Question

After Julien was stabilized and had been given antivenom, the attending physician came to talk to the Brahims. She explained that the mechanism of action for Chironex fleckeri, commonly known as the box jellyfish, is not well understood. However, based on what is known, it seems the toxin has multiple effects. One component of the toxin is thought to function as a calcium pore-forming protein that is inserted into the membrane of autorhythmic cells (and possibly other cells). Tanya thought back to her physiology course and remembered that intracellular calcium concentrations are much lower than extracellular concentrations. She knew that this ion plays a large role in cardiac cells and in muscles. When stung, Julien had felt lethargic and was having trouble breathing, and noticed something off with his heart rate. Tanya started to wonder about the pathophysiology of this venom. While she sat next to Julien she began running through various physiological scenarios in her head.

Question

11. Based on what the physician stated, how do you think this toxin would impact the action potentials and function of autorhythmic cells and the activity of the heart? State your hypothesis, and then draw a graph to accompany your predicted outcome.

Answer 1

Calcium ions are link between electrical activation and mechanical contraction. Cells maintain a negatively charged environment inside them relative to their outside environment.

The Ca2+ are released from the sarcoplasmic reticulum in each action potential. This helps to generate contraction for continuous heart beating. After contraction, calcium ions further decrease to normal level via absorption by the sarcoplasmic reticulum during the relaxation phase.

In the above case If the toxin makes calcium-pores, more calcium ions will enter into the cardiomyocytes hence Hypercalcaemia would occur. These calcium ions will cause permanent contraction of muscles, which is called tetanic contraction.

Increased level of calcium in Autorhymic cells would lead to more depolarisation thus disturbing heart activity. That would be leading to  development of hypertension which is difficult to control and lead to many serious problems such as left ventricular atrophy, heart failure etc.

The following graph is showing how the contraction of cardiac muscles is affected. The venom influence starts from summation, then reaches incomplete tetanus and finally at complete tetanus. If antivenom is not administered in time, the heart will stop due to complete tetanus.

THANK YOU :)