Effects of extravascular acidification and extravascular alkalinization on constriction and depolarization in rat cerebral arterioles in vitro

Characterising the protective vasodilatory effects of hypobaric hypoxia on the neurovascular coupling response

Neurovascular coupling (NVC) is the link between local neuronal activity and regional cerebral blood flow. High altitude (HA) ascent induces acute hypoxic vasodilation of the cerebral vasculature, with associated changes in CO2 and acid-base status. We aimed to characterise the effects of (a) acute removal of the HA-induced vasodilation and (b) rapid ascent to and residency at HA on NVC responses. In twelve healthy participants (7 M/5F), arterial blood gases and NVC were measured at baseline (1130 m) and on days two (<24 h at HA) and nine (post-acclimatisation) at 3800 m. Acute gas challenges were performed using end-tidal forcing, with (a) normoxia and isocapnic hypoxia at 1130 m and (b) poikilocapnic hypoxia and isocapnic hyperoxia on days two and nine at 3800 m. Posterior cerebral artery velocity (PCAv) was measured using transcranial Doppler ultrasound in each condition and time-point. NVC was assessed via a standardized 30 s intermittent strobe light visual stimulus (VS), and quantified as the peak and mean change from baseline in PCAv. No significant differences were observed for any NVC metric across all conditions and time points. Our results reveal remarkable stability of the NVC response following (a) acute removal of HA-induced hypoxic vasodilation and (b) rapid ascent to and residency at 3800 m.

Restoration of the response of the middle cerebral artery of the rat to acidosis in hyposmotic hyponatremia by the opener of large-conductance calcium sensitive potassium channels (BKCa)

Hyposmotic hyponatremia (the decrease of extracellular concentration of sodium ions from 145 to 121 mM and the decrease of hyposmolality from 300 to 250 mOsm/kg H₂O) impairs response of the middle cerebral artery (MCA) to acetylcholine and NO donor (S-nitroso-N-acetyl-DL-penicillamine). Since acidosis activates a similar intracellular signaling pathway, the present study was designed to verify the hypothesis that the response of the MCA to acidosis is impaired during acute hyposmotic hyponatremia due to abnormal NO-related signal transduction in vascular smooth muscle cells. Studies performed on isolated, cannulated, and pressurized rat MCA revealed that hyposmotic hyponatremia impaired the response of the MCA to acidosis and this was associated with hyposmolality rather than with decreased sodium ion concentration. Response to acidosis was restored by the BK_Ca but not by the K_ATP channel activator. Patch-clamp electrophysiology performed on myocytes freshly isolated from MCAs, demonstrated that hyposmotic hyponatremia does not affect BK_Ca currents but decreases the voltage-dependency of the activation of the BK_Ca channels in the presence of a specific opener of these channels. Our study suggests that reduced sensitivity of BK_Ca channels in the MCA to agonists results in the lack of response of this artery to acidosis during acute hyposmotic hyponatremia.

A role for ATP-sensitive potassium channels in the anticonvulsant effects of triamterene in mice

There are reports indicating that diuretics including chlorothiazide, furosemide, ethacrynic acid, amiloride and bumetanide can have anticonvulsant properties. Intracellular acidification appears to be a mechanism for the anticonvulsant action of some diuretics. This study was conducted to investigate whether or not triamterene, a K(+)-sparing diuretic, can generate protection against seizures induced by intravenous or intraperitoneal pentylenetetrazole (PTZ) models. And to see if, triamterene can withstand maximal electroshock seizure (MES) in mice. We also investigated to see if there is any connection between triamterene's anti-seizure effect and ATP-sensitive K(+) (KATP) channels. Five days triamterene oral administration (10, 20 and 40 mg/kg), significantly increased clonic seizure threshold which was induced by intravenous pentylenetetrazole. Triamterene (10, 20 and 40 mg/kg) treatment also increased the latency of clonic seizure and decreased its frequency in intraperitoneal PTZ model. Administration of triamterene (20 mg/kg) also decreased the incidence of tonic seizure in MES-induced seizure. Co-administration of a KATP sensitive channel blocker, glibenclamide, in the 6th day, 60 min before intravenous PTZ blocked triamterene's anticonvulsant effect. A KATP sensitive channel opener, diazoxide, enhanced triamterene's anti-seizure effect in both intravenous PTZ or MES seizure models. At the end, triamterene exerts anticonvulsant effect in 3 seizure models of mice including intravenous PTZ, intraperitoneal PTZ and MES. The anti-seizure effect of triamterene probably is induced through KATP channels.

Diverse Kir expression contributes to distinct bimodal distribution of resting potentials and vasotone responses of arterioles

The resting membrane potential (RP) of vascular smooth muscle cells (VSMCs) is a major determinant of cytosolic calcium concentration and vascular tone. The heterogeneity of RPs and its underlying mechanism among different vascular beds remain poorly understood. We compared the RPs and vasomotion properties between the guinea pig spiral modiolar artery (SMA), brain arterioles (BA) and mesenteric arteries (MA). We found: 1) RPs showed a robust bimodal distribution peaked at -76 and -40 mV evenly in the SMA, unevenly at -77 and -51 mV in the BA and ~-71 and -52 mV in the MA. Ba(2+) 0.1 mM eliminated their high RP peaks ~-75 mV. 2) Cells with low RP (~-45 mV) hyperpolarized in response to 10 mM extracellular K(+), while cells with a high RP depolarized, and cells with intermediate RP (~-58 mV) displayed an initial hyperpolarization followed by prolonged depolarization. Moderate high K(+) typically induced dilation, constriction and a dilation followed by constriction in the SMA, MA and BA, respectively. 3) Boltzmann-fit analysis of the Ba(2+)-sensitive inward rectifier K(+) (Kir) whole-cell current showed that the maximum Kir conductance density significantly differed among the vessels, and the half-activation voltage was significantly more negative in the MA. 4) Corresponding to the whole-cell data, computational modeling simulated the three RP distribution patterns and the dynamics of RP changes obtained experimentally, including the regenerative swift shifts between the two RP levels after reaching a threshold. 5) Molecular works revealed strong Kir2.1 and Kir2.2 transcripts and Kir2.1 immunolabeling in all 3 vessels, while Kir2.3 and Kir2.4 transcript levels varied. We conclude that a dense expression of functional Kir2.X channels underlies the more negative RPs in endothelial cells and a subset of VSMC in these arterioles, and the heterogeneous Kir function is primarily responsible for the distinct bimodal RPs among these arterioles. The fast Kir-based regenerative shifts between two RP states could form a critical mechanism for conduction/spread of vasomotion along the arteriole axis.

Neurovascular uncoupling under mild hypoxic hypoxia: an EEG-fMRI study in rats

The effects of oxygen availability on neurovascular coupling were investigated using simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), in addition to the monitoring of physiological parameters, in 16 α-chloralose-anesthetized rats. Mild hypoxic hypoxia (oxygen saturation=83.6±12.1%) induced significant reductions in fMRI responses (P<0.05) to electrical stimulation in the forepaw, but EEG responses remained unchanged. In addition, the changes in oxygen saturation were linearly correlated with the changes in the fMRI responses. These data further emphasize the importance of oxygen availability, which may regulate neurovascular coupling via the oxygen-dependent enzymatic synthesis of messenger molecules.

pCO(2) and pH regulation of cerebral blood flow

CO₂ serves as one of the fundamental regulators of cerebral blood flow (CBF). It is widely considered that this regulation occurs through pCO₂-driven changes in pH of the cerebral spinal fluid (CSF), with elevated and lowered pH causing direct relaxation and contraction of the smooth muscle, respectively. However, some findings also suggest that pCO₂ acts independently of and/or in conjunction with altered pH. This action may be due to a direct effect of CSF pCO₂ on the smooth muscle as well as on the endothelium, nerves, and astrocytes. Findings may also point to an action of arterial pCO₂ on the endothelium to regulate smooth muscle contractility. Thus, the effects of pH and pCO₂ may be influenced by the absence/presence of different cell types in the various experimental preparations. Results may also be influenced by experimental parameters including myogenic tone as well as solutions containing significantly altered HCO₃⁻ concentrations, i.e., solutions routinely employed to differentiate the effects of pH from pCO₂. In sum, it appears that pCO₂, independently and in conjunction with pH, may regulate CBF.

