small-conductance (SK(Ca)) and intermediate-conductance (IK(Ca)) K(Ca) channels and large-conductance K(Ca) (BK(Ca)) channels in endothelial cells (ECs) and smooth muscle cells (SMCs) of PAs. Both SK(Ca) and IK(Ca) channels conducted the outward current in isolated PA ECs (current densities, ~20 pA/pF and ~28 pA/pF at +40 mV, respectively), but these currents were not detected in PA SMCs. In contrast, BK(Ca) currents were prominent in PA SMCs (~154 pA/pF), but were undetectable in PA ECs. Pressurized PAs constricted to inhibition of SK(Ca) (~16%) and IK(Ca) (~16%) channels, but were only modestly affected by inhibition of BK(Ca) channels (~5%). Blockade of SK(Ca) and IK(Ca) channels decreased resting cortical cerebral blood flow (CBF) by ~15%. NS309 (6,7-dichloro-1H-indole-2,3-dione3-oxime), a SK(Ca)/IK(Ca) channel opener, hyperpolarized PA SMCs by ~27 mV, maximally dilated pressurized PAs, and increased CBF by ~40%. In conclusion, these data show that SK(Ca) and IK(Ca) channels in ECs profoundly modulate PA tone and CBF, whereas BK(Ca) channels in SMCs only modestly influence PA diameter.

Quantification of dynamic blood flow autoregulation in optic nerve head of rhesus monkeys

Autoregulation capacity has been classically assessed with a 'two-point' measurement or static autoregulation (sAR). In such an approach, stabilized hemodynamic parameters are determined before and after a perfusion pressure challenge. Analysis of dynamic autoregulation (dAR), an early phase of blood flow response to a sudden perfusion pressure change is emerging as a preferred approach to assess the capacity of autoregulation in many non-ocular tissues and has developed rapidly in the last decade. The purpose of this study was to develop a method to quantify dAR in the optic nerve head (ONH). In six pentobarbital (6-9 mg/kg/h, IV) anesthetized rhesus monkeys, dAR was elicited by increasing intraocular pressure (IOP) from 10 to 30 or 40 mmHg (IOP(10-30)/IOP(10-40)) manometrically via switch between reservoirs connected to the anterior chamber. Relative blood flow changes during dAR in the ONH, estimated with a laser speckle flowgraph (LSFG), were continuously measured for 1 min. Time-domain parameters of dAR response, including: BF(Deltamax) (maximal blood flow decrease, %), K(r) (descending slope of blood flow from baseline to BF(Deltamax)) and T(r) (descending time of blood flow from baseline to BF(Deltamax)) were extracted and analyzed offline. For each monkey, same procedure was repeated three times during three different visits. The test-retest repeatability and inter-ocular difference of the parameters was statistically evaluated. During IOP(10-30) and IOP(10-40), the mean arterial BP was 89 +/- 7 and 85 +/- 6 mmHg, respectively. Immediately after the reservoir was switched, the blood flow started to decline and reached maximal in approximately 4 s. The blood flow then returned back toward baseline despite continuous IOP increase, which took 8-11 s to reach the level of the raised reservoir. The general pattern of blood flow responses was similar between IOP(10-30) and IOP(10-40) and there was no statistically significant difference for T(r) (P > 0.05). However, IOP(10-40) caused greater BF(Deltamax) and deeper K(r) than IOP(10-30) (P < 0.0001 and P < 0.05, respectively). The blood flow during steady state, 5 min after IOP elevation, showed no statistically significant difference from baseline (P > 0.05). All dAR parameters (T(r), K(r) and BF(Deltamax)) showed no significant difference across the 3 visits (Repeat measures ANOVA, P = 0.7, 0.2 and 0.2, respectively); the corresponding coefficients of variance were 24%, 43% and 34% during IOP(10-30) and 11.8%, 30.3% and 19.0% during IOP(10-40). The mean dAR parameters between the eyes showed no statistically differences (P = 0.6) during both IOP(10-30) and IOP(10-40). The current study showed that a rapid ocular perfusion pressure decrease induced by a sudden IOP step increase evoked a transient and reproducible dAR response in the ONH of non-human primates measured with LSFG. Quantitative analysis of dAR may provide a direct view of vasomotorial activity in the resistant vessels and thus a new approach to assess the autoregulatory capacity in the ONH.

