Effects of age and exercise training on coronary microvascular smooth muscle phenotype and function

Abstract

Coronary microvascular function and blood flow responses during acute exercise are impaired in the aged heart but can be restored by exercise training. Coronary microvascular resistance is directly dependent on vascular smooth muscle function in coronary resistance arterioles; therefore, we hypothesized that age impairs contractile function and alters the phenotype of vascular smooth muscle in coronary arterioles. We further hypothesized that exercise training restores contractile function and reverses age-induced phenotypic alterations of arteriolar smooth muscle. Young and old Fischer 344 rats underwent 10 wk of treadmill exercise training or remained sedentary. At the end of training or cage confinement, contractile responses, vascular smooth muscle proliferation, and expression of contractile proteins were assessed in isolated coronary arterioles. Both receptor- and non-receptor-mediated contractile function were impaired in coronary arterioles from aged rats. Vascular smooth muscle shifted from a differentiated, contractile phenotype to a secretory phenotype with associated proliferation of smooth muscle in the arteriolar wall. Expression of smooth muscle myosin heavy chain 1 (SM1) was decreased in arterioles from aged rats, whereas expression of phospho-histone H3 and of the synthetic protein ribosomal protein S6 (rpS6) were increased. Exercise training improved contractile responses, reduced smooth muscle proliferation and expression of rpS6, and increased expression of SM1 in arterioles from old rats. Thus age-induced contractile dysfunction of coronary arterioles and emergence of a secretory smooth muscle phenotype may contribute to impaired coronary blood flow responses, but arteriolar contractile responsiveness and a younger smooth muscle phenotype can be restored with late-life exercise training. NEW & NOTEWORTHY Aging impairs contractile function of coronary arterioles and induces a shift of the vascular smooth muscle toward a proliferative, noncontractile phenotype. Late-life exercise training reverses contractile dysfunction of coronary arterioles and restores a young phenotype to the vascular smooth muscle.

Keywords: adiponectin; contractile phenotype; secretory phenotype; vascular smooth muscle; vasoconstriction.

Fig. 1.

Vasoconstriction to the thromboxane analog, U46619, in intact (A) and denuded (B) coronary resistance arterioles from young sedentary (YSED), old sedentary (OSED), young exercise-trained (YET), and old exercise-trained (OET) rats. Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). †Effect of exercise training (P ≤ 0.05).

Fig. 2.

Vasoconstriction to potassium chloride (KCl) in denuded coronary resistance arterioles from young sedentary (YSED), old sedentary (OSED), young exercise-trained (YET), and old exercise-trained (OET) rats. Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). †Effect of exercise training (P ≤ 0.05).

Fig. 3.

Expression of contractile proteins in denuded coronary arterioles from YSED, OSED, YET, and OET rats. A: vascular smooth muscle heavy chain isoform 1 (SM1). B: vascular smooth muscle heavy chain isoform 2 (SM2). Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). †Effect of exercise training (P ≤ 0.05).

Fig. 4.

Effects of age and exercise training on levels of total (A) and phosphorylated (p-; B) ribosomal protein S6 (rpS6) in denuded coronary arterioles from YSED, OSED, YET, and OET rats. Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). **Effect of age (P = 0.058). †Effect of exercise training (P ≤ 0.05).

Fig. 5.

Immunohistochemical analysis of vascular smooth muscle phenotype in coronary arterioles. A: representative cross-sections for YSED (a–d), OSED (e–h), YET (i–l), and OET (m–p). Merged images (d, h, l, and p) include blue DAPI-stained nuclei (DAPI; a, e, i, and m), red smooth muscle α-actin (SM actin; b, f, j, and n), and green phospho-histone H3-positive nuclei (PHH3; c, g, k, and o). Scale bar corresponds to 100 µm. B: smooth muscle nuclei/cross-sectional area. C: smooth muscle proliferation. %PHH pos, percentage positively stained for PHH3. Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). †Effect of exercise training (P ≤ 0.05).

Fig. 6.

Effects of age and exercise training on body weight, fat mass, and lean mass. A: weekly changes in body weight in YSED, OSED, YET, and OET rats. Absolute body fat mass (B) and percentage of body fat (C) in YSED, OSED, YET, and OET rats are shown. Absolute lean mass (B) and percentage of lean mass (C) in YSED, OSED, YET, and OET rats are also shown. Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). †Effect of exercise training (P ≤ 0.05).

Fig. 7.

A: circulating adiponectin in YSED, OSED, YET, and OET rats. Expressions of AMPK protein (B) and adiponectin R1 receptor (C and D) in coronary arterioles from YSED, OSED, YET, and OET rats are shown. In B, representative cross-sections of AdipoR1 staining in coronary arterioles were from YSED (a), OSED (b), YET (c), and OET (d) rats. Scale bar corresponds to 100 µm. Values are means ± SE; n = no. of rats. *Effect of age (P ≤ 0.05). †Effect of exercise training (P ≤ 0.05). AU, arbitrary units.