Differential expression of angiotensin II type 1 receptor subtypes within the cerebral microvasculature

Abstract

Arteries and arterioles constrict in response to intraluminal pressure to generate myogenic tone, but the molecular nature of the vascular force-sensing mechanism is not fully characterized. Here, we investigated the role of angiotensin II type 1 receptors (AT₁Rs) on vascular smooth muscle cells in the development of myogenic tone in cerebral parenchymal arterioles from mice. We found that pretreatment with the AT₁R blocker losartan inhibited the development of myogenic tone in these vessels but did not alter the luminal diameter of arterioles with preestablished tone. Rodents express two AT₁R isotypes: AT₁R_a and AT₁R_b. We previously demonstrated that AT₁R_b is expressed at much higher levels compared with AT₁R_a in cerebral pial arteries and is required for myogenic contractility in these vessels, whereas AT₁R_a is unnecessary for this function. Here, we found that AT₁R_a and AT₁R_b are expressed at similar levels in parenchymal arterioles and that genetic knockout of AT₁R_a blunted the ability of these vessels to generate myogenic tone. We also found that AT₁R_b and total AT₁R expression levels are much lower in parenchymal arterioles compared with pial arteries and that parenchymal arterioles are less sensitive to the vasoconstrictive effects of the endogenous AT₁R ligand angiotensin II (ANG II). We conclude that 1) AT₁Rs are critical for the initiation, but not the maintenance, of myogenic tone in parenchymal arterioles, and 2) lower levels of AT₁R_b and total AT₁R in parenchymal arterioles compared with pial arteries result in differences in myogenic and ANG II-induced vasoconstriction between these vascular segments.NEW & NOTEWORTHY Myogenic tone is critical for appropriate regulation of cerebral blood flow, but the mechanisms used by vascular smooth muscle cells to detect changes in intraluminal pressure are not fully characterized. Here, we demonstrate angiotensin II receptor type 1 (AT₁R) is indispensable to initiation, but not maintenance, of myogenic tone in cerebral parenchymal arterioles. Furthermore, we demonstrate differences in AT₁R expression levels lead to critical differences in contractile regulation between parenchymal arterioles and cerebral pial arteries.

Keywords: angiotensin II receptor type 1; cerebral arteries; myogenic tone; parenchymal arterioles; vasoconstriction.

Fig. 1.

Angiotensin II type 1 receptor (AT₁R) activity is required for the development, but not the maintenance, of myogenic tone in parenchymal arterioles. A: representative traces and summary data demonstrating that pretreatment with the selective AT₁R blocker losartan (1 µM) prevents the development of myogenic tone in parenchymal arterioles (N = 4 animals, n = 6 arterioles for both groups; two-way ANOVA with Sidak post hoc test, *P < 0.05). B: representative traces and summary data demonstrating that pretreatment with the angiotensin-converting enzyme inhibitor captopril (10 µM) does not alter the development of myogenic tone in parenchymal arterioles. (N = 4 animals, n = 6 arterioles for both groups; two-way ANOVA with Sidak post hoc test). C: representative traces and summary data demonstrating that pretreatment with water (vehicle for both losartan and captopril) does not alter the ability of parenchymal arterioles to develop myogenic tone (N = 4 animals, n = 6 arterioles for both groups; two-way ANOVA with Sidak post hoc test). D: representative trace and summary data demonstrating that acute administration of losartan (1 µM) to parenchymal arterioles with preestablished myogenic tone does not alter luminal diameter (N = 3 animals, n = 6 arterioles; paired t-test).

Fig. 2.

Parenchymal arterioles from Agtr1a^−/− mice exhibit a diminished myogenic response. A: representative traces and summary data showing that parenchymal arterioles isolated from Agtr1a^−/− mice generate significantly less myogenic tone compared with those isolated from wild-type (WT) mice (WT: N = 4 animals; Agtr1a^−/−: N = 3 animals; n = 6 arterioles for both groups; two-way ANOVA with Sidak post hoc test). B: summary data showing that constriction in response to elevated extracellular K⁺ concentration ([K⁺]; 60 mM) does not differ between parenchymal arterioles isolated from WT and Agtr1a^−/− mice (WT: N = 4 animals; Agtr1a^−/−: N = 3 animals; n = 6 arterioles for both groups; unpaired t-test, *P < 0.05). C: quantitative RT-PCR results showing angiotensin II type 1 receptor b (AT₁R_b) expression in parenchymal arterioles isolated from WT and Agtr1a^−/− mice. mRNA expression was normalized to Actb, encoding β-actin (n = 3 animals/group; unpaired t-test).

Fig. 3.

Expression of angiotensin II type 1 receptor a (AT₁R_a), AT₁R_b, and total AT₁R in parenchymal arterioles and pial arteries. A: quantitative RT-PCR results showing expression levels of mRNA encoding AT₁R_a and AT₁R_b in parenchymal arterioles and pial arteries (n = 3 animals/group; N.S., not significant; t-test, *P < 0.05). B: quantitative RT-PCR results showing total AT₁R expression in parenchymal arterioles and pial arteries. mRNA expression was normalized to Actb, encoding β-actin (n = 3 animals/group; N.S., not significant; unpaired t-test, *P < 0.05).

Fig. 4.

Rapid and persistent desensitization to the vasoconstrictor effects of ANG II. Representative trace (left) and summary (right) data showing that vasoconstriction in response to a second application of ANG II (100 nM), administered after 20 min, is significantly blunted compared with the first administration (N = 3 animals, n = 6 pial arteries; paired t-test, *P < 0.05).

Fig. 5.

Parenchymal arterioles are less sensitive to the contractile effects of ANG II compared with pial arteries. A: representative traces showing that constriction of parenchymal arterioles in response to ANG II (100 nM) is significantly lower compared with that of pial arteries. B: summary data comparing the vasoconstrictive effects of ANG II at different concentrations (N = 3–5 animals, n = 6 vessels for each group at each concentration of ANG II; two-way ANOVA with Sidak post hoc test, *P < 0.05). C: summary data showing constriction of parenchymal arterioles and pial arteries in response to elevated K⁺ concentration ([K⁺]; 60 mM) (n = 24 in both groups; unpaired t-test, *P < 0.05).