Kir Channel Function And Diet

The high potassium foods diet is a diet that has a high ratio of potassium to sodium and results in an alkaline urine. The alkalinity is critical to the ability of the kidney to recycle sodium and excrete large amounts of potassium. This allows our cells to be in electrical balance to prevent hypertension, osteoporosis, and cardiovascular disease. Multiple population based studies have shown the importance of the components of the high potassium foods diet. But basic science studies are needed to provide confirmation of the importance of the potassium to sodium ratio and alkalinity. Without a basic science model, only an association can be shown, not causation.

Potassium and sodium inside and outside the cell determine the electrical field of the cell. There is an optimal balance of this field across the cell membrane of each cell that allows the proper functioning of the cell. Channels, pumps and transporters move potassium, sodium, and other ions across this membrane to maintain this electrical balance. Some of these channels, pumps and transporters are sensitive to the pH (acidity, which is a measure of the concentration of hydrogen ions) in the cell. The proper function of these channels, pumps and transporters is critical to the proper function of the cell.

How One Kir Channel Controls Potassium

The study (1) to be discussed today looked at how one of these potassium channels that is sensitive to acidity/alkalinity (pH) functions. The channel is a Kir channel (an inwardly rectifying potassium channel) that is sensitive to the pH inside the cell. It moves potassium into the cell, which is the opposite direction of most potassium channels. And the opening and closing of the channel depends on how acid or alkaline the cell is.

The researchers examined what forces within the components of the channel affect the channel's structure as it opens and closes. They found that an entire network of hydrogen bonds determined the channel's structure and function. The network included pure hydrogen bonds and salt bridges (hydrogen bonds combined with electrostatic forces).

Previously the researchers showed that a hydrogen bond in the pore of the channel occurred when the channel closed. This study looked at the structure of the channel as it progressed from the closed state to the pre-open state to the open state. The researchers were able to use X-ray crystallography to determine the shape and structure of the molecule in these three states. Using computer simulations, they were able to calculate the free energy of these states, and to determine the number and locations of hydrogen bonds and salt bridges in these states.

The researchers mutated 189 of the 391 amino acids in the channel. Of the 189 positions mutated, 135 showed a functional change in the channel. The other mutations did not affect function. 49 of the mutations affected pH gating (opening and closing of the channel). These mutated channels opened and closed at a different level of acidity than the normal channel. Of the 49 mutations affecting pH gating, 47 caused an increase in pH sensitivity (a lower concentration of hydrogen ions closed the channel). 2 caused a decreased pH sensitivity.

There are three main portions of this potassium channel. The first portion is the selectivity filter that lets potassium in and keeps sodium (and other ions) out. The second is the pore that is the TransMembrane (TM) portion of the channel that allows potassium to pass through the cell membrane into the cell. There are two TM portions that slide on each other during opening and closing. And the third is the cytoplasmic (CTD) portion inside the cell.

How The Channel Opens And Closes

During the transition from closed to pre-open state, the portion of the channel in the cytoplasm moves up toward the pore. Then the 2 TM portions slide apart, opening the pore to allow potassium to pass through.

The researchers looked for interaction between the amino acids as the channel moved from closed to pre-open to open. If two or more amino acids formed hydrogen bonds or salt bridges it was considered a cluster.

They then looked at what happened to the clusters as the channel moved back and forth between closed and open. In the closed state, an unconnected series of small clusters was seen. The largest cluster involved 20 amino acids.

The Network

The clusters were located on the pore where it was near the cytoplasmic portion, and on the cytoplasmic portion near the pore. As the cytoplasmic portion moved toward the pore to open the pore, the clusters coalesced, physically connecting through hydrogen bonds and salt bridges to form a network.

A much larger change in physical connectivity between amino acids occurred during the transition from the closed to the pre-open state. Going from a series of small clusters in the closed state, the clusters connected to form a single large network in the pre-open state. In the open state the network enlarged slightly. In both the pre-open and open state, the amino acids formed a single large network with between 120 and 132 amino acids being connected via hydrogen bonds and salt bridges.

The researchers also found that a change of only one amino acid did not just affect the shape of the channel nearby. It had an effect on the shape of the channel at a great distance if the amino acid was in the network. A change outside the network had little effect. In other words, the entire network was coupled together with hydrogen bonds and salt bridges that depended on one another. Disrupting any one of these connections affected the entire network and the function of the channel.

This coupling indicates that a small mutation in the channel can have a large effect on the function of the channel. Such an effect explains how single mutations in the channel can have such major consequences to health. The large network present in the open state stabilizes it. When a mutation affects the network, the network becomes destabilized and results in a series of small clusters and a closed state.

How Alkalinity Fits In

Because the biggest change in the network occurs between the closed and pre-open state, this is the transition that is most likely to be affected by pH. The researchers found that most mutations increased pH sensitivity, shifting the channel to a closed state when there were fewer hydrogen ions (that is, with a more alkaline pH). This shift resulted in the channel being closed at physiological pH (the normal pH of the cell). Since the principal state of Kir channels is the open state at physiological pH, this results in worse function of the channel, and worse function of cellular processes. At physiological pH the channel no longer functions.

How a single mutational change in a single channel can affect health can be seen from a prior publication (2) that corresponds very well with the above study. The study was about mutations leading to type II Bartter's syndrome. Bartter's syndrome usually appears shortly after birth and involves low blood pressure, and excessive loss of fluids and potassium in the urine. It can lead to death if not caught early enough.

In this prior publication, researchers found mutations in the same channel that the present study reported on. The mutations the prior researchers reported on resulted in an alkaline shift in pH sensitivity with loss of function at physiological pH. In that study the mutations that the researchers discovered occurred in amino acids that are part of the network identified in the present study.

So we have a study showing the importance of maintaining the proper electric field in the cell. If there is malfunction of even one of the channels that balances potassium and sodium to provide this field, the result can be severe. It can result in Bartter's syndrome or, as will be discussed in future posts, in primary aldosteronism or adrenal tumors leading to hypertension. Such basic science studies as these provide the basic underpinnings for a scientific model that connects association with causation. The associations shown in epidemiological studies become causation when confirmed by a basic science model.
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1. State-dependent network connectivity determines gating in a K+ channel. Bollepalli MK, Fowler PW, Rapedius M, Shang L, Sansom MS, Tucker SJ, Baukrowitz T. Structure. 2014 Jul 8;22(7):1037-46. doi: 10.1016/j.str.2014.04.018. Epub 2014 Jun 26.

2. Inwardly rectifying potassium channels: their structure, function, and physiological roles. Hibino H1, Inanobe A, Furutani K, Murakami S, Findlay I, Kurachi Y. Physiol Rev. 2010 Jan;90(1):291-366. doi: 10.1152/physrev.00021.2009.