In most, if not all peripheral arteries increases in intraluminal flow elicit dilations. Much less is known regarding the nature of flow-induced responses in cerebral vessels. Previous studies reported both dilations and constrictions to flow in vessels isolated from different regions of the brain. We hypothesized that the nature of the response depends on the origin of cerebral vessels.Methods
Isolated middle cerebral arteries (MCA) and basilar arteries (BA) of rats were studied in a pressure/flow chamber. Changes of inner diameter to stepwise increases in intraluminal flow (at a constant intraluminal pressure of 80 mmHg) were measured by a microangiometer. Intraluminal flow was established by increasing the pressure difference throughout the vessels (ΔP = from 0 to 60 mmHg). Inhibitors of known mechanisms of action were used: Nω-nitro-L-arginine methyl ester (L-NAME) and indomethacin (INDO) to inhibit the synthesis of nitric oxide and prostaglandins, respectively. At the end of experiments the passive diameters (PD) of vessels (in Ca2+ free solution) were measured.Results
In the presence of 80 mmHg intraluminal pressure the basal diameter of MCA and BA were (181 ± 6μm and 328 ± 19μm, which were ∼58% and ∼78 % of PD, respectively). Increases in flow elicited significant constrictions in MCA (from 61 ± 1.2 to 50 ± 1.3% of PD, p < 0.05) and dilations in BA (from 281 ± 36 to 371 ± 21% of PD, p < 0.05). L-NAME, which reduced the dilation to acetylcholine, did not affect flow-induced constriction of MCA and dilation of BA. Indomethacin inhibited the constriction of MCA, but did not affect the dilation of BA to increases in flow, confirming the findings of previous studies.Conclusions
We propose that the nature and the mediation of responses of cerebral arteries to increases in flow are depending on the location of vessels in the brain. Furthermore, the opposite responses may have important physiological roles: dilations of basilar arteries could be important to amplify the increases in blood flow to the brain in case of increased demand, whereas constrictions of middle cerebral arteries could be important to limit increases of intracranial blood volume.Support
AHA-FA 0855910D, Hungarian Sci. Res. Funds (OTKA) K71591 and K67984.