Cerebral arteriolar tone is modulated in response to changes in transmural pressure and luminal flow. The effect of flow on the relation between pressure and diameter has not been fully evaluated in these vessels. This study was conducted to investigate this interaction and to determine the role of the endothelium in mediating it.
Rat pial arterioles from the territory of the posterior cerebral artery were mounted in a perfusion myograph. In some arterioles, the endothelium was removed by air perfusion. Diameters were recorded at pressures from 20 to 200 mmHg in the presence and absence of flow (10 microl/min). The response to flow (0-30 microl/min) was recorded at 60 and 120 mmHg.
In the absence of flow, endothelium-intact arterioles demonstrated tone at distending pressures between 40 and 140 mmHg. In the presence of flow, tone did not develop until pressure exceeded 100 mmHg, and the vessels remained active at pressures up to 200 mmHg. Endothelium-denuded arterioles developed tone at the same pressure when perfused as when unperfused, but perfused vessels were able to maintain active tone at higher pressures. At 60 mmHg, flow caused dilation if the endothelium was intact and constriction if it had been removed. At 120 mmHg, flow caused constriction. Endothelium-dependent flow-relaxation was inhibited by N(G)-nitro-L-arginine methyl ester (10(-5) M) and abolished by indomethacin (10(-5) M).
Flow inhibits the development of pial arteriolar tone at low intraluminal pressures through endothelium-dependent mechanisms. Conversely, perfusion extends the upper limit of the myogenically regulated pressure range through endothelium-independent activation of arteriolar smooth muscle contraction.
THE capacity of pial arterioles to adjust their tone in response to fluctuations in distending pressure contributes to autoregulation of cerebral blood flow. 1–3In most vascular beds, flow activates the release of endothelium-derived relaxing factors that oppose pressure-sensitive (myogenic) vasoconstriction and prevent it from overriding metabolic regulatory mechanisms. 4,5In cerebral arterial segments, in contrast, the response to flow changes to constriction as basal tone is increased, 6–9suggesting that, in this circulation, flow may enhance rather than oppose myogenic tone. However, the maintenance of an established level of tone involves different pathways of smooth muscle activation than does contraction evoked by an increase in transmural pressure. 10Therefore, the effect of basal tone on the flow–response and the effect of flow on myogenic contraction represent interactions between different mechanisms, and the hypothesis that luminal flow enhances the pressure-sensitive constriction needs to be specifically tested.
Factors that modulate pressure-sensitive tone may also alter the range of pressures over which the myogenic response is active. 11–14A change in the threshold pressure for arteriolar contraction would affect the capacity to maintain perfusion at low arterial pressures. Conversely, arterial pressures greater than that at which active tone is lost are associated with hemorrhagic stroke and hypertensive encephalopathy. 15–17Despite their relevance, the effects of luminal flow on the upper and lower limits of the myogenically regulated pressure range have not been assessed in cerebral arterioles. Accordingly, this study was conducted to determine the effect of perfusion on the relation between transmural pressure and diameter in rat pial arterioles and the role of the endothelium in mediating this interaction.