Neuronal nitric oxide mediates cerebral vasodilatation during acute hypertension

Parasympathetic nerves from the pterygopalatine ganglia provide nitroxidergic innervation to forebrain cerebral blood vessels. Disruption of that innervation attenuates cerebral vasodilatation seen during acute hypertension as does systemic administration of a non-selective nitric oxide synthase (NOS) inhibitor. Although such studies suggest that nitric oxide (NO) released from parasympathetic nerves participates in vasodilatation of cerebral vessels during hypertension, that hypothesis has not been tested with selective local inhibition of neuronal NOS (nNOS). We tested that hypothesis through these studies performed in anesthetized rats instrumented for continuous measurement of blood pressure, heart rate and pial arterial diameter through a cranial window. We sought to determine if the nNOS inhibitor propyl-L-arginine delivered directly to the outer surface of a pial artery would (1) attenuate changes in pial arterial diameter during acute hypertension and (2) block nNOS-mediated dilator effects of N-methyl-D-aspartate (NMDA) delivered into the window but (3) not block vasodilatation elicited by acetylcholine (ACh) and mediated by endothelial NOS dilator. Without the nNOS inhibitor arterial diameter abruptly increased 70+/-15% when mean arterial pressure (MAP) reached 183+/-3 mm Hg while with nNOS inhibition diameter increased only 13+/-10% (p<0.05) even when MAP reached 191+/-4 mm Hg (p>0.05). The nNOS inhibitor significantly attenuated vasodilatation induced by NMDA but not ACh delivered into the window. Thus, local nNOS inhibition attenuates breakthrough from autoregulation during hypertension as does complete interruption of the parasympathetic innervation of cerebral vessels. These findings further support the hypothesis that NO released from parasympathetic fibers contributes to cerebral vasodilatation during acute hypertension.

Cerebral blood flow autoregulation and edema formation during pregnancy in anesthetized rats

Eclampsia is considered a form of hypertensive encephalopathy in which an acute elevation in blood pressure causes autoregulatory breakthrough, blood-brain barrier disruption, and edema formation. We hypothesized that pregnancy predisposes the brain to eclampsia by lowering the pressure of autoregulatory breakthrough and enhancing cerebral edema formation. Because NO production is increased in pregnancy, we also investigated the role of NO in modulating autoregulation. Cerebral blood flow autoregulation was determined by phenylephrine infusion and laser Doppler flowmetry. Four groups were studied: untreated nonpregnant (n=7) and late-pregnant (days 19 to 21; n=8) Sprague-Dawley rats and nonpregnant (n=8) and late-pregnant (n=8) animals treated with an NO synthase inhibitor (N(G)-nitro-l-arginine methyl ester; 0.5 to 0.7 g/L). Brain water content and blood-brain barrier permeability to sodium fluorescein were determined after breakthrough. Pregnancy caused no change in autoregulation or the pressure of breakthrough. However, treatment with the NO synthase inhibitor significantly increased the pressure of autoregulatory breakthrough (nonpregnant: 183.6+/-3.0 mm Hg versus 212.0+/-2.8 mm Hg, P<0.05; late-pregnant: 180.8+/-3.2 mm Hg versus 209.3+/-4.7 mm Hg, P<0.05). After autoregulatory breakthrough, only late-pregnant animals showed a significant increase in cerebral edema formation, which was attenuated by NO synthase inhibition. There was no difference in blood-brain barrier permeability between nonpregnant and late-pregnant animals in response to acute hypertension, suggesting that pregnancy may predispose the brain to eclampsia by increasing cerebral edema through increased hydraulic conductivity.

The Effect of Superoxide Anion on Autoregulation of Cerebral Blood Flow

Background and Purpose— Recent studies have suggested that autoregulation of cerebral blood flow (CBF) is impaired after traumatic and ischemic brain injury. Given that the levels of superoxide anion (O 2 · − ) are increased in these conditions, we postulate that O 2 · − contributes to the impairment of CBF autoregulation.

Methods— CBF was monitored with laser Doppler flowmetry during increases in blood pressure.

Results— During the control period, CBF was well autoregulated after the increase in mean arterial pressure (MAP) from 98±3 to 140±6 mm Hg. The autoregulation index (AI; ΔCBF/ΔMAP) averaged 0.25±0.02 (n=6). O 2 · − in the brain was then increased by subdural perfusion of xanthine/xanthine oxidase (different concentrations) and catalase. Low concentrations of O 2 · − decreased basal CBF by 10±1.6% but had no effect on autoregulation (AI, 0.19±0.02; n=6). Higher concentrations of O 2 · − (0.2 mmol/L xanthine and either 3 or 20 mU xanthine oxidase) increased basal CBF by 30±2% and 42±4%, respectively, and impaired autoregulation of CBF (AI, 0.55±0.03 and 0.76±0.02; n=6). Inclusion of superoxide dismutase in the O 2 · − -generating system restored autoregulation (AI, 0.28±0.05; n=6). Neither inhibition of NO synthase nor the addition of deferioxamine had any effect on the ability of higher concentrations of O 2 · − to impair autoregulation of CBF (AI, 0.65±0.07 and 0.72±0.05 respectively; n=6). O 2 · − also increased the activity of K Ca channels in cerebral vascular smooth muscle cells (VSMCs; n=8).

