The angiotensin receptor blocker losartan reduces coronary arteriole remodeling in type 2 diabetic mice

Abstract

Cardiovascular complications are a leading cause of morbidity and mortality in type 2 diabetes mellitus (T2DM) and are associated with alterations of blood vessel structure and function. Although endothelial dysfunction and aortic stiffness have been documented, little is known about the effects of T2DM on coronary microvascular structural remodeling. The renin-angiotensin-aldosterone system plays an important role in large artery stiffness and mesenteric vessel remodeling in hypertension and T2DM. The goal of this study was to determine whether the blockade of AT1R signaling dictates vascular smooth muscle growth that partially underlies coronary arteriole remodeling in T2DM. Control and db/db mice were given AT1R blocker losartan via drinking water for 4 weeks. Using pressure myography, we found that coronary arterioles from 16-week db/db mice undergo inward hypertrophic remodeling due to increased wall thickness and wall-to-lumen ratio with a decreased lumen diameter. This remodeling was accompanied by decreased elastic modulus (decreased stiffness). Losartan treatment decreased wall thickness, wall-to-lumen ratio, and coronary arteriole cell number in db/db mice. Losartan treatment did not affect incremental elastic modulus. However, losartan improved coronary flow reserve. Our data suggest that Ang II-AT1R signaling mediates, at least in part, coronary arteriole inward hypertrophic remodeling in T2DM without affecting vascular mechanics, further suggesting that targeting the coronary microvasculature in T2DM may help reduce cardiac ischemic events.

Keywords: Angiotensin receptor blocker; Coronary arterioles; Type 2 diabetes mellitus; Vascular remodeling.

Fig. 1

AT₁R blockade and AT₁R KO reduce diabetes-induced coronary arteriole remodeling. H&E-stained sections of septal coronary arterioles from 16 week control (A), db/db (B), db/db + candesartan (C), db/db^AT1R+/− (D), and db/db^AT1R−/− (E) mice. Representative of n = 4 mice per group.

Fig. 2

Differential expression of AT₁R between vascular beds. mRNA expression was quantified by real-time PCR and normalized to the internal reference Rpl13a. Values are expressed as fold change (control aorta set to 1.0). Values are mean ± SEM, *p < 0.05 to control aorta; ##p < 0.01 to db/db aorta. N = 3 or 4 per group.

Fig. 3

T2DM coronary arteriole passive structural measurements: lumen diameter (A), external diameter (B), wall thickness (C), wall:lumen ratio (D), and medial cross sectional area (E). *p < 0.05; ***p < 0.001 vs. control; ^p < 0.05; ^^p < 0.01 vs. db/db untreated; n = 9–10 per group.

Fig. 4

Losartan blocked the growth of coronary arterioles in db/db mice. Decreased growth index of isolated coronary arterioles under a range of pressures (A). These indices are calculated as a percent over the averaged control; thus only db/db and Los db/db values are plotted and statistics are not performed. Losartan treatment reduced coronary arteriole cell numbers (cells positive for both alpha smooth muscle actin and DAPI) (B) Values are mean ± SEM, *p < 0.05 vs. control; ^p < 0.05 vs. untreated db/db; n = 3–5.

Fig. 5

T2DM coronary arteriole passive mechanical properties: circumferential stress (A), and strain (B) over a range of pressures. Coronary arteriole stress–strain curves (C) and incremental modulus of elasticity (D). *p < 0.05, ***p < 0.001 vs. control; ^p < 0.05 vs. db/db untreated; n = 9–10 per group.

Fig. 6

Differential expression of extracellular matrix components. mRNA expression was quantified by q-PCR and normalized to Rpl13a. Values are relative to control for each target. Values are mean ± SEM, *p < 0.05 relative to control. N = 6 per group.

Fig. 7

Elastin staining of septal coronary arterioles shows increase in db/db (B) over control (A). Losartan treatment (C) shows no difference in elastin staining compared to db/db mice. Representative of n = 3 per group.

Fig. 8

Losartan increases coronary flow reserve in db/db mice. Coronary blood flow was reduced in db/db mice during hyperemic conditions using 3% isoflurane (A). Coronary flow reserve was reduced in db/db mice. Losartan increased coronary flow reserve (B). ***p < 0.001 from control; ^p < 0.05 vs. db/db untreated; n = 7–8 per group.

Fig. 9

LV tissue sections were immunostained with an antibody to CD31 (A). Capillaries (<15 μm), labeled in brown, were counted in 2 fields per slide. Representative of n = 3 per group.