ASSESSMENT OF SMOOTH-MUSCLE CONTRIBUTION TO DESCENDING THORACIC AORTIC ELASTIC MECHANICS IN CONSCIOUS DOGS

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 (E(inc)) composed of the elastic modulus of elastin fibers (E(E)), the elastic modulus of collagen fibers (E(C)) affected by the fraction of collagen fibers (f(C)) recruited to support wall stress, and the elastic modulus of the vascular smooth muscle (E(SM)) according to the following formula: E(inc) = E(E) + E(C) x f(C) + E(SM). E(inc) 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 mug . kg-1 m min-1) were obtained by transient occlusions of the descending aorta and inferior vena cava. Results were as follows: E(E) was 4.99+/-1.58 X 10(6) dynes/cm2 (mean+/-SD), and E(C) was 965.8+/-399.8 x 10(6) 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<.005). Active muscle exhibited a unimodal stress-strain curve with a maximum stress of 0.949+/-0.57 x 10(6) dyneS/cm2 at a Corresponding strain value of 1.299+/-0.083. The maximum value observed correspond, on the pressure-diameter curve of the active artery, to a pressure of 234.28+/-46.6 mm Hg and a diameter of 17.94+/-1.6 mm. The maximum E(SM) derived from the stress-strain relation of the active muscle was 8.345+/-7.56 X 10(6) dynes/cm2 at a strain value of 1.283+/-0.079. This point was located at 208.01+/-40.8 mm Hg and 17.73+/-1.41 mm on the active pressure-diameter curve. During activation of vascular smooth muscle, E(inc) decreased (P<.05) when plotted against internal pressure but increased (P<.05) when plotted against strain, over the operative range. Our results show that (1) the analysis of the elastic behavior of active muscle is feasible in conscious dogs, (2) the isobaric analysis of elastic modulus is, in fact, a comparison of two different materials, ie, the vascular smooth muscle (with a very low elastic modulus) and the collagen fibers (whose high elastic modulus determines the mechanical properties of a largely distended artery), and (3) isometric analysis shows E(inc) to increase with smooth muscle activation, and therefore this type of analysis appears to be the most appropriate in functional terms.