Methods and Materials
The protocol was approved by the animal care committee of the Montreal Neurological Institute, Montreal, Quebec, Canada. Male Sprague-Dawley rats (200–250 g) were stunned and then killed by decapitation. The entire brain was immediately removed and placed in a silicone-lined dissecting dish containing cold (0–4°C), oxygenated, bicarbonate-buffered physiologic salt solution (PSS; NaCl 119 mm; KCl 4.7 mm, KH2PO41.18 mm; MgSO41.17 mm, NaHCO324.9 mm, EDTA 400 μm; CaCl23.7 mm, glucose 5 mm, pH 7.4). The posterior cerebral artery was followed until an unbranched arteriolar segment at least 1 mm in length could be identified. The segment was dissected free, cleared from the adhering tissue, and transferred to a plexiglas vessel chamber (Living Systems, Burlington, VT) containing PSS. Inflow and outflow micropipettes were matched for resistance to flow, and the system was arranged to have mirror symmetry with the axis of symmetry located at the middle of the arteriolar segment. This resulted in equal resistances of the two sides of the system (from pressure transducer to the tip of the pipette). The proximal end of the arteriole was mounted to the inflow cannula and secured with 12-0 suture. The perfusion pressure was then increased to 20 mmHg with a pressure-servo micropump system (Living Systems), taking its inflow from a reservoir of PSS. After the arteriole was cleared of clotted blood, its distal end was mounted to the outflow cannula. Both the inflow and outflow cannulae were connected to microflow pumps (Living Systems). The inflow pump was a constant flow pump that had been calibrated over a range of flows from 0 to 45 μl/min, whereas the outflow pump was regulated by a servo system to maintain a constant downstream pressure. This allowed the vessel to be perfused at constant flow set by the upstream pump while the midpoint pressure (calculated as the average of the upstream and downstream pressures) was maintained at the desired level by adjusting the pressure target for the servo mechanism regulating the downstream pump. In this way, flow is established by changing proximal and distal pressures by an equal amount but in opposite directions such that the average of the upstream and downstream pressures (midpoint luminal pressure) remains constant. 18,19In preliminary studies, we measured midpoint intraluminal pressure by micropuncture using a servo-null pressure transducer (micropressure system model 900; World Precision Instruments, Sarasota, FL) during incremental pressure changes from 0 to 100 mmHg (the range of the pressure transducer) at each of the flows used in the current study. As reported previously by other investigators, 18,19we found no difference between the average of upstream and downstream pressures and the directly measured midpoint pressure at any of the flow rates studied. The arteriole was set to its in situ length using an eyepiece micrometer. The inflow cannula was closed, and the transmural pressure (i.e. , intraluminal pressure relative to atmospheric pressure) was slowly increased to 60 mmHg at zero flow by adjusting the downstream pressure target for the servo mechanism regulating the outflow pump. The pressure–servo system was then placed in manual mode, where a stable pressure value indicated that there was no leak in the system. Vessels in which a leak was detected were discarded.
The apparatus was transferred to an inverted microscope (Nikon TMS-F, 20× objective; Nikon, Melville, NY). Steady state measurements of internal diameter at the midpoint of the arteriolar segment were made using a high resolution CCD video camera (Hitachi KPC503; Hitachi, San Jose, CA) and a video calliper (Living Systems) calibrated using a stage micrometer. The vessel was continuously superfused with PSS flowing through the chamber at a rate of 6 ml/min. The chamber was warmed to 37°C using a heat exchanger in line with the superfusion pump over 60 min and maintained at this temperature throughout the experimental protocol. Chamber temperature and pH were monitored continuously using a probe (Oakton series 35616, Singapore) and samples of the superfusing buffer were periodically drawn from the chamber for gas analysis (model 995, AVL Instruments, Graz, Austria). A plexiglas cover excluded ambient air from the chamber. The reservoir containing the superfusate and the vessel chamber itself were bubbled with gas, the composition of which was adjusted using separate tanks and regulators for each of oxygen, carbon dioxide, and nitrogen, to achieve oxygen and carbon dioxide partial pressure values in the vessel chamber of 100 mmHg and 40 mmHg, respectively. Under these conditions, the vessels gradually developed spontaneous tone independent of vasoconstrictor agents. Endothelium-dependent and -independent dilation was evaluated in all vessels by determining the ability of acetylcholine (10−5m, Sigma) and diethylamine–nitric oxide (DEA-NO, 10−4m, Research Biochemicals International, Nantick, MA), respectively, to inhibit intrinsic tone. On completion of each experiment, the internal diameter was recorded with the vessels in the passive state at a distending pressure of 60 mmHg. The passive state was achieved by bathing the arterioles in calcium-free PSS containing EGTA (4 mm) and adenosine (10−4m).
