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 und er isotonic conditions but increases it under isometric conditions, co ncomitant with increased pulse-wave velocity. We examined the individu al contributions of aortic constituents to the elastic modulus of the aortic wall to determine if isobaric analysis produces an accurate ass essment of vascular smooth muscle activation. We used a modified Maxwe ll model assuming an incremental elastic modulus (E(inc)) composed of the elastic modulus of elastin fibers (E(E)), the elastic modulus of c ollagen fibers (E(C)) affected by the fraction of collagen fibers (f(C )) recruited to support wall stress, and the elastic modulus of the va scular 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 ad ministration of phenylephrine (5 mug . kg-1 m min-1) were obtained by transient occlusions of the descending aorta and inferior vena cava. R esults 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 th e control state. Phenylephrine administration increased the theoretica l pulse-wave velocity (Moens-Korteweg equation) from 5.25+/-1.03 m/s d uring the control state to 7.57+/-2.53 m/s (P<.005). Active muscle exh ibited 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+/-0.0 83. The maximum value observed correspond, on the pressure-diameter cu rve of the active artery, to a pressure of 234.28+/-46.6 mm Hg and a d iameter of 17.94+/-1.6 mm. The maximum E(SM) derived from the stress-s train 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 pl otted 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 smoot h muscle (with a very low elastic modulus) and the collagen fibers (wh ose high elastic modulus determines the mechanical properties of a lar gely distended artery), and (3) isometric analysis shows E(inc) to inc rease with smooth muscle activation, and therefore this type of analys is appears to be the most appropriate in functional terms.