Senior Seminar: Gastric and Pyloric Sphincter Muscle Function (Essay Sample)

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Sobchak, C., Fajardo, A. F., Shifrin, Y., Pan, J., & Belik, J. (2016). Gastric and pyloric sphincter muscle function and the developmental-dependent regulation of gastric content emptying in the rat. American Journal of Physiology-Gastrointestinal and Liver Physiology, 310(11), G1169-G1175.
Background
Feeding intolerance has been found to be a major problem in care of preterm babies. By definition, preterm babies are neonates born at fewer than 37 weeks of the gestation period. The intolerance usually exhibits itself as delay in gastric contents emptying, which poses a problem since the causes of the condition are poorly understood. The condition can lead to malnutrition and premature newborn may lose up to 20% of their weight in the first week of their lives (Khashana & Moussa, 2016). Based on this findings the investigators of this articles sought to comparatively study age-related function of pyloric and gastric smooth muscles together with their regulators using rat samples to help understand the condition much better.
Recently collected data illustrates that gastric emptying function do not exist before 24 weeks of pregnancy and gradually develop with progression of the fetal development before the full term. According to pressure recordings of Duodenal- jejunal intraluminal obtained in premature newborns, the intestinal motor activities are characterized by poorly coordinated and random low- amplitude in prior 31 weeks of gestations. This age- dependent function has also been observed in the gastric antrum where the pressure of intragastric before 32 weeks of pregnancy amounted to 50% of those found in fully developed newborns. Contractions Gastric contents emptying are reliant on a coordinated functioning of the gastro-pyloric motor that involves antrum and fundus muscle contraction as pyloric sphincter relaxes.
Over the past two decades, age-dependent transformation in agonist-induced force has been observed in antrum muscle strips and gastric fundus of cats, guinea pigs, and rabbits. In this regard, the use of rats by the investigators was appropriate, and it makes the study fit the overall literature, as their gastric activity is comparable to human being. In fact, the gastrointestinal function of a 1-2 weeks old rat is comparable to that of 32 weeks gestation newborn. Theoretically, it is proposed that age-dependent changes are dependent on decreased intracellular mobilization of calcium ions following agonist stimulation in neonates compared to adult stomach muscle.
Calcium is known to influence activities of muscle myosin light chain (MLC) kinase and Rho-associated kinase two involved in gastric muscle contractions. Relaxation of muscle occurs when phosphorylation of MLC reduce due to decreased release of calcium ions in the cell or reduced entry into the cell with increased dephosphorylation initiated by NO. The vice versa of this happens during contraction coupled with inhibition of MLCP dephosphorylation by Rho kinase which leads to greater sensitization of myofilaments to calcium resulting in a powerful contraction.
Hypothesis
The investigators hypothesized that the reduced contraction of gastric muscle is the cause of delayed stomach emptying in the newborns. They anticipated that changes dependent on development in Rho-associated kinase 2 (ROCK-2) and muscle myosin light chain kinase (MLCK) expression in stomach muscle and pyloric sphincter leads to possibilities of having decreased gastric- emptying in the early life. In addition, they also predicted that Low amounts of NO early in life might lead to pyloric sphincter not relaxing enough
Experimental Design
Sobchak, Fajardo, Shifrin, Pan & Belik to test their hypothesis they used 1, 2, and >3 weeks (adult) of age rats. The animals gastric was first stimulated using KCI to contract, and observation was made to ascertain whether there are real differences between the force of contraction. This step was meant to determine whether there is a relationship between reduced gastric muscle contraction and the age of the animals. KCI is used as a stimulant since when potassium ions are increased outside a cell, hyperpolarization is suppressed which enables Calcium ions to depolarize the cell causing contraction. Thus by increasing KCL, K+ concentration rises causing greater sensitivity to calcium. They then used an electric field to stimulate contraction of the cells. Notably, electric field opens the voltage-gated Ca+ channels causing the entry of more calcium ions. The last stimulation involved use of Carbachol, which is a cholinergic agonist. The chemical acts like Acetylcholine and it binds to its receptors which lead to K + closure which consequently causes more calcium to enter into the cell and depolarize.
Following depolarization test experiment the investigators focused on determining the part of the stomach that had a significant difference in adult and infant muscle shortening. Fundus muscle shortening was compared to antrum muscle and pylorus muscles shortening. This test is critical in finding out whether shortening of the muscle causes the delay in gastric content emptying. They found a significant difference between the fundus and pylorus muscle shortening only which they then used to test the variation in protein composition that is involved in smooth muscle contraction. The protein examined includes ROCK-2, phosphorylated myosin phosphatase (pMYPT), neuronal nitric oxide synthase (nNOS) and phosphorylated vasodilator pVASP. This was essential, as it would assist in understanding whether the lower concentration of this proteins in the infant’s smooth muscle cell is the primary cause of delayed contraction.
Analysis
Analysis of data was done by comparing the results of tests made in the gastric tissue of one-week old rat with an adult. This study was mainly focused on first ascertaining whether there is a relationship between gastric muscle contraction and age. It was then supposed to establish if the cause of the delay is a lack of sufficient proteins involved in the process of contraction and specifically the Rho-associated kinase 2 (ROCK-2), muscle myosin light chain kinase as well as low amounts of NO. As asserted, the investigators in their first steps used stimulation of KCL, electric field and Carbachol. The three experiments are used to demonstrate whether the delay in muscle contraction is caused by calcium-dependent muscle activation, which forms the basis of the study.
The next step is to establish whether in the delay found to be emanating from calcium-activated pathway of smooth muscle contraction is caused by low concentration of MLCK protein. The investigators accomplish this by first examining the protein composition in the three major muscles involved in contraction of gastric that is fundus antrum and pylorus muscles. To make these work easier, the individuals first examined the differences between shortening of the three muscles from one week old rats compared to adult ones. This experiment assisted in determining which muscle specifically has delayed contraction by evaluating their shortening. They then did a western blot, which can be said to assist in determining if there is a variance in concentration of the protein.
Notably, they also examined in this muscles ROCK-2, phosphorylated myosin phosphatase, neuronal nitric oxide synthase (nNOS) and phosphorylated vasodilator pVASP. All the four proteins are involved in contraction of the muscle, and thus the experiment was appropriate in testing their hypothesis. NO is essential for muscle relaxation, and thus its synthesis can affect time taken to contract. This means that by comparing the amount of nNOS together with the other three enzymes, in adults and newborns can help in understanding whether the delay is caused by little or lack of the enzyme synthesis in the cells.
Results
When KCl was added to both the adult and one week old smooth muscle cell of rats, the adult's cell responded with a greater force of contraction. Upon electrical field stimulation to the cells, investigators found that adults cell leads to a greater force of contraction tha...