Myoendothelial coupling is unidirectional in guinea pig spiral modiolar arteries

Gap junctions (GJs) facilitate communication and promote transfer of signaling molecules or current between adjacent cells in various organs to coordinate cellular activity. In arteries, homocellular GJs are present between adjacent smooth muscle cells (SMCs) and between adjacent endothelial cells (ECs), whilst many arteries also exhibit heterocellular GJs between SMCs and ECs. To test the hypothesis that there is differential cell coupling in guinea pig spiral modiolar arteries (SMA), we used intracellular recording technique to record cellular activities simultaneously in ECs or SMCs in acutely isolated guinea pig SMA preparations. Cell types were identified by injection of a fluorescent dye, propidium iodide (PI), through recording microelectrodes. Stable intracellular recordings were made in 120 cells among which 61 were identified as SMCs and 28 as ECs. Dual intracellular recordings were conducted to detect the coexistence of the two distinct levels of resting potential (RP) and to estimate the intensity of electrical coupling between two cells by a current pulse of up to 0.5-1.5 nA. The electrotonic potential was detected not only in the current-injected cell, but also in the majority of non-injected cells. The electrical coupling ratios (ECRs) of homocellular cells were not significant (P>0.05) (0.084±0.032 (n=6) and 0.069±0.031 (n=7) for EC-EC and SMC-SMC pairs, respectively). By contrast, the ECRs of heterocellular cells were significantly different when a current pulse (1.5 nA, 2s) was injected into EC and SMC respectively (0.072±0.025 for EC; 0.003±0.001 for SMC, n=5, P<0.01). The putative gap junction blocker 18β-glycyrrhetinic acid significantly attenuated electrical coupling in both homocellular and heterocellular forms. The results suggest that homocellular GJs within SMCs or ECs are well coordinated but myoendothelial couplings between ECs and SMCs are unidirectional.

Coupling between gamma oscillation and fMRI signal in the rat somatosensory cortex: its dependence on systemic physiological parameters

The simultaneous recordings of neuronal and hemodynamic signals have revealed a significant involvement of high frequency bands (e.g., gamma range, 25-70 Hz) in neurovascular coupling. However, the dependence on a physiological parameter is unknown. In this study, we performed simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) recordings in 12 Wistar rats using a conventional forepaw stimulation paradigm and concurrently monitored the systemic physiological parameters of the partial pressure of arterial oxygen, partial pressure of arterial carbon dioxide, pH, mean arterial blood pressure, and heart rate through the rat femoral artery. The high frequency bands in the artifact-free EEG signals, especially those in the gamma range, demonstrated a maximum correlation with fMRI signals in the rat somatosensory cortex. A multiple linear regression analysis demonstrated that the correlation coefficient between the gamma power and fMRI signal depended on the actual values of the physiological parameters (R(2)=0.20, p<0.05), whereas the gamma power and fMRI signal by itself were independent. Among the parameters, the heart rate had a statistically significant slope (95% CI: 0.00027-0.0016, p<0.01) in a multiple linear regression model. These results indicate that neurovascular coupling is mainly driven by gamma oscillations, as expected, but coupling or potential decoupling is strongly influenced by systemic physiological parameters, which dynamically reflect the baseline vital status of the subject.

Fundamental increase in pressure-dependent constriction of brain parenchymal arterioles from subarachnoid hemorrhage model rats due to membrane depolarization

Intracerebral (parenchymal) arterioles are morphologically and physiologically unique compared with pial arteries and arterioles. The ability of subarachnoid hemorrhage (SAH) to induce vasospasm in large-diameter pial arteries has been extensively studied, although the contribution of this phenomenon to patient outcome is controversial. Currently, little is known regarding the impact of SAH on parenchymal arterioles, which are critical for regulation of local and global cerebral blood flow. Here diameter, smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)), and membrane potential measurements were used to assess the function of intact brain parenchymal arterioles isolated from unoperated (control), sham-operated, and SAH model rats. At low intravascular pressure (5 mmHg), membrane potential and [Ca(2+)](i) were not different in arterioles from control, sham-operated, and SAH animals. However, raising intravascular pressure caused significantly greater membrane potential depolarization, elevation in [Ca(2+)](i), and constriction in SAH arterioles. This SAH-induced increase in [Ca(2+)](i) and tone occurred in the absence of the vascular endothelium and was abolished by the L-type voltage-dependent calcium channel (VDCC) inhibitor nimodipine. Arteriolar [Ca(2+)](i) and tone were not different between groups when smooth muscle membrane potential was adjusted to the same value. Protein and mRNA levels of the L-type VDCC Ca(V)1.2 were similar in parenchymal arterioles isolated from control and SAH animals, suggesting that SAH did not cause VDCC upregulation. We conclude that enhanced parenchymal arteriolar tone after SAH is driven by smooth muscle membrane potential depolarization, leading to increased L-type VDCC-mediated Ca(2+) influx.

Mechanism of ATP-induced local and conducted vasomotor responses in isolated rat cerebral penetrating arterioles

BACKGROUND

Adenosine triphosphate (ATP), a potent vascular regulator in the cerebral circulation, initiates conducted vasomotor responses which may be impaired after pathological insults. We analyzed the mechanism of ATP-induced local vasomotor responses and their effect on conducted vasomotor responses in rat cerebral penetrating arterioles.

METHODS

Arterioles were cannulated and their internal diameter monitored. Vasomotor responses to ATP were observed in the presence or absence of inhibitors, or after endothelial impairment. Smooth muscle membrane potentials were measured in some vessels.

RESULTS

Microapplication of ATP produced a biphasic response (constriction followed by dilation), which resulted in conducted dilation preceded by a membrane hyperpolarization. alpha,beta-methylene-ATP or pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) blunted the ATP-mediated constriction and enhanced local and conducted dilation. N(omega)-monomethyl-L-arginine, endothelial impairment and N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH) reduced the local dilation caused by ATP. The conducted dilation was attenuated by MS-PPOH and endothelial impairment, but not N(omega)-monomethyl-L-arginine or indomethacin.

CONCLUSION

ATP-induced conducted dilation is preceded by membrane hyperpolarization. Local ATP induces initial local constriction via smooth-muscle P(2X1) and subsequent dilation via endothelial P(2Y) receptors. Nitric oxide, cytochrome P450 metabolites, and intermediate and large conductance K(Ca) channels mediate dilation caused by ATP. ATP-induced conducted dilation is dependent upon both the endothelium and cytochrome P450 metabolites.