In vessels in which the endothelium was to be removed, the responses to acetylcholine and DEA-NO were evaluated after the initial equilibration period as previously described. The intraluminal pressure was then reduced to 20 mmHg, the stopcock on the outflow cannula was opened, and the arterioles were perfused with 2 ml air. The arterioles were then perfused with PSS for 10–15 min at 20 mmHg to flush the separated endothelial layer from the vessel lumen and out of the cannula system. The outflow cannula was closed, the intraluminal pressure was restored to 60 mmHg, and the dilatory responses to acetylcholine and DEA-NO were once again determined. In a previous histologic study, 20elimination of vasodilation in response to acetylcholine with an intact vasodilatory response to nitric oxide donors after this procedure has been shown to be associated with ablation of the endothelial cell layer and, in the current study, was taken as evidence of successful endothelial removal.
Three protocols were undertaken to determine (1) the effect of perfusion on the steady state pressure–diameter relation; (2) the effect of changing transmural pressure on the response to flow; and (3) the roles of nitric oxide and prostaglandin release in mediating flow–dilation.
Isoflow Pressure–Diameter Relations.
In separate groups of arterioles, the relation between ID and intraluminal pressure was recorded during zero flow pressurization (n = 14) and during perfusion with the upstream pump set at a constant flow rate of 10 μl/min (n = 14). In half of the arterioles the endothelium was removed. After recording steady state diameter at 60 mmHg, the intraluminal pressure was stepped upward or downward to present pressures at intervals of 20 mmHg over a range from 20 to 140 mmHg in random order. Because, in preliminary studies, arterioles that stabilized at 60 mmHg and pressurized above 140 mmHg in the absence of luminal flow, dilated and did not regain their previous level of spontaneous tone when returned to the lower pressure, pressures greater than 140 were presented last. The highest intraluminal pressures that could be achieved with this system were 200 mmHg in the absence of flow and 180 mmHg during perfusion at 10 μl/min. Diameter was measured 10 min after each pressure change, at which time the vessel had reached steady state. On completion of the active measurements, the superfusion solution was changed to Ca2+-free PSS containing EGTA (4 mm) and adenosine (10−4m), and pressure–diameter data were collected for the vessel in the passive state.
A separate group of endothelium-intact (n = 6) and endothelium-denuded (n = 6) arterioles was equilibrated for 1 h at 37°C and a distending pressure of 60 mmHg. Perfusion at 10 μl/min was then initiated. After recording the ID under these conditions, intraluminal pressure was slowly increased to 180 mmHg by adjusting the upstream and downstream pressures as previously described. The internal diameter at this pressure was recorded, and then the perfusion pump was stopped. The diameter was again recorded after the vessels had reached steady state at 180 mmHg in the absence of flow. On completion of each experiment, the passive diameter was recorded under each of the experimental conditions.
Isopressure Flow–Diameter and Shear Stress–Diameter Relations.
In separate groups of arterioles (n = 14 per group), perfusate flow was increased from 0 to 30 μl/min in 5-μl/min steps while maintaining the intraluminal pressure at either 60 or 120 mmHg (i.e. , near the lower and upper ends of the plateau phase of the pressure–diameter relation). In half of the arterioles in each group, the endothelium was removed. Diameter was recorded after the vessel had reached steady state, 10 min after each flow step. On completion of each experiment, the passive diameter at 60 mmHg was recorded. As described previously, the passive state was achieved by bathing the arterioles with Ca2+-free PSS containing EGTA and adenosine. Because diameter is normally regulated to preserve shear stress at the endothelial–luminal interface within a narrow range, 18,21,22it has been argued that shear, not flow, is the stimulus for endothelial mediator release and should be plotted as the independent variable. Accordingly, the relation between shear stress and steady state diameter after each flow step was also plotted. Shear stress (τ) was calculated as:τ= 4ηQ/πr3, where η is viscosity (Poises), Q is flow (milliliters per second), and r is vessel radius (centimeters). Viscosity of the perfusate (PSS), measured against water in a viscometer (Cannon Instrument Co., State College, PA) at 37°C was 0.0071 Poises.
