8.8 Exploring the biochemical and mechanical effects of intestinal  (Page 3/6)

Hypothesis 1

SMC contraction can be initiated by agonist stimulation. The first hypothesis explored agonist stimulation in edematous conditions by ACH, as shown in figure 2, and its influence on the magnitude of cellular contraction. Neuron firing causes exocytosis of vesicles carrying ACH at the synaptic terminals. ACH then diffuses across the synaptic cleft to the plasma membrane of the target SMC. This distance is relatively small, with an average value of 12 - 20 nm [link] . ACH then binds to receptor proteins along the plasma membrane, which in turn leads to the formation of the secondary messenger Inositol 1,4,5-trisphosphate ( $I{P}_3$ ) within the cell [link] . Activated $I{P}_3$ stimulates the release of calcium ions ( $C{a}^{2+}$ ) from the sarcoplasmic reticulum, and these calcium ions work as a positive feedback mechanism in the release of more $C{a}^{2+}$ from the inner sarcoplasmic space, in a process known as calcium-induced calcium release (CICR) [link] . The increased calcium concentration affects the membrane potential of the cell along with potassium ( $K^{+}$ ) channels embedded in the plasma membrane [link] . Both the membrane potential and $K^{+}$ channels regulate [ $C{a}^{2+}$ ] as well.

According to the first hypothesis, the chemical pathways are interrupted by intestinal edema. It was proposed that as the volume of fluid increases in the muscularis, the fluid penetrates into the interstitial spaces between SMCs and increases the distance between a target cell and the neuron that activates its contraction. As stated earlier, a typical synaptic cleft ranges from 12-20nm in length, but under edematous conditions this gap can reach anywhere from 30-60 nm [link] . Because of this larger synaptic distance, the volume through which the neurotransmitter ACH can diffuse also increases, and consequently the final concentration reaching the cell membrane receptors is lowered. The magnitude of the contractile response was expected to decrease due to decreased stimulation of secondary reaction networks. In testing hypothesis one, diffusion of ACH across the synaptic space was modeled using the two-dimensional heat equation under normal conditions and edematous conditions of varying magnitude. An explanation of this implementation is found in the discussion of the mathematical model (Section 2.1.1).

Hypothesis 2

Edema causes increased fluid in the interstitial spaces between SMCs, which overlap with the extracellular matrix. The extracellular matrix is composed of different proteins and collagen fibers. These fibers are attached to the cell membranes of the muscle cells and behave similar to coiled springs, able to stretch and compress easily, allowing the attached cells to contract and relax [link] .

However due to excess fluid in the interstitial spaces in edema, the collagen fibers become stretched and uncoiled, increasing the elastic modulus for the collagen fibers. This increased elastic modulus was expected to decrease the magnitude of contraction in SMCs. In testing hypothesis two, additional mechanical tension was placed in the collagen fibers in the extracellular matrix to analyze changes in the magnitude of cellular contraction.