Intrahepatic cholestasis of pregnancy may be complicated by fetal arrhythmia, fetal hypoxia, preterm labor, and, in severe cases, intrauterine death. The precise etiology of fetal death is not known. However, taurocholate has been demonstrated to cause arrhythmia and abnormal calcium dynamics in cardiomyocytes. To identify the underlying reason for increased susceptibility of fetal cardiomyocytes to arrhythmia, we studied myofibroblasts (MFBs), which appear during structural remodeling of the adult diseased heart. In vitro, they depolarize rat cardiomyocytes via heterocellular gap junctional coupling. Recently, it has been hypothesized that ventricular MFBs might appear in the developing human heart, triggered by physiological fetal hypoxia. However, their presence in the fetal heart (FH) and their proarrhythmogenic effects have not been systematically characterized. Immunohistochemistry demonstrated that ventricular MFBs transiently appear in the human FH during gestation. We established two in vitro models of the maternal heart (MH) and FH, both exposed to increasing doses of taurocholate. The MH model consisted of confluent strands of rat cardiomyocytes, whereas for the FH model, we added cardiac MFBs on top of cardiomyocytes. Taurocholate in the FH model, but not in the MH model, slowed conduction velocity from 19 to 9 cm/s, induced early after depolarizations, and resulted in sustained re-entrant arrhythmias. These arrhythmic events were prevented by ursodeoxycholic acid, which hyperpolarized MFB membrane potential by modulating potassium conductance. Conclusion: These results illustrate that the appearance of MFBs in the FH may contribute to arrhythmias. The above-described mechanism represents a new therapeutic approach for cardiac arrhythmias at the level of MFB. (HEPATOLOGY 2011;54:1282-1292

The nucleophilicity parameters (N) of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene borane and 1,3-dimethylimidazol-2-ylidene borane are 9.55 and 11.88. This places N-heterocyclic carbene boranes (NHC-boranes) among the most nucleophilic classes of neutral hydride donors. Reductions of highly electron-poor C═N and C═C bonds provide hydrogenation products along with new, stable borylated products. The results suggest that NHC-boranes have considerable untapped potential as neutral organic reductants.

We previously reported changes in mechanical properties and collagen cross-linking of the ovine thoracic aorta during perinatal development and postnatal maturation, and we now report changes in biochemical composition (elastin, collagen, and DNA contents per mg wet wt) over the same developmental intervals. A comparison of results from the present and previous studies has yielded novel and important observations concerning the relationship between aortic mechanics and composition during maturation. Developmental changes in aortic incremental elastic modulus at low tensile stress (E(low)) closely followed changes in relative elastin content (i.e., per mg wet wt). An 89% increase in E(low) during the perinatal period was associated with a 69% increase in relative elastin content, whereas neither variable changed during postnatal life. Incremental elastic modulus at high tensile stress (E(high)) did not change during the perinatal period but increased 88% during postnatal life. This pattern closely paralleled changes in collagen cross-linking index, which did not change perinatally but almost doubled postnatally. In contrast, relative collagen content (per mg wet wt) increased only slightly from fetal to adult life, a trend that was unrelated to aortic mechanics. Substantial, progressive decreases in measures of wall viscosity (pressure wave attenuation coefficient and viscoelastic phase angle) from fetal to adult life followed the pattern observed for relative DNA (smooth muscle cell) content (per mg wet wt). Our findings suggest that accumulation of elastin per milligram wet weight contributes most to developmental changes in E(low), change in collagen cross-linking is the primary determinant of developmental changes in E(high), and cell accumulation contributes most to developmental changes in wall viscosity.

Pregnancy produces rapid, non-pathological volume-overload in the maternal circulation due to the demands of the growing fetus. Using a bovine model for human pregnancy, previous work in our laboratory has shown remarkable pregnancy-induced changes in leaflet size and mechanics of the mitral valve. The present study sought to relate these changes to structural alterations in the collagenous leaflet matrix. Anterior mitral valve leaflets were harvested from non-pregnant heifers and pregnant cows (pregnancy stage estimated by fetal length). We measured changes in the thickness of the leaflet and its anatomic layers via Verhoeff-Van Gieson staining, and in collagen crimp (wavelength and percent collagen crimped) via picrosirius red staining and polarized microscopy. Collagen concentration was determined biochemically: hydroxyproline assay for total collagen and pepsin-acid extraction for uncrosslinked collagen. Small-angle light scattering (SALS) assessed changes in internal fiber architecture (characterized by degree of fiber alignment and preferred fiber direction). Pregnancy produced significant changes to collagen structure in the mitral valve. Fiber alignment decreased 17% with an 11.5° rotation of fiber orientation toward the radial axis. Collagen fiber crimp was dramatically lost, accompanied by a 53% thickening of the fibrosa, and a 16% increase in total collagen concentration, both suggesting that new collagen is being synthesized. Extractable collagen concentration was low, both in the non-pregnant and pregnant state, suggesting early crosslinking of newly-synthesized collagen. This study has shown that the mitral valve is strongly adaptive during pregnancy, with significant changes in size, collagen content and architecture in response to rapidly changing demands.

