Question

After Julien was stabilized and had been given antivenom, the attending physician explained that the mechanism of action for Chironex fleckeri, commonly known as the box jellyfish, is not well understood. 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 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 felt lethargic and was having trouble breathing, and noticed something off with his heart rate.

QUESTION: 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?

Answer 1

Calcium play a great role in regulation of muscle contraction including muscle of heart.

Heart consist of billions of muscle that squeeze together during each heart beat and cause pumping of blood. Heart uses electrical signal to make sure that all muscles contract together. This electrical signal is send cell to cell like a wave.

Calcium ion are link between electrical activation and mechanical contraction. Calcium ion can flow in and out of cell through gate like channels called ion channels. These channels help to control the amount of calcium in and out of cell. Cells maintain a negatively charged environment inside them relative to their outside environment.

Calcium ion enter the cell through channel and causes calcium induced calcium release from sarcoplasmic reticulum make cell positively charged and due to which contraction of muscle. After that cell relax where there is outward flow of calcium ions(repolarisation).

Here in the given situation, calcium is inserted into the autorythmic muscle so calcium level rises i. e hypercalcemia. As we know calcium blocks sodium channel and also inhibit depolarisation of muscle,increased calcium lead to rise in thersold for depolarisation.

Autorhythemic cells have pacemaker potential. In these depolarisation is due to inward diffusion of calcium not of sodium ions. Increased level of calcium in these cell lead to more depolarisation thus disturbing heart activity.

Increased levels of calcium would affect heart activity a lot. It is associated with development of hypertension which is difficult to control and lead to many serious problems such as left ventricular atrophy, heart failure etc.

Note:-Autorhythmic muscle are located in SA node, AV node, Bundle of his,, right and left bundle branches, purkinje fibres.