Mouth breathing increases the pentylenetetrazole-induced seizure threshold in mice: a role for ATP-sensitive potassium channels

Nasal obstruction and consequent mouth breathing have been shown to change the acid-base balance, producing respiratory acidosis. Additionally, there exists a large body of evidence maintaining that acidosis affects the activity of ATP-sensitive potassium (K(ATP)) channels, which play a crucial role in the function of the central nervous system (CNS), for example, in modulating seizure threshold. Thus, in the study described here, we examined whether mouth breathing, induced by surgical ligation of nostrils, could affect the seizure threshold induced by pentylenetetrazole in male NMRI mice. Using the selective K(ATP) channel opener (diazoxide) and blocker (glibenclamide), we also evaluated the possible role of K(ATP) channels in this process. Our data revealed that seizure threshold was increased 6 to 72 hours after nasal obstruction, reaching a peak 48 hours afterward, compared with either control or sham-operated mice (P<0.01). There was a significant decrease in pH of arterial blood samples and increase in CO(2) partial pressure (PCO(2)) during this time. Systemic injection of glibenclamide (1 and 2mg/kg, ip, daily) significantly prevented the increase in seizure threshold in 48-hour bilaterally nasally obstructed mice, whereas it had no effect on seizure threshold in sham-operated mice. Systemic injection of diazoxide (25mg/kg, ip, daily) had no effect on seizure threshold in all groups, whereas higher doses (50 and 100mg/kg, ip, daily) significantly increased seizure threshold in both 48-hour-obstructed and sham-operated mice. The decrease in seizure threshold induced by glibenclamide (2mg/kg, ip, daily) was prevented by diazoxide (25mg/kg, ip, daily). These results demonstrate for the first time that mouth breathing, which could result in respiratory acidosis, increases seizure threshold in mice and K(ATP) channels may play a role in this effect.

Acidosis-induced relaxation of human internal mammary artery is due to activation of ATP-sensitive potassium channels

Metabolic acidosis is associated with various clinical situations including diabetes mellitus and renal diseases. The aim of this study was to investigate the effects of acidosis on the resting as well as precontracted human left internal mammary artery. The vessels were obtained from the patients undergoing coronary artery bypass grafting surgery at The Aga Khan University Hospital, Karachi. Left internal mammary artery was cut into rings and isometric tension recording experiments were performed. Decrease in pH of the bathing solution from 7.4 to 6.8 had no effect on the resting tension of left internal mammary artery, whereas, acidic pH markedly relaxed the contractions to 24.8 mM KCl and 300 nM phenylephrine. Interestingly, when the KCl- or phenylephrine-contracted rings were treated with 3 microM glibenclamide; an inhibitor of ATP-sensitive potassium (K(ATP)) channels, the relaxant effect of acidosis was abolished. Similarly, acidosis failed to cause relaxation of 100 nM endothelin-1-induced contraction in Ca2+-free bathing solution or in the presence of a voltage-dependent Ca2+ channel inhibitor, verapamil (10 microM), whereas, endothelin-1-induced contraction was attenuated by acidosis in Ca2+-containing normal solution. From all these data, it is concluded that under the acidic pH conditions, opening of K(ATP) channels occurs; resulting in the hyperpolarization, decrease in Ca2+ influx via voltage-dependent Ca2+ channels and subsequent relaxation of human left internal mammary artery.

Vasodilatory mechanisms in contracting skeletal muscle

Skeletal muscle blood flow is closely coupled to metabolic demand, and its regulation is believed to be mainly the result of the interplay of neural vasoconstrictor activity and locally derived vasoactive substances. Muscle blood flow is increased within the first second after a single contraction and stabilizes within ∼30 s during dynamic exercise under normal conditions. Vasodilator substances may be released from contracting skeletal muscle, vascular endothelium, or red blood cells. The importance of specific vasodilators is likely to vary over the time course of flow, from the initial rapid rise to the sustained elevation during steady-state exercise. Exercise hyperemia is therefore thought to be the result of an integrated response of more than one vasodilator mechanism. To date, the identity of vasoactive substances involved in the regulation of exercise hyperemia remains uncertain. Numerous vasodilators such as adenosine, ATP, potassium, hypoxia, hydrogen ion, nitric oxide, prostanoids, and endothelium-derived hyperpolarizing factor have been proposed to be of importance; however, there is little support for any single vasodilator being essential for exercise hyperemia. Because elevated blood flow cannot be explained by the failure of any single vasodilator, a consensus is beginning to emerge for redundancy among vasodilators, where one vasoactive compound may take over when the formation of another is compromised. Conducted vasodilation or flow-mediated vasodilation may explain dilation in vessels (i.e., feed arteries) not directly exposed to vasodilator substances in the interstitium. Future investigations should focus on identifying novel vasodilators and the interaction between vasodilators by simultaneous inhibition of multiple vasodilator pathways.

Vasodilation of brain surface arterioles by blockade of Na–H+ antiport and its inhibition by inhibitors of KATP channel openers

Pial artrioles of rats were monitored in vivo and found to dilate in dose-dependent fashion upon application of either benzamil or ethyl isopropyl amiloride, both of which are inhibitors of the sodium-hydrogen antiport. Antiport blockade is known to decrease the internal pH of vascular smooth muscle (VSM). The dilation was blocked by 1 microm glibenclamide, which in that dose is a selective inhibitor of ATP sensitive potassium channels (K(ATP)). The nitric oxide synthase inhibitor nitro-l arginine (l-NNA) also blocked the response. Previous studies of this preparation under the same experimental conditions showed that l-NNA inhibited dilation by K(ATP) openers and that nitric oxide had no permissive action in this setting. Moreover, one study by others has demonstrated a pH sensitive site on the internal surface of K(ATP) while another study by others has demonstrated that sodium propionate, a direct acidifier of the cell, dilates rat basilar artery in K(ATP)-dependent fashion. Therefore, the present data support the following conclusions: decrease of internal pH dilates brain arterioles; the response is K(ATP) dependent; in some situations, inhibitors of nitric oxide synthase can inhibit K(ATP) and K(ATP)-dependent dilations including those produced by decrease of internal pH.

Influence of the glia limitans on pial arteriolar relaxation in the rat

We examined whether damage to the glia limitans (GL), via exposure to the gliotoxin l-alpha-aminoadipic acid (l-alphaAAA), alters hypercapnia-induced pial arteriolar dilation in vivo. Anesthetized female rats were prepared with closed cranial windows. Pial arteriolar diameters were measured using intravital microscopy. l-alphaAAA (2 mM) was injected into the space under the cranial windows 24 h before the study, and injury to the GL was confirmed by light microscopy. l-alphaAAA was associated with a reduction in pial arteriolar CO(2) reactivity to 40-50% of the level seen in vehicle-treated controls, with no further reduction in the CO(2) response after nitric oxide (NO) synthase (NOS) inhibition via N(omega)-nitro-l-arginine (l-NNA). Subsequent blockade of prostanoid synthesis, via indomethacin (Indo), reduced CO(2) reactivity to 10-15% of normal. In vehicle-treated controls, l-NNA, followed by Indo, reduced the response to approximately 50% and then to 15-20% of the normocapnic value, respectively. On the other hand, l-alphaAAA had no effect on vascular responses to the endothelium-dependent vasodilator acetylcholine or the NO donor SNAP and did not alter cortical somatosensory evoked responses. This indicates an absence of any direct l-alphaAAA actions on pial arterioles or influence on neuronal transmission. Furthermore, l-alphaAAA did not alter the vasodilation elicited by topical application of an acidic artificial cerebrospinal fluid solution, suggesting that the GL influences the pial arteriolar relaxation elicited by hypercapnic, but not local extracellular (EC), acidosis. That differences exist in the mechanisms mediating hypercapnia- versus EC acidosis-induced pial arteriolar dilations was further exemplified by the finding that topical application of a neuronal NOS (nNOS)-selective blocker (ARR-17477) reduced the response to hypercapnia (by approximately 65%) but not the response to EC acidosis. Disruption of GL gap junctional communication, using an antisense oligodeoxynucleotide (ODN) connexin43 knockdown approach, was accompanied by a 33% lower CO(2) reactivity versus missense ODN-treated controls. These results suggest that the GL contribution to the hypercapnic vascular response appears to involve the NO-dependent component rather than the prostanoid-dependent component and may involve gap junctional communication. We speculate that the GL may act to facilitate the spread, to pial vessels, of hypercapnia-induced vasodilating signals arising in the comparatively few scattered nNOS neurons that lie well beneath the GL.