Little is known about how valvular tissues grow and remodel in response to altered loading. In this work, we used the pregnancy state to represent a non-pathological cardiac volume overload that distends the mitral valve (MV), using both extant and new experimental data and a modified form of our MV structural constitutive model. We determined that there was an initial period of permanent set-like deformation where no remodelling occurs, followed by a remodelling phase that resulted in near-complete restoration of homeostatic tissue-level behaviour. In addition, we observed that changes in the underlying MV interstitial cell (MVIC) geometry closely paralleled the tissue-level remodelling events, undergoing an initial passive perturbation followed by a gradual recovery to the pre-pregnant state. Collectively, these results suggest that valvular remodelling is actively mediated by average MVIC deformations (i.e. not cycle to cycle, but over a period of weeks). Moreover, tissue-level remodelling is likely to be accomplished by serial and parallel additions of fibrillar material to restore the mean homeostatic fibre stress and MVIC geometries. This finding has significant implications in efforts to understand and predict MV growth and remodelling following such events as myocardial infarction and surgical repair, which also place the valve under altered loading conditions.

Pregnancy produces rapid, dramatic volume-overload changes to the maternal circulation. This paper examines pregnancy-induced structural-mechanical changes in bovine aortic and mitral heart valve leaflets. Valve leaflets were harvested from non-pregnant heifers and pregnant cows. Dimensions, biaxial extensibility and creep resistance were assessed and related to changes in the collagen network: histological leaflet and anatomic layer thicknesses plus collagen crimp, and biochemical collagen content. Collagen stability and crosslinking were assessed thermomechanically. Pregnancy altered both aortic and mitral valve leaflets. Both valves demonstrated biphasic changes in leaflet stretch, decreasing in early pregnancy and recovering by late pregnancy. Creep in leaflets from both valves was minimal and decreased even further with pregnancy in the mitral valve. There were valve-specific changes in preconditioning areal extension with pregnancy: increasing in the aortic valve and decreasing in the mitral valve. Leaflet area increased dramatically (84% aortic, 56% mitral), with thickening mainly in the fibrosa, accompanied by increases in collagen content (8% aortic, 16% mitral): together suggesting synthesis of new collagen. Collagen crimp was almost completely lost in pregnancy, with the denaturation temperature decreased by approximately 2 °C. Mature and total crosslinking increased, curiously without a significant increase in immature crosslinking. Mature aortic and mitral heart valve leaflets in the maternal cardiovascular system remodel substantially and similarly-despite their different embryological origins.

The mammalian aorta undergoes rapid remodeling during the perinatal period and more gradual remodeling during subsequent development, but the implications of this remodeling for arterial mechanics are poorly understood. In this study in vivo and in vitro techniques were used to determine the static and viscoelastic properties of the thoracic aortas of 119-day-gestation fetal sheep (full term = 145 days), 21-day-old lambs, and adult sheep at control distending pressures and after 70% increases or 30% decreases in pressure. In the weeks surrounding birth, aortic wall tissue became substantially stiffer (static elastic modulus in vitro increased by 28%, and pressure wave velocity in vivo increased by 61%) but less viscous (pressure wave attenuation in vivo decreased by 46%, and viscoelastic phase angle in vitro decreased by 15%), whereas the wall thickness-to-radius ratio was unchanged. By contrast, modest changes in tissue viscoelasticity from neonatal to adult life were accompanied by a halving of the wall thickness-to-radius ratio from 0.19 ± 0.01 to 0.10 ± 0.01. The relative thinning of the vessel wall, combined with a doubling of blood pressure after birth, resulted in a 265% increase in aortic wall tensile stress over the period of study. We concluded that rapid remodeling in the perinatal period primarily alters the viscoelastic properties of aortic wall tissues, whereas more gradual postnatal remodeling largely affects vessel geometry.

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