Effect of Indomethacin and N G-nitro-l -arginine Methyl Ester on Flow–Dilation.
The effect of the nitric oxide synthase antagonist N G-nitro-l-arginine methyl ester (l-NAME) and of indomethacin on the dilatory response to acetylcholine, DEA-NO, and prostaglandin E2(PGE2), were evaluated in endothelium-intact vessels. This was performed to determine if differences in responsiveness to flow after treatment with these agents could be attributed to nonspecific effects. 23At the end of the stabilization period, the responses to acetylcholine (10−5m), DEA-NO (10−4m), and PGE2(10−9m) were determined at a distending pressure of 60 mmHg in the absence of luminal flow by infusing these agents into the superfusion line using the syringe pump. The infusion was stopped, and after the arterioles had returned to their baseline diameters, l-NAME (10−5m, n = 7) or indomethacin (10−5m, 24n = 7) was added to the superfusion solution. After a further 15-min equilibration period, the responses to acetylcholine, DEA-NO, and PGE2were again recorded.
In separate groups of arterioles, at the end of the stabilization period and after evaluating the responses to acetylcholine and DEA-NO, indomethacin (10−5m, n = 7), l-NAME (10−5m, n = 7), or both (n = 7) were added to the superfusion and perfusion solutions. After a further 15-min equilibration period, the flow–diameter relation was determined at distending pressure of 60 mmHg.
Differences among multiple means were detected by analysis of variance corrected for repeated measures where appropriate and analyzed post hoc using the Student-Neuman-Keuls procedure. Unless otherwise stated, data are presented as mean ± SD in n number of animals with P less than 0.05 representing statistical significance.
The IDs for all vessels studied averaged 88 ± 11 μm at the end of the equilibration period. This value is less than the diameter recorded under passive conditions at an intraluminal pressure of 60 mmHg (152 ± 13 μm), indicating that by the end of the equilibration period the arterioles had developed spontaneous tone. The average diameters during relaxation with acetylcholine (129 ± 11 μm) and DEA-NO (149 ± 12 μm) were also significantly greater than that at the end of the equilibration period. The diameter during treatment with DEA-NO did not differ from that recorded in the passive condition.
Figure 1illustrates the relation between intraluminal pressure and ID for endothelium-intact and -removed vessels under active and passive conditions in the absence of luminal flow. Arterioles developed tone at intraluminal pressures greater than 40 mmHg, as evidenced by a significant difference in their diameter at this pressure compared with their corresponding passive diameter. At pressures exceeding 140 mmHg, the vessels dilated toward the passive relation. Results obtained in arterioles from which the endothelium was removed did not differ from those obtained in vessels in which the endothelium was intact.
The top of figure 2presents the pressure–diameter relations for endothelium-intact and -removed vessels that were perfused at 10 μl/min. In the presence of flow, endothelium-intact arterioles dilated passively as intraluminal pressure was increased until it exceeded 100 mmHg. Above this level, further increase in pressure elicited constriction. The perfused vessels were able to maintain a constant diameter at pressures at which arterioles pressurized in the absence of flow were forced to dilate. Endothelium-removed arterioles perfused at 10 μl/min developed tone at pressures above 40 mmHg, and the vessels continued to demonstrate active tone at the highest pressure studied. The effect of stopping flow on the ID of vessels perfused at 10 μl/min at a distending pressure of 180 mmHg is illustrated in the bottom of figure 2. Consistent with the results illustrated in the top of figure 2, cessation of flow was associated with dilation.
Flow–Diameter and Shear Stress–Diameter Relations
The top of figure 3illustrates the relations between flow and diameter and between shear stress and diameter in endothelium-intact and -removed arterioles at an intraluminal pressure of 60 mmHg. In endothelium-intact vessels, pressurized to 60 mmHg, increases in flow and shear stress elicited dilation with diameters approaching those recorded under passive conditions at flow rates greater than 20 μl/min. In endothelium-removed arterioles pressurized to 60 mmHg, increasing flow and shear stress elicited vasoconstriction. The bottom of figure 3presents the results obtained in endothelium-intact and -removed arterioles studied at an intraluminal pressure of 120 mmHg. At this level of intraluminal pressure, increases in flow and shear stress are associated with vasoconstriction that is more pronounced in vessels from which the endothelium had been removed.
