Re: How and why is the heart rate of daphnia effected by high and low pH?

Dear Alisha,

As you probably already know, Daphnia magna are small crustaceans that are frequently used as "bioindicators" to explore the toxicity of various chemicals in the environment. There are a large number of sites on the Web that have general information on Daphnia. I have listed a few of these in the References section below. Also, the question of changes in pH and the effect on heart rate in Daphnia has been addressed previously on our site, and I refer you to this link for further information. In my answer, I have included some general information on cardiac electrophysiology in humans that may help you interpret your data on Daphnia.

A brief introduction to ionic equilibria and resting membrane potentials
At rest, most animal cells have an electrical charge differential, or voltage, across their plasma membranes (usually around 90 millivolts). This is due mainly to differences in ionic distribution across cell membranes. This difference is known as the resting membrane potential. For most cells, the cytoplasm is usually more electrically negative when compared to the extracellular fluid. The resting membrane potential is maintained in part by ion channels within cell membranes. Since the lipid bilayer itself is impermeable to charged molecules, the movement of ions across the membrane requires protein ion channels that span the membrane. Some of these ion channels are highly specific with respect to the types of ions that are allowed to pass, whereas others allow all ions below a certain size to pass. Furthermore, some ion channels are regulated by the voltage difference across the cell membrane, and others are controlled by specific secreted regulatory molecules.

Basic electrophysiology of the human heart
In general, the electrical behavior of cardiac cells is dependent on the concentration of various ions both intracellularly and in the extracellular medium. These ions are primarily sodium (Na+), potassium (K+), and calcium (Ca++). The intracellular K+ concentration is much greater than the extracellular K+ concentration, while both Na+ and Ca++ are present in greater concentration extracellularly.

The rhythmic contraction of the heart is dependent upon the organized propagation of electrical impulses. The hallmark of such electrical stimulation is the action potential. An action potential is defined as a rapid depolarization in the cell membrane, followed by a return to the resting membrane potential. Voltage-dependent ion channels in the cell membrane are largely responsible for producing action potentials, and different cell types have different action potentials due to their unique populations of voltage-dependent ion channels.

Within the heart, there are two main types of action potentials that can be recorded. One type, the fast response, occurs in atrial and ventricular muscle cells as well as in specialized conducting fibers called Purkinje fibers. There is another type of action potential, the slow response that occurs in the sinoatrial node, the pacemaker region of the heart, and in the atrioventricular node, the specialized region that conducts impulses from atria to ventricles. I will not discuss the details of these action potentials here, but you can certainly read about them in any cardiac physiology text.

Disturbances in ion concentrations
In humans, the concentrations of ions, including H+, are maintained within a relatively narrow range, due to the coordinated actions of a number of organs, notably the lungs and the kidneys. Most organisms thrive in an environment that is maintained at a relatively constant pH. When the pH is outside of the normal range, some organisms including humans, have homeostatic mechanisms that can compensate for this alteration. However, other organisms, like Daphnia, do not have such sophisticated coping mechanisms and often die in extreme environments.

In humans, disturbances in the concentration of H+ can also impact the concentration of other ions in the body. For example, acute increases in H+ can result in a retention of K+. This increased concentration of K+ in body fluids (referred to as hyperkalemia) can impact the contractility of cardiac muscle cells.

I hope this information is helpful! Please feel free to contact me if you have further questions.

-Nikki

References

http://www.wku.edu/~stokes/zoology/225daphnia.html: Physiological Ecology of Daphnia Magna (website now defunct)

Daphnia: An Aquarist's Guide

Some general references on cardiac physiology:

Principles of Physiology, 2nd edition, Berne R. M. and Levy M. N., copyright 1996 by Mosby-Year Book, Inc.

Pathophysiology of Heart Disease, 2nd edition, Lilly, L. S. (ed.), copyright 1998 Williams & Wilkins.