Re: why does potasssium used in a high concentration cause cardiacfibrillation

Hello Natasha

Increasing extracellular potassium concentration in the human heart causes 
a decrease in the membrane potential for the conduction of electrical 
signals around the heart.  Under normal physiological conditions, the 
resting membrane potential is negative.  An increase in extracellular 
potassium concentration results in a decrease in this membrane potential 
because the electrochemical gradient for potassium has been made smaller.  
The rate of rise and the overall amplitude of the action potentials, which 
propagate the electrical signal around the heart, consequently diminish.  
This slows conduction velocity and may cause atrioventricular block, i.e. 
the electrical signal is unable to pass the atrioventricular node.  The 
slowing of conduction velocity will result in a slowing of the heart.  In 
addition, as the resting potential in pacemaker cells becomes less negative 
it approaches the threshold potential, which tends to increase the firing 
rate.  This increase in firing rate will result in fibrillation.  When the 
resting potential is very close to or beyond the threshold level, 
excitability becomes depressed or is abolished.
Unfortunately, I don't know about Daphnia hearts so these may have 
different mechanisms controlling their heart rates.  However, you do not 
specify which ion increases daphnia heart rate and there are different ions 
that have different effects on human heart rate.  The three main ions are 
sodium, potassium, and calcium.  Increasing sodium will also decrease 
contractility of the heart since this competes with calcium to get into the 
cell and therefore increase sodium will mean decreased calcium.  Increasing 
extracellular calcium concentrations increases cardiac contractility.  The 
increased calcium tends to produce a greater calcium influx during the 
action potential and it is the beat to beat maximum intracellular calcium 
concentration that determines the force of contractility.  However, 
increased extracellular calcium concentrations increase the threshold for 
propagation of action potentials which slows the heart rate.  So increased 
extracellular calcium increases the force and decreases the rate of heart 
contractions.

I hope that this has answered your question, sorry I cannot be more 
specific about drosophila.