Effect of Indomethacin and N G-nitro-l-arginine Methyl Ester on Flow–Dilation
Table 1presents the effects of l-NAME (10−5m) and indomethacin (10−5m) on the change in diameter (response in micrometers) of endothelium-intact vessels (intraluminal pressure = 60 mmHg) after addition of acetylcholine, PGE2, and DEA-NO to the superfusion solution. l-NAME inhibited dilation to acetylcholine without significantly altering the responses to PGE2or DEA-NO. Indomethacin in this concentration had no effect on the responses to acetylcholine, PGE2, or DEA-NO.
Figure 4illustrates the effect of treatment with indomethacin, with l-NAME, and with both agents on diameter during increases in flow and shear stress in endothelium-intact arterioles at an intraluminal pressure of 60 mmHg. Lines representing the results obtained in untreated endothelium-intact and -removed arterioles (presented in fig. 2) are included in figure 3for reference. l-NAME attenuated dilation in response to increases in flow and shear stress (P < 0.05 for difference from untreated endothelium-intact arterioles) but did not reproduce the effect of endothelial ablation. Responses in endothelium-intact arterioles treated with indomethacin or with both indomethacin and l-NAME did not differ from those in arterioles from which the endothelium had been removed.
The main findings of this study are that, in rat pial arterioles, (1) the transmural pressure required for the development of active tone is higher in the presence than in the absence of luminal flow; (2) the upper limit of the myogenically regulated pressure range is higher in perfused than in unperfused vessels; (3) flow elicits endothelium-dependent dilation at low intraluminal pressures (60 mmHg) and endothelium-independent constriction at high intraluminal pressures (120 mmHg); and (4) both l-NAME and indomethacin inhibit flow dilation in vessels pressurized to 60 mmHg, and the effect of indomethacin alone or in combination with l-NAME is similar to that of endothelial ablation.
This is the first study to assess the effect of perfusion on the entire steady state pressure–diameter relation in pial arterioles. The vessels selected for study were from the territory of the posterior cerebral artery from which blood flow to the occipital lobes is derived. These were chosen for their physiologic relevance in the regulation of cerebral blood flow and microvascular pressure 25,26and because previous studies have indicated that they demonstrate a high level of basal pressure-sensitive tone at intraluminal pressures within the physiologic range. 27However, it should be recognized that the intrinsic and neurohumoral mechanisms that regulate tone vary markedly between pial and intraparenchymal arterioles 28,29between arterioles from different regions of the brain 28and among vessels of different size. 30This heterogeneity limits the ability to generalize conclusions based on our findings to other components of the cerebral vasculature.
In the absence of luminal flow, the vessels in the current study maintained a relatively constant diameter over a range of transmural pressures from 40 to 140 mmHg. A similar pressure range has been reported in porcine coronary arterioles 25and in arterioles isolated from the rat diaphragm, 32,33cremaster muscle, 34and mesentery. 35In accord with these previous reports and with results obtained in some, 36but not all, 37,38large artery preparations, this response does not require the presence of an intact endothelial cell layer.
In endothelium-intact pial arterioles perfused at a constant rate of 10 μl/min, we found that the part of the pressure–diameter relation less than 100 mmHg resembles the passive curve. At pressures greater than 100 mmHg, perfused vessels developed active tone that persisted at the highest pressure studied (180 mmHg). In contrast, endothelium-removed arterioles perfused at 10 μl/min developed tone at a distending pressure similar to that recorded in the absence of flow. As in endothelium-intact vessels, perfused arterioles from which the endothelium had been removed maintained their level of tone as transmural pressure was increased to levels above that at which, in the absence of flow, unperfused vessels were forced to dilate.
