Life and death in the microcirculation: a role for angiotensin II.
OBJECTIVE
Angiotensin II (ANGII) plays a critical role in the maintenance of the microcirculation and in the anatomical loss of microvessels (rarefaction) that occurs in low renin forms of hypertension and in animals fed a high-salt diet. Elevations in sodium intake can trigger a series of hemodynamic and hormonal responses culminating in a substantial rarefaction of small arterioles and capillaries in both normal and reduced renal mass hypertensive rats.


METHODS
Immunohistochemistry, Northern blot, and reverse transcription-polymerase chain reaction (RT-PCR) analysis of microdissected blood vessels were used to localize ANGII receptors in the microcirculation. Chronic infusion of ANGII and other physiologic and pharmacologic manipulations of the reninangiotensin system in rats was combined with morphologic and mathematical analysis of the network architecture.


RESULTS
We have shown that rarefaction of the microcirculation can cause an increase in total peripheral resistance, reduced tissue perfusion, decreased oxygen delivery, and impaired organ function. Although the mechanisms by which this occurs are not well understood, a number of key observations point to a role for the renin-angiotensin system in this effect. First, ANGII infused systemically at subpressor levels, or locally into the skeletal muscle interstitium, can induce significant microvessel growth. Second, localization of ANGII receptor proteins by immunohistochemistry and Western blotting and RNA localization by RT-PCR confirm the presence of AT1 receptors, which are growth-stimulatory, and AT2 receptors, which are growth-inhibitory in the microcirculation. Third, maintenance of ANGII at normal levels during periods of hypertension or high-salt diet completely eliminates rarefaction.


CONCLUSIONS
Taken together, these results support the hypothesis that ANGII acting through AT1- and AT2-receptor mechanisms modulate vessel density during high-salt diet and hypertension.
Greene
A S
AS
Department of Physiology, Medical College of Wisconsin, Milwaukee 53226, USA.
D000804:Angiotensin II / Q000008:administration & dosage / Q000502:physiology*; D000818:Animals; D004195:Disease Models, Animal; D006977:Hypertension, Renal / Q000473:pathology / Q000503:physiopathology; D008833:Microcirculation / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats; D011945:Receptors, Angiotensin / Q000235:genetics / Q000378:metabolism; D012084:Renin-Angiotensin System / Q000187:drug effects / Q000502:physiology; D012982:Sodium, Dietary / Q000008:administration & dosage
D012333:RNA, Messenger; D011945:Receptors, Angiotensin; D012982:Sodium, Dietary; D000804:Angiotensin II