Conclusion— These results suggest that O 2 · − increases basal CBF and impairs autoregulation of CBF, likely through the activation of K Ca channels in cerebral VSMCs.

Functional alterations in cerebrovascular K+ and Ca2+ channels are comparable between simulated microgravity rat and SHR

Exposure to microgravity leads to a sustained elevation in transmural pressure across the cerebral vasculature due to removal of hydrostatic pressure gradients. We hypothesized that ion channel remodeling in cerebral vascular smooth muscle cells (VSMCs) similar to that associated with hypertension may occur and play a role in upward autoregulation of cerebral vessels during microgravity. Sprague-Dawley rats were subjected to 4-wk tail suspension (Sus) to simulate the cardiovascular effect of microgravity. Large-conductance Ca2+-activated K+ (BKCa), voltage-gated K+ (KV), and L-type voltage-dependent Ca2+ (CaL) currents of Sus and control (Con) rat cerebral VSMCs were investigated with a whole cell voltage-clamp technique. Under the same experimental conditions, KV, BKCa, and CaL currents of cerebral VSMCs from adult spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were also investigated. KV current density decreased in Sus rats vs. Con rats [1.07 ± 0.14 ( n = 22) vs. 1.31 ± 0.28 ( n = 16) pA/pF at +20 mV ( P < 0.05)] and BKCa and CaL current densities increased [BKCa: 1.70 ± 0.37 ( n = 23) vs. 0.88 ± 0.22 ( n = 19) pA/pF at +20 mV ( P < 0.05); CaL: −2.17 ± 0.21 ( n = 35) vs. −1.31 ± 0.10 ( n = 26) pA/pF at +10 mV ( P < 0.05)]. Similar changes were also observed in SHR vs. WKY cerebral VSMCs: KV current density decreased [1.03 ± 0.33 ( n = 9) vs. 1.62 ± 0.64 ( n = 9) pA/pF at +20 mV ( P < 0.05)] and BKCa and CaL current densities increased [BKCa: 2.54 ± 0.47 ( n = 11) vs. 1.12 ± 0.33 ( n = 12) pA/pF at +20 mV ( P < 0.05); CaL: −3.99 ± 0.53 ( n = 12) vs. −2.28 ± 0.20 ( n = 10) pA/pF at +20 mV ( P < 0.05)]. These findings support our hypothesis, and their impact on space cardiovascular research is discussed.

Cerebral artery reactivity changes during pregnancy and the postpartum period: a role in eclampsia?

Eclampsia is thought to be similar to hypertensive encephalopathy, whereby acute elevations in intravascular pressure cause forced dilatation (FD) of intrinsic myogenic tone of cerebral arteries and arterioles, decreased cerebrovascular resistance, and hyperperfusion. In the present study, we tested the hypothesis that pregnancy and/or the postpartum period predispose cerebral arteries to FD by diminishing pressure-induced myogenic activity. We compared the reactivity to pressure (myogenic activity) as well as factors that modulate the level of tone of third-order branches (<200 μm) of the posterior cerebral artery (PCA) that were isolated from nonpregnant (NP, n = 7), late-pregnant (LP, 19 days, n = 10), and postpartum (PP, 3 days, n = 8) Sprague-Dawley rats under pressurized conditions. PCAs from all groups of animals developed spontaneous tone within the myogenic pressure range (50–150 mmHg) and constricted arteries at 100 mmHg (NP, 30 ± 3; LP, 39 ± 4; and PP, 42 ± 7%; P > 0.05). This level of myogenic activity was maintained in the NP arteries at all pressures; however, both LP and PP arteries dilated at considerably lower pressures compared with NP, which lowered the pressure at which FD occurred from >175 for NP to 146 ± 6.5 mmHg for LP ( P < 0.01 vs. NP) and 162 ± 7.7 mmHg for PP ( P < 0.01 vs. NP). The amount of myogenic tone was also significantly diminished at 175 mmHg compared with NP: percent tone for NP, LP, and PP animals were 35 ± 2, 11 ± 3 ( P < 0.01 vs. NP), and 20 ± 7% ( P < 0.01 vs. NP), respectively. Inhibition of nitric oxide (NO) with 0.1 mM Nω-nitro-l-arginine (l-NNA) caused constriction of all vessel types that was significantly increased in the PP arteries, which demonstrates significant basal NO production. Reactivity to 5-hydroxytryptamine (serotonin) was assessed in the presence of l-NNA and indomethacin. There was a differential response to serotonin: PCAs from NP animals dilated, whereas LP and PP arteries constricted. These results suggest that both pregnancy and the postpartum period predispose the cerebral circulation to FD at lower pressures, a response that may lower cerebrovascular resistance and promote hyperperfusion when blood pressure is elevated, as occurs during eclampsia.