In previous studies, partial pressure–diameter relations have been generated in the presence of luminal flow in porcine coronary arterioles, 39rabbit renal afferent arterioles, 40and rat intracerebral arterioles. 41Flow was found to inhibit tone at low pressures in those studies and, as a result, the lower end of the pressure–diameter relation was shifted upward. Therefore, under physiologic conditions (i.e. , in the presence of luminal flow), the endothelium plays an important role in determining the pressure threshold for arteriolar smooth muscle activation in both cerebral and noncerebral systemic circulations.
The effect of perfusion on the upper limit of the myogenically regulated pressure range has not previously been assessed in arterioles from any vascular bed. Our current results indicate that, in pial arterioles, flow provides an additional endothelium-independent stimulus for contraction and permits diameter to be maintained in the face of high distending pressures. This may protect the vessel from the deleterious effects of pressurization by preventing a disproportionate increase in wall tension.
In arterioles in which flow was increased while intraluminal pressure was maintained at 60 mmHg, perfusion was accompanied by dilation, but tone was not eliminated (diameter did not equal that recorded under passive conditions) until flow exceeded 20 μl/min. This is at variance with our observation that, in arterioles in which pressure–diameter relations were generated, tone is completely suppressed by perfusion at 10 μl/min at pressures less than 100 mmHg. The difference in the results of the two protocols suggests that flow is less effective at inhibiting preexisting arteriolar tone than it is in preventing its development after changes in transmural pressure. In skeletal muscle arterioles, the inhibitory effect of L-type Ca2+channel antagonists on the response to changes in distending pressure was found to differ from that on basal tone. 10Therefore, the mechanisms that mediate myogenic responses and those that maintain the existing level of tone appear to involve different pathways of Ca2+entry. Differences in the susceptibility of these pathways to inhibition by endothelium-derived mediators could account for the dissimilarity in the effect of flow between the two protocols in the current study.
Flow has been shown to elicit vasodilation, which is eliminated by endothelial ablation, in large arteries 42,43and in isolated arterioles 19,44from a number of vascular beds. In previous studies conducted in the cerebral vasculature, branches of the middle and posterior cerebral arteries of the rabbit were found to relax in response to perfusion when the intraluminal pressure was less than 60 mmHg. 6,45Flow–dilation was attenuated but not eliminated after removal of the endothelium, indicating that a mechanism exists by which flow may trigger vascular smooth muscle relaxation directly. However, this response may be species-specific rather than a characteristic of the cerebral circulation, because similar results have been obtained in arterial segments from other vascular beds in the rabbit. 46In rat intracerebral arterioles, Ngai and Winn 41found that at a constant transmural pressure of 60 mmHg, flows less than 10 μl/min elicit dilation, whereas higher perfusion rates do not. The flow response in endothelium-removed vessels was not investigated in that study. Our current results indicate that in rat pial arterioles, at an intraluminal pressure of 60 mmHg, flows up to 30 μl/min elicit dilation, with the result that shear stress remained constant. Flow–dilation, and the tight regulation of shear stress demonstrated in endothelium-intact vessels, was eliminated after endothelial ablation. We conclude that the mechanisms that mediate flow–dilation in these vessels are localized to the endothelium.
