Evidence for a K(ATP) ion channel link in the inhibition of hypercapnic dilation of pial arterioles by 7-nitroindazole and tetrodotoxin

Strengthening the basics: acids and bases influence vascular structure and function, tissue perfusion, blood pressure, and human cardiovascular disease

Acids and their conjugate bases accumulate in or dissipate from the interstitial space when tissue perfusion does not match the metabolic demand. Extracellular acidosis dilates most arterial beds, but associated acid-base disturbances-e.g., intracellular acidification and decreases in HCO3- concentration-can also elicit pro-contractile influences that diminish vasodilation and even dominate in some vascular beds to cause vasoconstriction. The ensemble activities of the acid-base-sensitive reactions in vascular smooth muscle and endothelial cells optimize vascular resistance for blood pressure control and direct the perfusion towards active tissue. In this review, we describe the mechanisms of intracellular pH regulation in the vascular wall and discuss how vascular smooth muscle and endothelial cells sense acid-base disturbances. We further deliberate on the functional effects of local acid-base disturbances and their integrated cardiovascular consequences under physiological and pathophysiological conditions. Finally, we address how mutations and polymorphisms in the molecular machinery that regulates pH locally and senses acid-base disturbances in the vascular wall can result in cardiovascular disease. Based on the emerging molecular insight, we propose that targeting local pH-dependent effectors-rather than systemic acid-base disturbances-has therapeutic potential to interfere with the progression and reduce the severity of cardiovascular disease.

Endothelium-dependent responses in the microcirculation observed in vivo

Endothelium-dependent responses were first demonstrated 40 years ago in the aorta. Since then, extensive research has been conducted in vitro using conductance vessels and materials derived from them. However, the microcirculation controls blood flow to vital organs and has been the focus of in vivo studies of endothelium-dependent dilation beginning immediately after the first in vitro report. Initial in vivo studies employed a light/dye technique for selectively damaging the endothelium to unequivocally prove, in vivo, the existence of endothelium-dependent dilation and in the microvasculature. Endothelium-dependent constriction was similarly proven. Endothelium-dependent agonists include acetylcholine (ACh), bradykinin, arachidonic acid, calcium ionophore A-23187, calcitonin gene-related peptide (CGRP), serotonin, histamine and endothelin-1. Normal and disease states have been studied. Endothelial nitric oxide synthase, cyclooxygenase and cytochrome P450 have been shown to generate the mediators of the responses. Some of the key enzyme systems generate reactive oxygen species (ROS) like superoxide which may prevent EDR. However, one ROS, namely H₂ O₂ , is one of a number of hyperpolarizing factors that cause dilation initiated by endothelium. Depending upon microvascular bed, a single agonist may use different pathways to elicit an endothelium-dependent response. Interpretation of studies using inhibitors of eNOS is complicated by the fact that these inhibitors may also inhibit ATP-sensitive potassium channels. Other in vivo observations of brain arterioles failed to establish nitric oxide as the mediator of responses elicited by CGRP or by ACh and suggest that a nitrosothiol may be a better fit for the latter.

Acid-base regulation and sensing: Accelerators and brakes in metabolic regulation of cerebrovascular tone

Metabolic regulation of cerebrovascular tone directs blood flow to areas of increased neuronal activity and during disease states partially compensates for insufficient perfusion by enhancing blood flow in collateral blood vessels. Acid-base disturbances frequently occur as result of enhanced metabolism or insufficient blood supply, but despite definitive evidence that acid-base disturbances alter arterial tone, effects of individual acid-base equivalents and the underlying signaling mechanisms are still being debated. H+ is an important intra- and extracellular messenger that modifies cerebrovascular tone. In addition, low extracellular [HCO3-] promotes cerebrovascular contraction through an endothelium-dependent mechanism. CO2 alters arterial tone development via changes in intra- and extracellular pH but it is still controversial whether CO2 also has direct vasomotor effects. Vasocontractile responses to low extracellular [HCO3-] and acute CO2-induced decreases in intracellular pH can counteract H+-mediated vasorelaxation during metabolic and respiratory acidosis, respectively, and may thereby reduce the risk of capillary damage and cerebral edema that could be consequences of unopposed vasodilation. In this review, the signaling mechanisms for acid-base equivalents in cerebral arteries and the mechanisms of intracellular pH control in the arterial wall are discussed in the context of metabolic regulation of cerebrovascular tone and local perfusion.