Cerebrovascular vasodilation to extraluminal acidosis occurs via combined activation of ATP-sensitive and Ca2+-activated potassium channels

Albeit controversial, it has been suggested by several authors that nitric oxide (NO) serves as a permissive factor in the cerebral blood flow response to systemic hypercapnia. Potassium channels are important regulators of cerebrovascular tone and may be modulated by a basal perivascular NO level. To elucidate the functional targets of the proposed NO modulation during hypercapnia-induced vasodilation, the authors performed experiments in isolated, cannulated, and pressurized rat middle cerebral arteries (MCA). Extracellular pH was reduced from 7.4 to 7.0 in the extraluminal bath to induce NO dependent vasodilation. Acidosis increased vessel diameter by 35 +/- 10%. In separate experiments, ATP-sensitive potassium channels (KATP) were blocked by extraluminal application of glibenclamide (Glib), Ca2+-activated potassium channels (KCa) by tetraethylammonium (TEA), voltage-gated potassium channels (Kv) by 4-aminopyridine, and inward rectifier potassium channels (KIR) by BaCl2. Na+-K+-ATP-ase was inhibited by ouabain. Application of TEA slightly constricted the arteries at pH 7.4 and slightly but significantly attenuated the vasodilation to acidosis. Inhibition of the other potassium channels or Na+-K+-ATP-ase had no effect. Combined blockade of KATP and KCa channels further reduced resting diameter, and abolished acidosis induced vasodilation. The authors conclude that mainly KCa channels are active under resting conditions. KATP and KCa channels are responsible for vasodilation to acidosis. Activity of one of these potassium channel families is sufficient for vasodilation to acidosis, and only combined inhibition completely abolishes vasodilation. During NO synthase inhibition, dilation to the KATP channel opener pinacidil or the KCa channel opener NS1619 was attenuated or abolished, respectively. The authors suggest that a basal perivascular NO level is necessary for physiologic KATP and KCa channel function in rat MCA. Future studies have to elucidate whether this NO dependent effect on KATP and KCa channel function is a principle mechanism of NO induced modulation of cerebrovascular reactivity and whether the variability of findings in the literature concerning a modulatory role of NO can be explained by different levels of vascular NO/cGMP concentrations within the cerebrovascular tree.

ATP-sensitive potassium channels in the cerebral circulation

BACKGROUND

In brain blood vessels, electrophysiological studies proving the existence of ATP-sensitive potassium channels (KATP) are scarce. However, numerous pharmacological studies establish the importance of KATP channels in these blood vessels. This review emphasizes the data supporting the importance of vascular KATP in the responses of brain blood vessels.

SUMMARY OF REVIEW

Electrophysiological data show the existence of KATP in smooth muscle and endothelium of brain vessels. A much larger number of studies in virtually all experimental species have shown that classic openers of KATP dilate brain arteries and arterioles. This response can by blocked by glibenclamide, a selective inhibitor of KATP opening. Several physiological or pathophysiological responses are also blocked by glibenclamide. KATP contains a multiplicity of potential sites of interaction with drugs of diverse, sometimes unrelated, structures. Drugs with imidazole or guanidinium groups are particularly likely to have effects on KATP. This complicates interpretation of the actions of such drugs when used as supposedly selective pharmacological probes for other putative targets. A pH-sensitive site on the internal surface of cloned channels may explain the glibenclamide-inhibitable dilation produced by intracellular acidosis and perhaps by CO2. In some situations KATP appears to be involved in either the synthesis/release or action of endothelium-derived mediators of cerebrovascular tone. The importance of KATP may be dependent on the portion of the cerebrovascular tree being studied and on diverse experimental conditions, age, species, and the presence of disease.

CONCLUSIONS

KATP have been shown to mediate a wide range of cerebrovascular response in physiologic or pathologic circumstances in a variety of experimental conditions. Their relevance to cerebrovascular responses in humans remains to be explored.

ATP-sensitive potassium channels mediate dilatation of basilar artery in response to intracellular acidification in vivo

BACKGROUND AND PURPOSE

During cerebral ischemia, both hypoxia and hypercapnia appear to produce marked dilatation of the cerebral arteries. Hypercapnia and hypoxia may be accompanied by extracellular and intracellular acidosis, which is another potent dilator of cerebral arteries. However, the precise mechanism by which acidosis produces dilatation of the cerebral arteries is not fully understood. The objective of the present study was to examine the mechanisms by which intracellular acidosis produces dilatation of the basilar artery in vivo.

METHODS

Using a cranial window in anesthetized rats, we examined responses of the basilar artery to sodium propionate, which was used to cause intracellular acidosis specifically. Expression of subunits of potassium channels was determined by reverse transcription and polymerase chain reaction (RT-PCR).

RESULTS

Topical application of propionate increased diameter of the basilar artery in a concentration-related manner. Propionate-induced dilatation of the artery was attenuated by glibenclamide, an inhibitor of ATP-sensitive potassium channels. However, inhibitors of nitric oxide synthase (N(G)-nitro-L-arginine), large-conductance calcium-activated potassium channels (iberiotoxin), and cyclooxygenase (indomethacin) did not affect the vasodilatation. Expression of mRNA for SUR2B and Kir6.1 was detected, with the use of RT-PCR, in the cultured basilar arterial muscle cells.

CONCLUSIONS

The findings suggest that intracellular acidification may produce dilatation of the basilar artery through activation of ATP-sensitive potassium channels in vivo. Kir6.1/SUR2B may be the major potassium channels that mediate propionate-induced dilatation of the artery.

Alkaline pH shifts Ca2+ sparks to Ca2+ waves in smooth muscle cells of pressurized cerebral arteries

The effects of external pH (7.0-8.0) on intracellular Ca(2+) signals (Ca(2+) sparks and Ca(2+) waves) were examined in smooth muscle cells from intact pressurized arteries from rats. Elevating the external pH from 7.4 to 7.5 increased the frequency of local, Ca(2+) transients, or "Ca(2+) sparks," and, at pH 7.6, significantly increased the frequency of Ca(2+) waves. Alkaline pH-induced Ca(2+) waves were inhibited by blocking Ca(2+) release from ryanodine receptors but were not prevented by inhibitors of voltage-dependent Ca(2+) channels, phospholipase C, or inositol 1,4,5-trisphosphate receptors. Activating ryanodine receptors with caffeine (5 mM) at pH 7.4 also induced repetitive Ca(2+) waves. Alkalization from pH 7.4 to pH 7.8-8.0 induced a rapid and large vasoconstriction. Approximately 82% of the alkaline pH-induced vasoconstriction was reversed by inhibitors of voltage-dependent Ca(2+) channels. The remaining constriction was reversed by inhibition of ryanodine receptors. These findings indicate that alkaline pH-induced Ca(2+) waves originate from ryanodine receptors and make a minor, direct contribution to alkaline pH-induced vasoconstriction.

Mechanism of extracellular K+-induced local and conducted responses in cerebral penetrating arterioles

BACKGROUND AND PURPOSE

Extracellular concentration of potassium ion ([K+]o) may have a significant influence on the cerebral circulation in health and disease. Mechanisms of [K+]o-induced conducted vasomotor responses in cerebral arterioles, possibly linking microvascular regulation to neuronal activity, have not been examined.

