Re: How, and why does caffeine affect the rate of a daphnia's heart rate?

Hi Lizzie,
	I hope I can answer your question. Just like you, a daphnia’s heart 
will beat faster when it receives a dose of caffeine. Caffeine belongs to a 
class of compounds called methylxanthines and can block a receptor on the 
surface of heart muscle cells for adenosine. In fact, it is caffeine's 
blockade of the A1 adenosine receptor in the heart that causes the heart to 
pound after a significant caffeine dose. Caffeine and similar compounds 
also inhibit a class of enzymes known as cyclic nucleotide 
phosphodiesterases. These enzymes are, in part  responsible for degrading a 
stimulatory signal produced when excitatory  neurotransmitters activate 
different neurons in the central nervous system (CNS). Thus, when they are 
inhibited by caffeine, the stimulatory signal remains active for a longer period 
of time resulting in a greater sense of alertness (a CNS effect) but  also a 
higher heart rate, blood pressure and respiratory rate.

Take care,

Terry