Differential activation of potassium channels in cerebral and hindquarter arteries of rats during simulated microgravity

The purpose of this study was to test the hypothesis that differential autoregulation of cerebral and hindquarter arteries during simulated microgravity is mediated or modulated by differential activation of K(+) channels in vascular smooth muscle cells (VSMCs) of arteries in different anatomic regions. Sprague-Dawley rats were subjected to 1- and 4-wk tail suspension to simulate the cardiovascular deconditioning effect due to short- and medium-term microgravity. K(+) channel function of VSMCs was studied by pharmacological methods and patch-clamp techniques. Large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated K(+) (K(v)) currents were determined by subtracting the current recorded after applications of 1 mM tetraethylammonium (TEA) and 1 mM TEA + 3 mM 4-aminopyridine (4-AP), respectively, from that of before. For cerebral vessels, the normalized contractility of basilar arterial rings to TEA, a BK(Ca) blocker, and 4-AP, a K(v) blocker, was significantly decreased after 1- and 4-wk simulated microgravity, respectively. VSMCs isolated from the middle cerebral artery branches of suspended rats had a more depolarized membrane potential (E(m)) and a smaller K(+) current density compared with those of control rats. Furthermore, the reduced total current density was due to smaller BK(Ca) and smaller K(v) current density in cerebral VSMCs after 1- and 4-wk tail suspension, respectively. For hindquarter vessels, VSMCs isolated from second- to sixth-order small mesenteric arteries of both 1- and 4-wk suspended rats had a more negative E(m) and larger K(+) current densities for total, BK(Ca), and K(v) currents. These results indicate that differential activation of K(+) channels occur in cerebral and hindquarter VSMCs during short- and medium-term simulated microgravity. It is further suggested that different profiles of channel remodeling might occur in VSMCs as one of the important underlying cellular mechanisms to mediate and modulate differential vascular adaptation during microgravity.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

Actin filaments regulate the stretch sensitivity of large-conductance, Ca2+-activated K+ channels in coronary artery smooth muscle cells

Using the inside-out patch-clamp technique, large-conductance Ca2+ -activated K+ channel (BK(Ca)) currents were recorded from coronary artery smooth muscle cells. Cytochalasin D, an actin filament disrupter, increased channel activity ( NP(o), where N is the number of channels and P(o) the open probability), and this increase was reversed by phalloidin, an actin filament stabilizer. NP(o) was also increased by colchicine, a microtubule disrupter, and decreased by taxol, a microtubule stabilizer. With the stepwise increase of negative pressure in the patch pipettes, the activity of BK(Ca) gradually increased: the maximum effect (527% increase in NP(o)) was achieved at -40 cmH(2)O and the half-maximum effect at -25 cmH(2)O. The increase in NP(o) in response to negative pressure was abolished by phalloidin but not by taxol. These results imply that both actin filaments and microtubules inhibit the opening of BK(Ca) in coronary artery smooth muscle cells, but that only actin filaments are involved in the stretch sensitivity of BK(Ca).

Differences in the dynamic cerebrovascular response between stepwise up tilt and down tilt in humans

We studied dynamic cerebrovascular responses in eight healthy humans during repetitive stepwise upward tilt (SUT) and stepwise downward tilt (SDT) maneuvers between supine and 70 degrees standing at intervals of 60 s. Mean cerebral blood flow velocity (FV(MCA)) was measured at the middle cerebral artery (MCA) with transcranial Doppler ultrasonography. Mean arterial blood pressure (ABP) was measured via the radial artery and adjusted at the level of the MCA (ABP(MCA)). Cerebral critical closing pressure (P(CC)) was estimated from the systolic-diastolic relationship between FV(MCA) and ABP(MCA). ABP(MCA) minus P(CC) was considered the cerebral perfusion pressure (CPP). The tilt maneuvers produced stepwise changes in both CPP and FV(MCA). The FV(MCA) response to SUT was well characterized by a linear second-order model. However, that to SDT presented a biphasic behavior that was described significantly better (P < 0.05) by the addition of a slowly responding component to the second-order model. This difference may reflect both different cardiovascular responses to SUT or SDT and different cerebrovascular autoregulatory behaviors in response to decreases or increases in CPP.