METHODS

We analyzed vascular responses to small increases of [K+]o (up to 5 mmol/L) in isolated, cannulated, and pressurized rat cerebral arterioles (36.5+/-1.4 micro m). [K+]o was elevated globally through extraluminal application or locally through micropipette, while arteriolar diameter was measured online.

RESULTS

Elevation of [K+]o (5 mmol/L) produced dilation that was inhibited by ouabain but not BaCl2. Locally applied [K+]o (3 to 5 mmol/L) produced a biphasic response (initial constriction followed by dilation), both of which were conducted to the remote site (distance 1142+/-68 microm). Endothelial impairment inhibited conducted but not local biphasic responses. Extraluminal ouabain attenuated local and conducted secondary dilation but not initial constriction. The local biphasic response was unaffected by extraluminal or intraluminal BaCl2. Extraluminal but not intraluminal BaCl2 impaired both conducted constriction and dilation.

CONCLUSIONS

In rat penetrating arteriole, (1) [K+]o (3 to 5 mmol/L) strongly regulates arteriolar tone and causes conducted vasomotor responses; (2) local responses to elevated [K+]o are endothelium independent but conducted responses are dependent on an intact endothelium; (3) smooth muscle Na+-K+-ATPase activation is the generator of conducted dilation; and (4) smooth muscle inward rectifier potassium channels sustain conduction. Our findings suggest that potassium-induced conducted vasomotor responses may link local neuronal activity to microvascular regulation, which may be attenuated in pathological conditions.

Nitric oxide induces hyperpolarization by opening ATP-sensitive K(+) channels in guinea pig spiral modiolar artery

Nitric oxide (NO) hyperpolarizes vascular smooth muscle cells and dilates blood vessels of various beds, but little is known on cochlear vasculatures. Using in vitro preparations of the spiral modiolar artery (SMA), intracellular electrical recording and labeling techniques, we found that the NO donor DPTA-NONOate (10 microM) caused a hyperpolarization of approximately 9 mV in all the cells that had a low resting potential (RP) level near -40 mV. The hyperpolarization amplitude was concentration-dependent, with a 50% effect concentration (EC(50)) of 1 microM. The responses occur in both smooth muscle and endothelial cells, neither of which was blocked by 18beta-glycyrrhetinic acid. The induced hyperpolarization was completely blocked by glipizide, but not by charybdotoxin, apamin, barium, 4-aminopyridine or tetraethylammonium. The hyperpolarizing responses were imitated by pinacidil (EC(50)=30 microM). The pinacidil-induced response was also blocked by glipizide but not by the other K(+) channel blockers mentioned above. Both DPTA-NONOate and pinacidil had little membrane potential effect on cells that had a high RP level near -75 mV. However, when the high RP cells were depolarized to a level beyond -45 mV by barium, both DPTA-NONOate and pinacidil hyperpolarized these cells not differently from those that initially had a low RP. It is concluded that NO hyperpolarizes the SMA primarily by activating K(ATP) channels in both muscle and endothelial cells.

Role of endothelial nitric oxide and smooth muscle potassium channels in cerebral arteriolar dilation in response to acidosis

BACKGROUND AND PURPOSE

Potassium channels or nitric oxide or both are major mediators of acidosis-induced dilation in the cerebral circulation. However, these contributions depend on a variety of factors such as species and vessel location. The present study was designed to clarify whether potassium channels and endothelial nitric oxide are involved in acidosis-induced dilation of isolated rat cerebral arterioles.

METHODS

Cerebral arterioles were cannulated and monitored with an inverted microscope. Acidosis (pH 6.8 to 7.4) produced by adding hydrogen ions mediated dilation of the cerebral arterioles in a concentration-dependent manner. The role of nitric oxide and potassium channels in response to acidosis was examined with several specific inhibitors and endothelial damage.

RESULTS

The dilation was significantly inhibited by potassium chloride (30 mmol/L) and glibenclamide (3 micromol/L; ATP-sensitive potassium channel inhibitor). We found that 30 micromol/L BaCl2 (concentration-dependent potassium channel inhibitor) also affected the dilation; however, an additional treatment of 3 micromol/L glibenclamide did not produce further inhibition. Tetraethylammonium ion (1 mmol/L; calcium-activated potassium channel inhibitor) and 4-aminopyridine (100 micromol/L; voltage-dependent potassium channel inhibitor) as well as ouabain (10 micromol/L; Na-K ATPase inhibitor) and N-methylsulphonyl-6-(2-proparglyloxyphenyl) hexanamide (1 micromol/L; cytochrome P450 epoxygenase inhibitor) did not alter acidotic dilation. N(omega)-Monomethyl-L-arginine (10 micromol/L) and N(omega)-nitro-L-arginine (10 micromol/L) as nitric oxide synthase inhibitor blunted the dilation. Furthermore, the dilation was significantly attenuated after the endothelial impairment. Additional treatment with glibenclamide (3 micromol/L) further reduced the dilation in response to acidosis.

CONCLUSIONS

Endothelial nitric oxide and smooth muscle ATP-sensitive potassium channels contribute to acidosis-induced dilation of rat cerebral arterioles. Endothelial damage caused by pathological conditions such as subarachnoid hemorrhage or traumatic brain injury may contribute to reduced blood flow despite injury-induced cerebral acidosis.

Two resting potential levels regulated by the inward-rectifier potassium channel in the guinea-pig spiral modiolar artery

1. Intracellular in vitro recordings were made from 771 cells from the spiral modiolar artery (SMA). The initial resting potentials (RPs) displayed a bimodal distribution that was well modelled as a mixture of two Gaussian distributions. About half of the cells had an average RP of -74 mV, and were termed high-RP cells, whereas the other half had an average RP around -41 mV, and were termed low-RP cells. Preparations that were incubated for longer than 24 h contained significantly more high-RP cells than those incubated for less than 8 h. 2. When labelled with the fluorescent dye propidium iodide, 68 and 36 cells were identified as smooth muscle cells (SMC) and endothelial cells (EC), respectively. The RP and input resistance were not significantly different between these two types of cell. Dye coupling was observed only in ECs. Dual cell recordings with 0.2-1.0 mm separation demonstrated the simultaneous existence of high- and low-RP cells and a heterogeneous low-strength electrical coupling. 3. The high-RP cells were depolarized by ACh and by high extracellular potassium concentration (high K(+)). The low-RP cells were usually hyperpolarized by moderately high K(+) (7.5-20 mM) and by ACh. The high K(+)-induced hyperpolarization was suppressed by barium (Ba(2+), 10-50 microM). The putative gap junction blocker 18 beta-glycyrrhetinic acid suppressed the ACh-induced responses in SMCs, but not in ECs. 4. Low-RP cells could rapidly shift the membrane potential to a permanent high-RP state spontaneously or, more often, after a brief application of hyperpolarizing agents including high K(+), ACh, nitric oxide and pinacidil. Once shifted to a high-RP state, the responses of these cells to high K(+) and ACh became similar to those of the original high-RP cells. 5. High-RP cells occasionally shifted their potentials to a low-RP state either spontaneously or after a brief application of 10-50 microM Ba(2+) or 100 microM ouabain. Once shifted to the low-RP state, the response of these cells to high K(+) and ACh became a hyperpolarization. The shift between high- and low-RP states was largely mimicked by wash-in and wash-out of low concentrations of Ba(2+). The shift often showed a regenerative process as a fast phase in its middle course. 6. It is concluded that the cochlear SMA in vitro is composed of poorly and heterogeneously coupled SMCs and ECs, simultaneously resting in one of two distinct states, one a high-RP state and the other a low-RP state. The two RP states are exchangeable mainly due to all-or-none-like conductance changes of the inward-rectifier K(+) channel.

