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IONS AND VOLTAGESIONS AND VOLTAGES

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THE POTASSIUM GRADIENT AND THE RESTING VOLTAGE

Ions are electrically charged. This fact has two consequences for

membranes. First, the movement of ions across

a membrane will tend to change the voltage across that membrane.

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If positive ions leave the cytosol, they will leave the cytosol with a negative voltage, and vice versa.

Second, a voltage across a membrane will exert a force on all the ions present.

If the cytosol has a negative voltage, then positive ions such as sodium and potassium will be attracted in from the extracellular medium.

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Typical Concentrations for Five Important Ions in Mammalian Cytosol and Extracellular Medium

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The positively charged potassium ion cannot cross the lipid bilayer but passes easily through a water-filled tube in the potassium channel

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GENERAL PROPERTIES OF CHANNELS

Channels are integral membrane proteins that form water-filled tubes through the membrane.

the gap junction channel (page 55), porin (page 262), and the potassium channel.

Channels that, like the potassium channel, are selective for particular ions can set up transmembrane voltages.

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The gap junction channel is much less selective than the potassium channel.

It forms a tube, 1.5 nm in diameter, through which any solute of Mr ≤ 1000 can pass.

The gap junction channel is not always open.

It opens only when it connects with a second gap junction channel on another cell, forming a tube through which solutes can pass from the cytosol of one cell to the cytosol of the other.

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Channels that are sometimes open and sometimes shut are said to be gated.

When a gap junction channel contacts another on another cell, its gate opens and solute can pass through

at other times the gate is shut. The usefulness of gating is obvious: if the

gap junction channels not contacting others were open, many solutes, including ATP and sodium, would leak out into the extracellular fluid exhausting the cell’s energy currencies.

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Porin in the outer mitochondrial membrane plays an important role in energy conversion.

It forms a very large diameter tube that allows all solutes of Mr ≤ 10,000 to pass and seems to spend a large fraction of time open under most circumstances.

This is why the outer mitochondrial membrane is permeable to most solutes and ions.

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Cytochrome c —Vital But Deadly

the electron carrier cytochrome c resides in the intermembrane space between the outer and inner mitochondrial membranes and helps the electron transport chain to convert energy as NADH to energy as the hydrogen ion electrochemical gradient across the mitochondrial inner membrane (page 266).

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Although cytochrome c is a soluble protein of relative molecular mass 12,270, it cannot escape from the intermembrane space into the cytosol because porin, the channel of the outer mitochondrial membrane, only allows solutes of Mr ≤ 10,000 to pass.

Although cytochrome c is essential for mitochondrial function, it has another, deadly role.

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If cytochrome c comes into contact with a class of cytosolic enzymes called caspases, it activates them, turning on the process of cell suicide called apoptosis (page 417).

Under certain conditions, porin can associate with other proteins to form a channel of larger diameter; when this happens, cytochrome c can leak out and the cell dies by apoptosis.

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This process seems to occur in hearts during heart attacks, and in the brain during a stroke: there is therefore a considerable research effort aimed at preventing this from occurring.

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Poisoned Hearts Are Stronger Digitalis is used to treat heart failure. Digitalis inhibits the sodium/potassium

ATPase and is extremely toxic. Nevertheless, a small dose, which

inhibits the sodium/potassium pump just a little, causes the heart muscle to beat more strongly.

The reason is that inhibiting the sodium/potassium pump just a little causes a small increase of cytosolic sodium concentration.

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Because the sodium/calcium exchanger has three binding sites for sodium, its activity is extremely sensitive to sodium concentration, and even a small increase of cytosolic sodium reduces its activity significantly.

The calcium concentration in the cytosol therefore rises.

The mechanical motor that drives heart contraction (Chapter 18) is controlled by calcium, so that a small increase of cytosolic calcium makes the heart beat more strongly.

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All the cells of our bodies have one of these two calcium pumps—the sodium/calcium exchanger or the calcium ATPase—and many have both.

Because of the action of these carriers, the calcium concentration in the cytosol is much less than the concentration in the extracellular medium: usually about 100 nmol liter−1 compared with 1 mmol liter−1.

Because the resting voltage is attracting the positively charged calcium ions inward, the overall result is a large electrochemical gradient favoring calcium entry into cells.

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Measuring the Transmembrane Voltage

In 1949 Gilbert Ling and Ralph Gerard discovered that when a fine glass micropipette filled with an electrically conducting solution impaled a cell, the plasma membrane sealed to the glass, so that the transmembrane voltage was not discharged.

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The voltage difference between a wire inserted into the micropipette and an electrode in the extracellular fluid could then be measured.

By passing current through the micropipette, the transmembrane voltage could be altered.

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Twenty-five years later Erwin Neher and Bert Sakmann showed that the micropipette did not have to impale the cell.

If it just touched the cell, a slight suction caused the plasma membrane to seal to the glass.

The technique, called cell-attached patch clamping, can measure currents through the few channels present in the tiny patch of membrane within the pipette.

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Stronger suction bursts the membrane within the pipette.

The transmembrane voltage can now be measured.

Alternatively, current can be passed through the micropipette to change the transmembrane voltage—this is the whole cell patch clamp technique.

In 1991, Neher and Sakmann received the Nobel prize for medicine.

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Action of the calcium ATPase.