TY  - JOUR
A1  - Bhattarai, Aroj
A1  - Horbach, Andreas
A1  - Staat, Manfred
A1  - Kowalczyk, Wojciech
A1  - Tran, Thanh Ngoc
T1  - Virgin passive colon biomechanics and a literature review of active contraction constitutive models
JF  - Biomechanics
N2  - The objective of this paper is to present our findings on the biomechanical aspects of the virgin passive anisotropic hyperelasticity of the porcine colon based on equibiaxial tensile experiments. Firstly, the characterization of the intestine tissues is discussed for a nearly incompressible hyperelastic fiber-reinforced Holzapfel–Gasser–Ogden constitutive model in virgin passive loading conditions. The stability of the evaluated material parameters is checked for the polyconvexity of the adopted strain energy function using positive eigenvalue constraints of the Hessian matrix with MATLAB. The constitutive material description of the intestine with two collagen fibers in the submucosal and muscular layer each has been implemented in the FORTRAN platform of the commercial finite element software LS-DYNA, and two equibiaxial tensile simulations are presented to validate the results with the optical strain images obtained from the experiments. Furthermore, this paper also reviews the existing models of the active smooth muscle cells, but these models have not been computationally studied here. The review part shows that the constitutive models originally developed for the active contraction of skeletal muscle based on Hill’s three-element model, Murphy’s four-state cross-bridge chemical kinetic model and Huxley’s sliding-filament hypothesis, which are mainly used for arteries, are appropriate for numerical contraction numerical analysis of the large intestine.
KW  - virgin passive
KW  - strain energy function
KW  - smooth muscle contraction
KW  - viscoelasticity
KW  - damage
Y1  - 2022
U6  - http://dx.doi.org/10.3390/biomechanics2020013
SN  - 2673-7078
VL  - 2
IS  - 2
SP  - 138
EP  - 157
PB  - MDPI
CY  - Basel
ER  - 
TY  - CHAP
A1  - Akimbekov, Nuraly S.
A1  - Digel, Ilya
A1  - Sherelkhan, Dinara K.
A1  - Razzaque, Mohammed S.
T1  - Vitamin D and Phosphate Interactions in Health and Disease
T2  - Phosphate Metabolism
N2  - Vitamin D plays an essential role in calcium and inorganic phosphate (Pi) homeostasis, maintaining their optimal levels to assure adequate bone mineralization. Vitamin D, as calcitriol (1,25(OH)2D), not only increases intestinal calcium and phosphate absorption but also facilitates their renal reabsorption, leading to elevated serum calcium and phosphate levels. The interaction of 1,25(OH)2D with its receptor (VDR) increases the efficiency of intestinal absorption of calcium to 30–40% and phosphate to nearly 80%. Serum phosphate levels can also influence 1,25 (OH)2D and fibroblast growth factor 23 (FGF23) levels, i.e., higher phosphate concentrations suppress vitamin D activation and stimulate parathyroid hormone (PTH) release, while a high FGF23 serum level leads to reduced vitamin D synthesis. In the vitamin D-deficient state, the intestinal calcium absorption decreases and the secretion of PTH increases, which in turn causes the stimulation of 1,25(OH)2D production, resulting in excessive urinary phosphate loss. Maintenance of phosphate homeostasis is essential as hyperphosphatemia is a risk factor of cardiovascular calcification, chronic kidney diseases (CKD), and premature aging, while hypophosphatemia is usually associated with rickets and osteomalacia. This chapter elaborates on the possible interactions between vitamin D and phosphate in health and disease.
KW  - Vitamin D
KW  - PTH
KW  - Phosphate
KW  - FGF23
KW  - Klotho
Y1  - 2022
SN  - 978-3-030-91621-3
U6  - http://dx.doi.org/10.1007/978-3-030-91623-7_5
SP  - 37
EP  - 46
PB  - Springer
CY  - Cham
ER  -