MadSci Network: General Biology

Re: Why is the heart rate of Daphnia affected by the presence of ions?

Date: Tue May 2 08:07:21 2000
Posted By: Terry Hebert, Faculty, Universite de Montreal, Biochemistry, Montréal Heart Institute
Area of science: General Biology
ID: 956572367.Gb

Dear Karen,

I hope my answer helps. We'll start with the basics. As you've said it's 
all about depolarization. First of all, we need to say that the cells 
responsible for the heartbeat, the cardiac muscle cells or myocytes have a 
resting membrane potential which is caused by the distribution of ions 
inside and outside the cell- high potassium, low sodium inside and high 
sodium, low potassium outside. In a cardiac action potential, the upswing 
or depolarization is caused by an influx of sodium ions into the cell 
carried along its diffusion gradient by a protein called the sodium 
channel. Shortly after the sodium channels open, they close and another set 
of channels (permeable to potassium) opens and lets potassium out, thus 
repolarizing the cell. In addition there are a number of pumps and other 
channels which help maintain the resting potential between beats. So, why 
does increased external potassium cause the heart to beat faster? Well, if 
you increase the external potassium concentration you change the resting 
potential and thus you change the threshold for firing an action potential. 
Basically, you make it easier for the cell to fire an action potential (it 
requires less of a depolarization) and thus it can beat faster. The second 
thing this change in external potassium concentration does is shorten the 
time between action potentials. It does this by effecting the driving force 
(i.e. the difference between external and internal potassium concentration) 
for potassium and thus many of the potassium channels which are opened upon 
repolarization act differently than normal thus causing the cell to 
repolarize faster than normal. I've simplified a very complicated process 
here but I think you get the general idea. 

Good luck,


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