SCT: Understanding the Importance of Osmoregulation

Cells and bodies could not exist without fluid control. The processes involved are complex and amazing, especially for salmon.

This article was first published in Science & Culture Today.

How Salmon Adjust from Fresh Water to Salt Water, and Back Again
by David F. Coppedge
Science & Culture Today, August 28, 2015

In the salmon sequence of Living Waters: Intelligent Design in the Oceans of the Earth, one of the stories documentary producer Lad Allen wanted to tell was about osmoregulation. That is, the control of body fluids and ions during the transition from fresh water to salt water and back again. This is an important transition in the life of every Pacific salmon.

Allen’s crew researched this fascinating topic, but in the final cut they only had time to mention it briefly, stating, “In preparation for their time at sea, these small fresh water fish must adapt their biology to survive in salt water, then enter an ocean they’ve never seen.” How exactly does it work?

Three main things must occur for the young salmon, called a smolt, to prepare for life in the salty ocean. First, it must start drinking a lot of water. Second, the kidneys have to drop their urine production dramatically. Third, and very important, molecular pumps in the cells of the gills have to shift into reverse, pumping sodium out instead of in. All these physiological changes have to change back when then the mature fish re-enters the freshwater river on its way to spawn. The fish will spend a few days in the intertidal zone as these changes are made automatically.

From a design perspective, the third change is very interesting. The gills contain specialized pumping cells called chloride cells. The membranes of these cells contain molecular machines with the bulky name, Na+/K+ ATPase. We’ll call them NKA for short. As the name implies, the machine spends ATP to operate on sodium (Na) and potassium (K) ions. This machine is important in all animal cells. In fact, they are at work in your brain right now. The Nobel Prize was awarded to Jens Christian Skou in 1997 for his discovery of NKA back in the 1950s. Wikipedia has some simplified diagrams of the pump.

NKA is essential for numerous functions. It helps control cell volume. It contributes to cell signaling. And it regulates the cell’s “resting potential” (or resting voltage), its default electrical charge from which modulations indicate activity, such as the spikes in neurons (see this short Neurotech video overview, and the Wikipedia discussion of the complexities of this operation). Here’s an interesting factoid: most animal cells spend a fifth of their energy on these pumps. Nerve cells, because they depend on ionic charges to transmit signals, spend two-thirds of their energy budget running their NKA pumps.

For all these functions, NKA has to regulate the amount of sodium and potassium inside the cell membrane. Because fresh water is low in sodium, salmon need their gill cells to pump it in as they swim downstream. But once they enter the ocean, sodium is overly abundant, requiring them to pump it out. Salmon also have to pump out the chloride ions (Cl-) that result from dissolved ocean salt….

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