Nitric oxide from perivascular nerves modulates cerebral arterial pH reactivity

In the isolated rat middle cerebral artery (MCA) we investigated the role of nitric oxide (NO)/cGMP in the vasodilatory response to extraluminal acidosis. Acidosis increased vessel diameter from 140 +/- 27 microm (pH 7.4) to 187 +/- 30 microm (pH 7.0, P < 0.01). NO synthase (NOS) inhibition by N(omega)-nitro-L-arginine (L-NNA, 10 microM) reduced baseline diameter (103 +/- 20 microm, P < 0.01) and attenuated response to acidosis (9 +/- 8 microm). Application of the NO-donors 3-morpholinosydnonimine (1 microM) or S-nitroso-N-acetylpenicillamine (1 microM), or of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP, 100 microM) reestablished pre-L-NNA diameter at pH 7.4 and reversed L-NNA-induced attenuation of the vessel response to acidosis. Restoration of pre-L-NNA diameter (pH 7.4) by papaverine (20 microM) or nimodipine (30 nM) had no effect on the attenuated response to acidosis. Guanylyl cyclase inhibition with 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (5 microM) or NOS-inhibition with 7-nitroindazole (7-NI, 100 microM) reduced baseline vessel diameter (109 +/- 8 or 127 +/- 11 microm, respectively) and vasodilation to acidosis, and restoration of baseline diameter with 8-BrcGMP (30 microM) completely restored dilation to pH 7.0. Chronic denervation of NOS-containing perivascular nerves in vivo 14 days before artery isolation significantly reduced pH-dependent reactivity in vitro (diameter increase sham: 48 +/- 14 microm, denervated: 14 +/- 8 microm), and 8-BrcGMP (30 microM) restored dilation to pH 7.0 (denervated: 49 +/- 31 microm). Removal of the endothelium did not change vasodilation to acidosis. We conclude that NO, produced by neuronal NOS of perivascular nerves, is a modulator in the pH-dependent vasoreactivity.

Interactions between the endothelium-derived relaxing factor/nitric oxide system and the endogenous opiate system in the modulation of cerebral and spinal vascular CO2 responsiveness

The role of the L-arginine-nitric oxide (NO) system, the role of the endogenous morphine-like substances (endorphins), and the possible interaction between these two systems in the modulation of regional cerebral and spinal CO2 responsiveness was investigated in anesthetized, ventilated, normotensive, normoxic cats. Regional cerebral blood flow was measured with radiolabeled microspheres in hypocapnic, normocapnic, and hypercapnic conditions in nine individual cerebral and spinal cord regions. General opiate receptor blockade by 1 mg/kg naloxone intravenously alone or NO synthase blockade by 3 mg/kg N(omega)-nitro-L-arginine-methyl ester (L-NAME) intravenously alone caused no changes in regional CO2 responsiveness. Combined administration of these two blocking agents in the very same doses, however, resulted in a strong potentiation, with a statistically significant reduction of the CO2 responsiveness observed. Separation of the blood flow response to hypercapnia and hypocapnia indicates that this reduction occurs only during hypercapnia. Specific mu and delta opiate receptors were blocked by 0.5 mg kg(-1) IV beta-funaltrexamine and 0.4 mg kg(-1) IV naltrindole, respectively. The role of specific mu and delta opiate receptors in the NO-opiate interaction was found to be negligible because neither mu nor delta receptor blockade along with simultaneous NO blockade were able to decrease CO2 responsiveness. The current findings suggest a previously unknown interaction between the endothelium-derived relaxing factor/nitric oxide (EDRF/NO) system and the endogenous opiate system in the cerebrovascular bed during hypercapnic stimulation, with the phenomenon not mediated by mu or delta opiate receptors.

Effects of luminal shear stress on cerebral arteries and arterioles

The effect of luminal shear stress was studied in cerebral arteries and arterioles. Middle cerebral arteries (MCA) and penetrating arterioles (PA) were isolated from male Long-Evans rats, mounted in a tissue bath, and pressurized. After the development of spontaneous tone, inside diameters were 186 +/- 5 microm (n = 28) for MCA and 65 +/- 3 microm (n = 37) for PA. MCA and PA constricted approximately 20% with increasing flow. Flow-induced constriction persisted in MCA and PA after removal of the endothelium. After removal of the endothelium, the luminal application of a polypeptide containing the Arg-Gly-Asp amino acid sequence (inhibitor of integrin attachment) abolished the flow-induced constriction. Similarly, an antibody specific for the beta(3)-chain of the integrin complex significantly inhibited the flow-induced constriction. The shear stress-induced constriction was accompanied by an increase in vascular smooth muscle Ca(2+). For example, a shear stress of 20 dyn/cm(2) constricted MCA 8% (n = 5) and increased Ca(2+) from 209 +/- 17 to 262 +/- 29 nM (n = 5). We conclude that isolated cerebral arteries and arterioles from the rat constrict to increased shear stress. Because the endothelium is not necessary for the response, the shear forces must be transmitted across the endothelium, presumably by the cytoskeletal matrix, to elicit constriction. Integrins containing the beta(3)-chain are involved with the shear stress-induced constrictions.

Vasodilatory effect of basic fibroblast growth factor in isolated rat cerebral arterioles: mechanisms involving nitric oxide and membrane hyperpolarization

Basic fibroblast growth factor (bFGF), a potent mitogen, acutely dilates cerebral blood vessels and may be effective in reducing cerebral infarction. However, the vasodilatory mechanism, which may involve nitric oxide (NO), is not completely understood. This study investigated whether membrane hyperpolarization is also involved in this mechanism. Membrane potential (MP) of smooth muscle cells and vessel diameter of isolated intracerebral arterioles were simultaneously measured following extraluminal application of bFGF in rats. The involvement of NO and adenosine triphosphate-sensitive potassium (KATP) channels in bFGF-induced vasodilation and membrane hyperpolarization was evaluated using specific inhibitors, NG-monomethyl-L-arginine (L-NMMA, 10(-4) M) and glibenclamide (GB, 10(-5) M), respectively. The resting MP was recorded at a mean value of -31.9 +/- 4.5 mV. bFGF (1 to 1000 ng/ml) produced significant vasodilation and hyperpolarization. Treatment with L-NMMA caused vasoconstriction and significantly attenuated bFGF-induced vasodilation without affecting membrane hyperpolarization. In the presence of GB, the membrane potential was significantly depolarized but the vessel diameter was only marginally reduced, so bFGF-induced membrane hyperpolarization was inhibited while arteriolar dilation was attenuated. These results suggest that bFGF-induced vasodilation is mediated by a mechanism involving both NO and membrane hyperpolarization, and that membrane hyperpolarization is caused by the activation of KATP channels.

Role of potassium channels in regulation of brain arteriolar tone: comparison of cerebrum versus brain stem

BACKGROUND AND PURPOSE

Potassium channels are important regulators of resting tone in large cerebral arteries, but their activity and distribution may vary according to vessel location and species studied. In the cerebral microcirculation in vivo, however, these channels appear to be silent at rest. Our goal was to determine the activity of potassium channels of brain arterioles from 2 origins under basal conditions in vitro.

