Assessment of smooth muscle contribution to descending thoracic aortic elastic mechanics in conscious dogs

Abstract

Early investigators found contradictory evidence that vascular smooth muscle activation reduces the elastic modulus of the arterial wall under isotonic conditions but increases it under isometric conditions, concomitant with increased pulse-wave velocity. We examined the individual contributions of aortic constituents to the elastic modulus of the aortic wall to determine if isobaric analysis produces an accurate assessment of vascular smooth muscle activation. We used a modified Maxwell model assuming an incremental elastic modulus (Einc) composed of the elastic modulus of elastin fibers (EE), the elastic modulus of collagen fibers (Ec) affected by the fraction of collagen fibers (fc) recruited to support wall stress, and the elastic modulus of the vascular smooth muscle (ESM) according to the following formula: Einc=EE+Ec×fc+ESM. Einc was assessed in eight conscious dogs using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. Stress-strain relations in the control state and during activation of smooth muscle by continuous administration of phenylephrine (5 μG · kg-1 · min-1) were obtained by transient occlusions of the descending aorta and inferior vena cava. Results were as follows: EE was 4.99±1.58×106 dynes/cm2 (mean±SD), and Ec was 965.8±399.8×106 dynes/cm2, assessed during the control state. Phenylephrine administration increased the theoretical pulse-wave velocity (Moens-Korteweg equation) from 5.25±1.03 m/s during the control state to 7.57±2.53 m/s (P