The Effect of Caffeine on the Heart Rate of Daphnia

Caffeine is widely known as a stimulant. Stimulants cause an increase in the metabolism of the human body resulting in an increase in the rate of the heart and lungs. In small doses they wake you up and give you a high by exciting the central nervous system. In high doses they can make you anxious, irritable and even psychotic. Caffeine perks you up by stimulating the heart and suppressing the effect of adenosine, one of the brains inhibitory chemicals. Caffeine also increases blood pressure and promotes urine formation. I will only be investigating the effect of caffeine on the heart rate.

Daphnia are typically freshwater creatures but are capable of surviving in slightly salty water.

There is a big difference in the size of a Daphniidae, depending on the species. Newly hatched Moina are slightly larger than newly hatched brine shrimps and twice as large as average adult rotifers. Most daphnia populations consist entirely of females that reproduce asexually. A female can produce 100 eggs per brood and can have a brood every 3 days. The female will begin reproducing at about 4 days old. When there is a low food supply, low temperature or lack of oxygen, males are produced and sexual reproduction occurs. Daphnia need oxygen for respiration to occur but they can survive in water with a very low oxygen concentration. This is largely due to their ability to produce haemoglobin.

To reproduce Daphnia require a pH of between 6.5 and 9.5. Daphnia can actually survive outside of this pH, it is only the reproduction that is restricted.

Daphnia are quite hardy when it comes to extremes of temperature although the optimum temperature is 18-22?C. Daphnia are cold blooded which means they do not need to thermoregulate. Their body temperature is that of the water surrounding them.

My research has led me to the conclusion that Daphnia are a suitable organism to investigate the heart rate of when affected by caffeine. There will be little variation between individual daphni of the same species and their outer layer and internal organs are semi-permeable meaning the caffeine can easily be taken up. Their temperature can also be easily regulated.

Hypothesis

The heart rate of daphnia will increase as the concentration of caffeine is increased. This is because the caffeine actually blocks the adenosine (form in which broken down glucose energy is stored) receptive sites on the heart muscle cells causing an increase in heart rate.

Another reason is that caffeine inhibits the production of an enzyme called cyclic nucleotide. Cyclic nucleotide’s purpose is to neutralise a stimulatory signal produced when excitatory neurotransmitters activate different neurons in the central nervous system. Therefore, when cyclic nucleotide is inhibited (by caffeine) the stimulatory signal can’t be “turned off” meaning, not only a greater sense of alertness but also a higher heart rate, blood pressure and respiratory rate.

Plan

I intend to use caffeine of 0.01 mol because this is the most concentrate that I am allowed to use. It will gradually be diluted down with distilled water. I will use similar sized daphnia because this is the closest indicator of age. Different aged daphnia will have even more varied metabolic rates and heart rates.

I will keep the body temperature of the daphnia constant because at different temperatures cell metabolism will occur at different rates. All chemical reactions are at a faster rate at a higher temperature because the molecules have more energy, move faster and therefore collide more often. It will be easy to maintain a constant body temperature because daphnia are cold-blooded and don’t thermoregulate. It will simply mean monitoring the external water temperature.

The pH will loosely be set at as the pH of the tap water I am using. PH doesn’t really need to be controlled because a change in pH won’t actually affect the daphnia, only the amount of reproduction that can occur.

The salinity of the water will be kept constant by using tap water each time. The only variable I will be changing is the concentration of caffeine. Each caffeine concentration will be used 3 times in order to obtain more reliable results.

When counting the heart beat under the microscope I will count for a minute using a clicker counter so that I don’t lose count.

I will take every care not to stress the daphnia by being gentle as possible with them and not disturbing their habitat. If they became sufficiently stressed their heart pressure would rise and caffeine would again rise this amount giving an anomalous result.

My preliminary research informs me that daphnia die at about 0.02 mol caffeine concentration when the heart rate is about 340 bpm. I will therefore go no higher than 0.01 caffeine concentration. Preliminary work also informed me that the most efficient and least stressful method of retrieving the daphnia is to use a medicine dropper and to then place it in the well of a depression slide. The caffeine solution will be constructed using distilled water and caffeine mixed to give a total volume of 10cm3. The daphnia will then be immersed in the solution for a set period. The heart rate will then be monitored under a microscope for a minute whilst using a clicker timer to keep count.