METHODS

Penetrating cerebral (40. 9+/-2.2 microm control diameter) and brain stem (36.2+/-1.2 microm) arterioles of rats were prepared from middle cerebral and basilar arteries, respectively. The internal diameter of cannulated and pressurized vessel was monitored with the inverted microscope before and after administration of potassium channel inhibitors. In addition, we studied the effect of nitric oxide synthase inhibition on potassium channel activity.

RESULTS

Cerebral and brain stem arterioles were significantly constricted by 4-aminopyridine and low concentration of BaCl(2) but not by glibenclamide. The addition of N:(omega)-nitro-L-arginine to 4-aminopyridine further decreased diameters of both arterioles. Tetraethylammonium ion caused a significant constriction of brain stem but not cerebral arteriole. The brain stem arteriole was further constricted by additional N:(omega)-nitro-L-arginine.

CONCLUSIONS

Voltage-dependent and inward-rectifier, but not ATP-sensitive, potassium channels are active under basal conditions of rat cerebral and brain stem arterioles. There is a regional difference in the activity of calcium-activated potassium channels, which, at rest, are open in brain stem but silent in cerebral arterioles. In addition, basal endogenous nitric oxide may not contribute to the activation of voltage-dependent and calcium-activated potassium channels.

Extracellular pH, Ca(2+) influx, and response of vascular smooth muscle cells to 5-hydroxytryptamine

BACKGROUND AND PURPOSE

Cerebral vascular smooth muscle cells (VSMCs) contract on extracellular pH (pH(o)) increases and relax on pH(o) decreases. These changes in tone are believed to result from changes in [Ca(2+)](i), although the responsible mechanisms are not fully understood. VSMCs also contract in response to 5-hydroxytryptamine (5-HT), which increases [Ca(2+)](i) via both Ca(2+) release and influx. We hypothesized that examining effects of pH(o) decreases on 5-HT-induced [Ca(2+)](i) changes would allow us to identify mechanisms whereby pH(o) influences tone. Accordingly, we compared [Ca(2+)](i) increases in cerebral VSMCs, evoked by 5-HT, with increases evoked by increased pH(o) and examined 5-HT-dependent [Ca(2+)](i) increases at normal and decreased pH(o).

METHODS

We monitored [Ca(2+)](i,), using the Ca(2+)-sensitive dye fura 2, in cultured rat cerebral VSMCs obtained by enzymatic digestion of middle cerebral arteries and their branches (passages 1 to 3) grown on glass coverslips and superfused with physiological saline.

RESULTS

Increasing pH(o) from 7.3 to 7.8 increased [Ca(2+)](i), and these increases were prevented in Ca(2+)-free solutions. Decreasing pH(o) from 7.3 to 6.9 did not alter [Ca(2+)](i) unless [Ca(2+)](i) was first raised by treatment with 5-HT (10 micromol/L). 5-HT resulted in biphasic [Ca(2+)](i) increases characterized by transient peaks blocked by the Ca(2+)-ATPase inhibitor thapsigargin (10 nmol/L) and prolonged plateaus blocked by the Ca(2+) channel blocker Ni(2+) (1 mmol/L). Acidification did not alter the transient peaks but significantly reduced 5-HT-induced Ca(2+) influx.

CONCLUSIONS

We conclude that increasing pH(o) induces Ca(2+) influx in rat cerebral VSMCs and decreasing pH(o) inhibits 5-HT-stimulated Ca(2+) entry but not intracellular Ca(2+) release.

Inhibitory effects of halothane, isoflurane, sevoflurane, and pentobarbital on the constriction induced by hypocapnia and bicarbonate in isolated canine cerebral arteries

The effects of halothane, isoflurane, sevoflurane (0.5, 1 and 2 MAC) and pentobarbital (10(-5) M, 10(-4) M and 3 x 10(-4) M) on hypocapnia- and bicarbonate-induced constriction of isolated dog middle cerebral arteries were investigated in vitro. The isometric tension of isolated cerebral arterial rings was measured in an organ bath containing Krebs bicarbonate solution, aerated with 5% CO2 and 95% O2. Hypocapnia, induced by replacing the bathing solution with one that had been equilibrated with 2.5% CO2 and 97.5% O2, produced a sustained vasoconstriction (268 +/- 36 mg, mean +/- SEM). Exposure of arterial rings to a bathing solution that contained double the concentration of NaHCO3 (50 mM) elicited a phasic constriction followed by a gradual decrease in tension (309 +/- 34 mg). Although halothane, isoflurane, and sevoflurane attenuated both hypocapnia- and bicarbonate-induced constrictions in a dose-dependent manner, the inhibition of these constrictions was greater in rings treated with halothane than in those treated with isoflurane or sevoflurane when compared at equipotent concentrations. These alkaline-induced constrictions were attenuated by pentobarbital only at the highest concentration of 3 x 10(-4) M. Halothane (1 and 2 MAC) attenuated the constriction induced by hypocapnia to a greater extent than that induced by 15 mM KCl, whereas pentobarbital (10(-4) M and 3 x 10(-4) M) attenuated hypocapnia-induced constriction less than KCl-induced constriction. These results indicate that alkaline-induced constriction is more vulnerable to halothane than other volatile anesthetics and pentobarbital. The mechanisms of the inhibitory effects of halothane and pentobarbital on alkaline-induced cerebral vasoconstriction seem to differ; the inhibitory effect of pentobarbital, but not of halothane may be, in part, ascribed to its inhibitory effect on the Ca++ influx.

Interactions between Ca(2+) and H(+) and functional consequences in vascular smooth muscle

Ca(2+) and H(+) ions can profoundly alter vascular tone. In many physiological and pathological processes, changes in the concentration of both ions occur. Thus, to understand the processes and mechanisms that modify force, it is necessary to understand what changes occur in these ions and, importantly, how they interact with each other. In this minireview, we highlight the quantitatively important mechanisms involved in the contractile responses of vascular tissues to pH change and discuss the cellular and molecular reasons underlying these responses.

On the cellular mechanism for the effect of acidosis on vascular tone

The role of smooth muscle [Ca2+]i and membrane potential for the relaxation to hypercapnic (increased CO2) and normocapnic (unchanged CO2) acidosis is not complete understood. It is often stated that membrane hyperpolarization plays an important role but this has not been vigorously tested. In this study we investigated isolated rat cerebral small arteries under isobaric conditions. Lumen diameter was measured simultaneously with either [Ca2+]i or membrane potential, and acidosis was induced by increasing PCO2 or reducing HCO3- of the bathing solution or by adding HCI to a nominally bicarbonate-free solution. Confocal microscopy verified loading of smooth muscle cells with fluorescent dyes. Acidosis always reduced myogenic tone at transmural pressures between 20 and 120 mmHg. Acidification at a transmural pressure of 40 mmHg caused an increase in diameter and a decrease in [Ca2+]i. This was also seen in the presence of L-NNA and after depolarization with 50 mM K+. The response to hypercapnic and normocapnic acidosis was similar. However, while hypercapnic acidosis caused hyperpolarization, normocapnic acidosis caused depolarization. Dilatation, decrease of [Ca2+]i and depolarization, was also seen with reduction of pH in bicarbonate-free solution. We conclude that the isobaric relaxation to both hypercapnic and normocapnic acidosis is most likely mediated by a reduction of [Ca2+]i. Membrane potential may on the other hand not play a major role for this reduction of [Ca2+]i and it is possible that molecular CO2 has an effect on the membrane potential.

