Zamir, M.

Patients with congenital heart disease who have a morphological right ventricle (RV) serving as a systemic ventricle have an increased incidence of RV dysfunction. A different structural response of micro‐vessels to increased stress in the RV wall is a possible mechanism for this dysfunction. We examined this possibility by measuring the volume of myocardium perfused by arteries less than 200μm in lumen diameter (V) 7 weeks after a 5mg/kg IV injection of monocrotaline in three 2‐week old and three 18‐week old pigs. This information was obtained by use of 3D micro‐CT of transmural segments (~ 2cm 3 ) of the RV wall. The table provides the average values of V (mm 3 /mm 2 ± SD). Pulmonary artery pressures in the mature, treated pigs were 35±6 mmHg and in the juvenile were 25±2 mmHg. V did not differ from control in the treated 2.5‐month old pigs but trended down in the 6‐month old pigs. We conclude that the volume of myocardium perfused by an arteriole in the RV wall may be an indicator of the perfusion reserve available to the microcirculation in response to increased wall stress. A more definitive answer would require delayed study of the early onset treatment group at the age of 6 months. (Supported in part by NIH grant EB000305)

AbstractMicrovasculature associated with the sciatic nerve was examined using high‐resolution micro‐CT scanning in one group of rats and surgical exploration in another. The results indicate that blood supply to the sciatic nerve is an “open‐ended” system in which the vessels run longitudinally within the epineurium and connect with external vasculature primarily at junction points. Although the range of vasculature found extended down to 4–5 μ, only a few isolated vessels of this size were found, with no capillary “mesh” as such, possibly because of the close proximity of the intrinsic vessel to nerve fibers within the epineurium. While the study did not include direct measurements of flow or nerve function, the “open‐ended” pattern of vasculature found has important implications regarding the relationship between the two. Specifically, the nerve is less vulnerable to a severe or complete disruption in blood supply than it would be under a close‐ended system such as that of the heart or brain, where a severe disruption can occur with the obstruction of only a single vessel. Indeed, the pattern of vasculature found, subject to further study of vasculature at the capillary level, suggests that flow within the intrinsic vessels may be in either direction, depending on circumstances, somewhat like flow within the circle of Willis in the cerebral circulation.

We tested the hypothesis that changes in the arteriolar branching architecture contributed to increased running capacity of rats subjected to two-way artificial selection for intrinsic aerobic endurance treadmill running capacity resulting in strains of low-capacity and high-capacity endurance rats. Hearts and gastrocnemius muscles were harvested from each strain, and the microvasculature's branching geometry measured from micro-CT images. The vascular branching geometry of the hearts and skeletal muscle from the high capacity was indistinguishable from low-capacity rats. Our hypothesis was not supported. Neither remodeling nor an increase in arteriolar microvasculature branching appears to play a role in the enhanced performance of the high capacity rats. We are led to speculate that endothelial tolerance for shear stress and/or increased coupling of myocardial muscle fiber metabolic-to-contractile function is increased in the high-capacity runner strain to the effect of allowing either higher flow rate per unit volume of muscle or more efficient use of oxygen and nutrients in the high-capacity endurance rats.