Does hypercapnia-induced impairment of cerebral autoregulation affect neurovascular coupling? A functional TCD study

Neurovascular coupling (NVC) and dynamic cerebral autoregulation (dCA) are both impaired in the acute phase of ischemic stroke, but their reciprocal interactions are difficult to predict. To clarify these aspects, the present study explored NVC in a healthy volunteer population during a surrogate state of impaired dCA induced by hypercapnia. This study aimed to test whether hypercapnia leads to a depression of NVC through an impairment of dCA. Continuous recordings of middle cerebral arteries cerebral blood flow velocity (CBFv), blood pressure (BP), heart rate, and end-tidal CO2 were performed in 19 right-handed subjects (aged >45 yr) before, during, and after 60 s of a passive paradigm during normocapnia and hypercapnia. The CBFv response was broken down into subcomponents describing the relative contributions of BP (VBP), critical closing pressure (VCrCP), and resistance area product (VRAP). VRAP reflects myogenic activity in response to BP changes, whereas VCrCP is more indicative of metabolic control. The results revealed that hypercapnia significantly affected NVC, with significant reductions in the relative contribution of VCrCP to the paradigm-induced increase in CBFv. The present study suggests that hypercapnia impairs both dCA and NVC, probably acting through an impairment of the metabolic component of CBF control.

Optimizing the intraoperative management of carbon dioxide concentration

PURPOSE OF REVIEW

This review assesses whether there is a carbon dioxide concentration range that provides optimum benefit to the patient intraoperatively. It includes the physiological effects of carbon dioxide on various organ systems in awake and anesthetized individuals and its clinical effects in the ischemia/reperfusion setting. This review will present views on end-tidal or arterial carbon dioxide tension management in the perioperative period.

RECENT FINDINGS

Hypocapnia reduces intracranial pressure and is used by clinicians during acute traumatic brain injury, acute intracranial hemorrhage, and acutely growing brain tumors. There is mounting evidence, however, that hypercapnia improves tissue perfusion and oxygenation. Therefore, clinicians may want to induce mild-to-moderate hypercapnia during reperfusion states such as major vascular surgery, organ transplantation, tissue-graft surgery, and cases managed with low mean arterial pressures to control bleeding. As hypercapnia preserves cerebral blood flow even under relatively low perfusion pressures, it may be beneficial during global reperfusion scenarios. This hypothesis needs to be tested extensively before being considered for clinical applications. From a different perspective, current American Heart Association Guidelines recommend 12-15 breaths/min during cardiopulmonary resuscitation and stress the potential negative role of inadvertent hyperventilation on survival outcome. The importance of this concept is discussed briefly.

SUMMARY

Overall, the benefits of managing carbon dioxide concentration intraoperatively for the maintenance of cardiac output, tissue oxygenation, perfusion, intracranial pressure, and cerebrovascular reactivity are well defined.

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.

Ca(2+)-activated potassium (K(Ca)) channel inhibition decreases neuronal activity-blood flow coupling

A number of possible mediators have been proposed to couple neuronal activity with local cerebral metabolic activity and blood flow, but the mechanisms by which these mediators act is still unclear. In order to explore these coupling mechanisms, we used the rodent whisker-barrel cortex (WBC) model to test the hypothesis that modulation of K(Ca) channels is an important step in this coupling process. Anesthetized rats were prepared for laser-Doppler flowmetry (LDF) or evoked potential recordings utilizing a thinned cranial window over WBC. Superfusion of the K(Ca) channel blockers tetraethylammonium (TEA) or iberiotoxin directly onto WBC attenuated the magnitude of the whisker evoked LDF changes. Similar effects were seen after intravenous administration of TEA. Although attenuated, neither the temporal profile of the elicited blood flow responses nor the evoked electrical activity in WBC were affected by K(Ca) blockade. These data suggest that the process of cerebral metabolism/blood flow coupling in the rodent WBC involves K(Ca) channels.

Dilation of rat brain arterioles by hypercapnia in vivo can occur even after blockade of guanylate cyclase by ODQ

1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) is an inhibitor of guanylate cyclase and has been reported to inhibit dilation of cerebral blood vessels by hypercapnia. This supports the hypothesis that this dilation is dependent upon guanylate cyclase, activated by nitric oxide (NO) released from neural tissue. However, there are conflicting reports concerning the role of guanylate cyclase in response to hypercapnia. Therefore, we tested the effect of topically applied ODQ (10 microM) on rat pial arterioles observed with a microscope through a closed cranial window. In one study, we tested ODQ ability to inhibit both the dilation produced by hypercapnia (3% and 5% inspired CO(2)) and, in the same rats, the dilation produced by N-methyl-D-aspartate (NMDA). In another experiment, we tested the ability of ODQ to inhibit dilation produced by hypercapnia and the dilation produced by 3-morpholinosydnonimine (SIN-1), a donor of NO. The responses to NMDA and to NO are known to depend upon activation of guanylate cyclase and were both blocked in the present study. However, the response to hypercapnia was not affected. These findings provide evidence that hypercapnic dilation can occur independently of guanylate cyclase activation.