Intracellular acidosis differentially regulates KV channels in coronary and pulmonary vascular muscle

Decreases in intracellular pH (pHi) potently dilate coronary resistance arteries but constrict small pulmonary arteries. To define the ionic mechanisms of these responses, this study investigated whether acute decreases in pHi differentially regulate K+ currents in single vascular smooth muscle (VSM) cells isolated from rat coronary and pulmonary resistance arteries. In patch-clamp studies, whole cell K+ currents were elicited by 10-mV depolarizing steps between -60 and 0 mV in VSM cells obtained from 50- to 150-micrometers-OD arterial branches, and pHi was lowered by altering the NH4Cl gradient across the cell membrane. Progressively lowering pHi from calculated values of 7.0 to 6.7 and 6.4 increased the peak amplitude of K+ current in coronary VSM cells by 15 +/- 5 and 23 +/- 3% but reduced K+ current in pulmonary VSM cells by 18 +/- 3 and 21 +/- 3%, respectively. These changes were reversed by returning cells to the control pHi of 7.0 and were eliminated by dialyzing cells with pipette solution containing 50 mmol/l HEPES to buffer NH4Cl-induced changes in pHi. Pharmacological block of ATP-sensitive K+ channels and Ca2+-activated K+ channels by 1 micromol/l glibenclamide and 100 nmol/l iberiotoxin, respectively, did not prevent changes in K+ current levels induced by acidotic pHi. However, block of voltage-gated K+ channels by 3 mmol/l 4-aminopyridine abolished acidosis-induced changes in K+ current amplitudes in both VSM cell types. Interestingly, alpha-dendrotoxin (100 nmol/l), which blocks only select subtypes of voltage-gated K+ channels, abolished the acidosis-induced decrease in K+ current in pulmonary VSM cells but did not affect the acidosis-induced increase in K+ current observed in coronary VSM cells. These findings suggest that opposing, tissue-specific effects of pHi on distinct subtypes of voltage-gated K+ channels in coronary and pulmonary VSM membranes may differentially regulate vascular reactivity in these two circulations under conditions of acidotic stress.

Morphological characteristics and distribution pattern of the arterial vessels in human cerebral cortex: a scanning electron microscope study

BACKGROUND

The human cerebral cortex is supplied by vessels that arise from the pial arteries. These vessels give rise to a dense vascular network that is highly interconnected. Cortical arteries have been classified in different categories. Both their angioarchitectonic pattern and anatomical structures involved in their regulation are not fully understood.

METHODS

Twelve fresh human brains were studied by scanning electron microscopy of vascular corrosion casts.

RESULTS

Four types of arterial vessels in the cerebral cortex--short, middle, long, and transcortical--were identified. The cortical vascular network was formed by several interconnected clusters of vessels, which were arranged in four vascular layers parallel to the pial surface and characterized by different vascular densities. The greatest vascular density corresponded to the middle and deep vascular layers. Circular constrictions were found at the origin of cortical arteries and at their branching sites, probably related to vascular sphincters. Connections between cortical arteries were observed at their initial course. Plastic strips, occasionally related to constrictions, were observed around both middle and long cortical arteries. Other plastic structures, morphologically similar to pericytes, were found around capillary vessels.

CONCLUSIONS

The blood supply to the human cerebral cortex depends on the short, middle, and long cortical arteries, which give rise to a highly anastomosed capillary network. There exist vascular connections between pial arteries and occasionally between cortical arteries. Blood flow autoregulation is probably mediated by smooth muscle cells at the arteriolar level and by pericytes at the capillary level, through endothelial connections.

Role of potassium channels in relaxations of isolated canine basilar arteries to acidosis

BACKGROUND AND PURPOSE

Concentration of hydrogen ions is an important regulator of cerebral arterial tone under physiological and pathological conditions. Previous studies demonstrated that in cerebral arteries, relaxations to hypercapnia are due to decrease in extracellular pH. The present study was designed to determine the role of potassium channels in mediation of cerebral arterial relaxations induced by extracellular acidosis.

METHODS

Rings of canine basilar arteries without endothelium were suspended for isometric force recording. Acidosis (pH 7.3 to 7.0) was produced by incremental addition of hydrochloric acid (1.0N). The concentration of hydrogen ions was continuously monitored with a pH meter.

RESULTS

During contractions to UTP, acidosis (pH 7.3 to 7.0) induced pH-dependent relaxations. These relaxations were abolished in arteries contracted by potassium chloride (20 mmol/L). A nonselective potassium channel inhibitor, BaCl2 (10(-4) and 10(-4) mol/L), and an ATP-sensitive potassium channel inhibitor, glyburide (5 x 10(-6) mol/L), significantly reduced relaxations to acidosis. Furthermore, BaCl2 (10(-4) mol/L) and glyburide (5 x 10(-6) mol/L) abolished relaxations to an ATP-sensitive potassium channel opener, cromakalim (10(-8) to 3 x 10(-5) mol/L). However, these potassium channel inhibitors did not affect relaxations to a voltage-dependent calcium channel inhibitor, diltiazem (10(-8) to 10(-4) mol/L), and glyburide (5 x 10(-6) mol/L) did not alter relaxations to a nitric oxide donor, SIN-1 (10(-9) to 10(-4) mol/L). A calcium-activated potassium channel inhibitor, charybdotoxin (10(-7) mol/L), and a delayed rectifier potassium channel inhibitor, 4-aminopyridine (10(-3) mol/L), did not affect relaxations to acidosis.

CONCLUSIONS

These results suggest that extracellular acidosis causes relaxations of cerebral arteries in part by activation of potassium channels. ATP-sensitive potassium channels appear to contribute to acidosis-induced decrease in cerebral arterial tone.

Acidosis-induced coronary arteriolar dilation is mediated by ATP-sensitive potassium channels in vascular smooth muscle

Although a decrease in extravascular pH has been suggested to be involved in coronary flow regulations during hypoxia, ischemia, and increased metabolic demand of the heart, its vasomotor control mechanism has not been elucidated. To examine the effect of acidosis of vasomotor tone, porcine coronary arterioles (40 to 110 microns) were isolated, cannulated, and pressurized to 60 cm H2O intraluminal pressure without flow for in vitro study. Acidosis (pH 7.4 to 7.0) was produced by adding HCl to the extravascular solution. The involvement of potassium channels in the vasomotor response to acidosis was evaluated by using BaCl2 (100 mumol/L, nonspecific potassium channel inhibitor), glibenclamide (5 mumol/L, ATP-sensitive potassium channel inhibitor), and iberiotoxin (100 nmol/L, calcium-activated potassium channel inhibitor). To determine whether endothelial hyperpolarization contributes to the acidosis-induced dilation, the pH-diameter relation of the vessel was examined under a high intraluminal concentration of KCl (40 mmol/L). The involvement of nitric oxide and prostaglandins was assessed by using NG-monomethyl-L-arginine (L-NMMA, 10 mumol/L) and indomethacin (10 mumol/L), respectively. To evaluate the role of endothelium in the acidosis-induced dilation, the pH-diameter relation was studied after endothelial removal. All vessels developed a similar level of spontaneous tone (internal diameter, 75 +/- 4 microns [approximately 69 +/- 1% of maximum diameter) and dilated to HCl in dose-dependent manner. Glibenclamide completely abolished vasodilation to a mild level of acidosis (pH 7.2 to 7.3) and attenuated the vasodilation by 70% at pH 7.0. Acidosis-induced dilation was also inhibited by BaCl2 but not by iberiotoxin. L-NMMA, indomethacin, and intraluminal KCl did not alter the pH-diameter relation. Vasodilation to acidosis of the endothelium-denuded vessels was identical to that of the endothelium-intact vessels. In addition, glibenclamide attenuated the acidosis-induced arteriolar dilation of endothelium-denuded vessels. These results suggest that the opening of ATP-sensitive potassium channels in vascular smooth muscle mediates the coronary arteriolar dilation during acidosis.