Vascular smooth muscle cell contraction and relaxation in the isolated aorta: a critical regulator of large artery compliance

Abstract

Over the past few decades, isometric contraction studies of isolated thoracic aorta segments have significantly contributed to our overall understanding of the active, contractile properties of aortic vascular smooth muscle cells (VSMCs) and their cross-talk with endothelial cells. However, the physiological role of VSMC contraction or relaxation in the healthy aorta and its contribution to the pulse-smoothening capacity of the aorta is currently unclear. Therefore, we investigated the acute effects of VSMC contraction and relaxation on the isobaric biomechanical properties of healthy mouse aorta. An in-house developed set-up was used to measure isobaric stiffness parameters of periodically stretched (10 Hz) aortic segments at an extended pressure range, while pharmacologically modulating VSMC tone and endothelial cell function. We found that the effects of α1-adrenergic stimulation with phenylephrine on the pressure-stiffness relationship varied in sensitivity, magnitude and direction, with the basal, unstimulated NO production by the endothelium playing a pivotal role. We also investigated how arterial disease affected this system by using the angiotensin-II-treated mouse. Our results show that isobaric stiffness was increased and that the aortic segments demonstrated a reduced capacity for modulating the pressure-stiffness relationship. This suggests that not only increased isobaric stiffness at normal pressure, but also a reduced capacity of the VSMCs to limit the pressure-associated increase in aortic stiffness, may contribute to the pathogenesis of this mouse model. Overall, this study provides more insight in how aortic VSMC tone affects the pressure-dependency of aortic biomechanics at different physiological and pathological conditions.

Keywords: Aortic stiffness; basal NO; mouse; vascular smooth muscle tone.

Figure 1

Contribution of basal VSMC tone to the pressure‐dependency of diastolic diameter (A), compliance (B), and Ep (C). The x‐axis represents the average pressure at which the continuously oscillating vessel (with a stretch amplitude of 40 mmHg), was studied. Removal of extracellular Ca²⁺ (0Ca) from the Krebs‐Ringer (KR) solution did not significantly affect the pressure‐dependency of any of these parameters. Symbols and error bars represent the mean and 95% confidence intervals, respectively

Figure 2

Average diameter distension of one cycle (100 msec) of the oscillating thoracic aorta segments (n = 5–6) at normal (80–120 mmgH, A) and high (180–220 mmHg, B) pressure. VSMC stimulation using either 1 μmol/L phenylephrine (PE) or PE in the presence of 300 μmol/L L‐NAME (PE+LN) decreased isobaric diameter distension as compared to the unstimulated distention in Krebs‐Ringer solution (KR) at normal pressure (80–120 mmHg), while pre‐contraction of the VSMCs increased isobaric diameter distension at high pressure (180–220 mmHg), independently from L‐NAME. Tracings were downsampled to 5 msec intervals. Line and error bars represent the mean and 95% confidence intervals.

Figure 3

Pressure‐dependency of diastolic diameter (A), compliance (B), and Ep (C) in unstimulated conditions (KR) and after stimulation with 1 μmol/L phenylephrine (PE), in the absence or presence of 300 μmol/L L‐NAME (LN). Panels from the first, second, and third row represent the absolute values (A–C), the isobaric change from baseline (KR) (D–F) and the relative isobaric change from baseline (KR) (G–I), respectively. Symbols and error bars represent the mean and 95% confidence intervals, respectively. For clarity, symbols describing the level of significance were omitted from this figure and can be found in Table S1.

Figure 4

Dose response curves showing the compliance (A,C) and Ep (B,D) response to pre‐contraction with PE at different concentrations at normal pressure (80–120 mmHg) and high pressure (180–220 mmHg) in the absence (‐LN, black) or presence (+LN, gray) of 300 μmol/L L‐NAME. The pressure‐dependent shifts in PE sensitivity in the absence and presence of L‐NAME are indicated with solid black and dashed gray lines and arrows, respectively. Dose‐response curves were fitted and EC50 (E, F) and Emax (G, H) values for compliance (E, G) and Ep (F, H) were calculated. Two‐way ANOVA with Bonferroni post‐hoc test, *,**,*** P < 0.05, 0.01, 0.001 versus ‐LN. ##, ### P < 0.01, P < 0.001 for the pressure factor. Symbols and error bars represent the mean and 95% confidence intervals.

Figure 5

Pressure‐dependency of isobaric aortic stiffness in an angII‐mouse model. Maximal aortic distension at 80–120 mmHg (A) and 180–220 mmHg (B) was significantly different between 1‐week ang‐II‐treated (Ang‐II) and vehicle‐treated (Veh) mice. The pressure‐dependency of the diastolic diameter was not significantly different between groups in unstimulated conditions (C). Pressure‐Ep relationship in Krebs‐Ringer (KR), (D) or after pre‐contraction with 2 μmol/L phenylephrine (PE) in the absence (E) or presence (F) of 300 μmol/L L‐NAME (LN) to block basal NO production. The isobaric change in Ep (vs. KR) with PE or PE+LN is shown in (G) and (H), respectively. Tracings in (A, B) are downsampled to 5 msec intervals. Two‐way ANOVA with Bonferroni post‐hoc test for multiple comparisons. #, ### P < 0.05, 0.001 for the treatment factor in (A, B). *,**,*** P < 0.05, 0.01, 0.001 versus vehicle. Symbols and error bars represent the mean and 95% confidence intervals.