Question

1. Hyperkalemia

(a) Provide an understanding of how abnormally elevated blood serum K⁺ levels can lead to abnormalities in the resting membrane potential

(b) Describe how hyperkalemia impacts a neurons ability to produce typical action potentials

(c) From your acquired understanding of how hyperkalemia leads to abnormalities in the resting membrane potential, describe how hyperkalemia may result in cardiac arrhythmias.

Answer 1

Hyperkalemia is an elevated level of potassium (K+) in the blood which is caused due to excessive production or ineffective elimination of K+ ion due to several reasons like oral intake, tissue breakdown kidney failure, hypoaldosteronism, and rhabdomyolysis or several medications like NSAIDs.

A) It can lead to abnormalities in the resting membrane potential. It decreases the resting potential i.e. makes it less negative from -90 mV to -80mV and the threshold potential decreases from −75 mV to −70 mV this 5-mV decrease, however, is less than the decrease in resting potential. Therefore, the difference between the resting and threshold potentials decreases to approximately 10 mV therby by making the cell hyperexcitable.But as the potassium levels increase further, the resting membrane potential continues to become less negative and change in threshold potential now parallel the changes in resting potential, and the difference between the two reaches a constant value of approximately 15 mV. Finally, the resting potential decreases to less than the threshold potential leading to depolarization where repolarization cannot occur, and the cell is no longer excitable.

B)Hyperkalemia impacts a neurons ability to produce typical action potentials. initally it makes the cell hyperexcitable but later on gradually the resting potential becomes less than the threshold potential leading to prolonged depolarization from where a cell can't reploarize and can't generate any more of the action potential. The depolarization of membrane inactivates sodium channels and opens potassium channels thus the cells become refractory.

C) Hyperkalemia may result in cardiac arrhythmias. As the potassium levels increase in the extracellular space, the magnitude of the concentration gradient for potassium across the myocyte diminishes, it leads to decrease in the resting membrane potential and the percentage of available sodium which leads to a decrement in the inward sodium current and a concurrent decrease in the Vmax. This decrease in Vmax slow down the impulse conduction through the myocardium and a prolongation of membrane depolarization; as a result, the QRS duration is prolonged. In short, we can say that effect of mild hyperkalemia on myocyte function is to increase myocyte excitability by shifting the resting membrane potential to a less negative value and thus closer to threshold potential; but as potassium levels continue to rise, myocyte depression occurs and Vmax continues to decrease leading to the prolonged depolarization which can't reploarize.