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

I carried out an investigation where I placed Daphnia is different
solutions of NaCL and KCl and recorded their heart rates. I found that as
the concentration of potassium ions was increased and the concentration of
sodium ions was decreased, the heart rate of the Daphnia decreased.  I am
guessing this has something to do which the wave of depolarisation which
once iniated controls the heart beat.  However, I am not exactly sure how
this links with action potentials and sodium and potassium ions. Please
could you help to explain this.  I have already submitted this question 
but unfortunately, the search you recommended did not prove successful.  I 
replied to the email you sent me but I was unsure if you would receive it, 
so I thought I would re-submit the question.  I have searched your site, 
but there is no information on how and why potassium and sodium ions 
affect the heart rate of Dapnia. There is only information about the 
effects of temperature, drugs, etc. which is not appropriate to my 
investigation.  I really hope you can help.
Thanks,
Karen Parker

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