In previous studies, Gaw and Bevan 45and Thorin-Trecases and Bevan 6showed that antagonists of nitric oxide synthesis inhibit flow relaxation in endothelium-intact but not endothelium-removed rabbit cerebral artery segments. Similarly, Ngai and Winn 41and Shimoda et al. 9found that l-arginine analogs eliminate flow–dilation in rat intracerebral arterioles and piglet cerebral arteries, respectively. In the latter study, indomethacin had no effect on flow–dilation, and the investigators concluded that, in the cerebral vasculature in these species, this response is entirely mediated by nitric oxide. In contrast, in arterioles from the rat cremaster muscle, indomethacin was found to eliminate flow dilation 18and, although the effect of inhibiting nitric oxide synthesis was not evaluated, it was concluded that the response was entirely mediated by endothelial prostaglandin release. All of these previous results are at variance with the findings of Koller et al. , 47who studied the effects of indomethacin and N G-nitro-[scap]l-arginine on flow–dilation in arterioles from the rat gracilis muscle. In that study, these antagonists each reduced flow dilation, 39but simultaneous treatment with both agents was required to abolish the response, indicating that the interaction between nitric oxide and cyclooxygenase pathways is additive. The present results demonstrate that, in arterioles from the rat pial circulation, both nitric oxide and prostaglandin release must be intact for the dilatory response to occur. Our finding that treatment with l-NAME blocked flow–dilation even though inhibition of prostaglandin synthesis (with indomethacin) was equivalent to endothelial ablation suggests that, in this circulation, the interaction between these two pathways is not simply additive, as has been reported in arterioles isolated from gracilis muscle. 47This result is not caused by a nonspecific effect of l-NAME on prostaglandin-mediated relaxation because, in the concentrations used, this agent had no effect on the dilator response to PGE2. Administration of exogenous nitric oxide has been shown to modulate prostaglandin synthesis in several cell types, 48,49and activation of cyclooxygenase by endogenously produced nitric oxide has been demonstrated in cultured endothelial cells. 50A permissive role for nitric oxide in pial arterioles, such that its production within the endothelial cell is required for full activation prostaglandin release by flow, would provide an explanation for our results. Further studies are required to evaluate this hypothesis.
Perfusion has been previously reported to elicit constriction in rabbit mesenteric, 51femoral, 52and cerebral arteries, 6,7piglet cerebral arteries, 9and cat pulmonary arteries. 53In rabbit femoral arteries, preconstriction did not affect this response. 52In contrast, rabbit cerebral arteries demonstrate endothelium-dependent flow–dilation at low levels of basal tone and endothelium-independent flow–constriction when tone is enhanced either by increasing transmural pressure 6,7,9,54or with agonists. 6,55In porcine coronary arterioles, increasing basal tone inhibits flow–dilation 39,56but has not been shown to reverse the response from dilation to constriction as it does in the cerebral circulation. The results of the present study indicate that rat pial arterioles resemble qualitatively rabbit cerebral arteries in that the direction of the flow response depends on whether transmural pressure, and hence basal myogenic tone, exceeds a certain threshold. The threshold pressure for reversal of the response was 100 mmHg in the arterioles used in the current study compared with 60 mmHg in the larger cerebral arteries studied previously. 6,7,54This variability could be species-dependent. Nevertheless, it is attractive to speculate that greater sensitivity of the flow response to pressure in larger upstream arteries compared with smaller downstream arterioles would shift the locus of pressure dissipation to more muscular, thicker-walled vessels during episodes of acute arterial hypertension.
Because increasing intraluminal pressure enhances arteriolar tone directly, it is difficult to determine, either from the results of previous studies or from our current findings, the extent to which concomitant impairment of endothelium-derived relaxing factor release contributes to the pressure-dependent reversal of the flow response from dilation to constriction. In the present study, however, we found that at a distending pressure of 120 mmHg, flow–constriction was more marked in endothelium-intact than in endothelium-denuded vessels. Therefore, the endothelium continues to play some role in modulating arteriolar tone at high pressures, either through the release of vasoactive mediators or by providing a physical barrier between the smooth muscle layer and mechanical stresses applied at the luminal surface.
The interaction between the effects of pressure and flow on pial arteriolar tone is inherently protective. In the presence of an intact endothelial cell layer, myogenic tone is eliminated in perfused vessels at low pressures. This is required if cerebral perfusion is to be optimized during episodes of hypotension. Above a critical level, further pressure elevations elicit vasoconstriction, and the capacity to sustain active tone in the face of very high transmural pressures is enhanced. This may be an important mechanism limiting exposure of exchange vessels to the elevated arterial pressure levels that occur during intense muscular contraction, emotional stress, and hypertensive crises.