Every day, human kidneys squeeze two litres of urine from our blood by forcing it through microscopic tubules under high pressure. But insects have no such luck. With low blood pressure, and only blind-ending Malpighian tubules to speak of for kidneys, clearing the blood of unwanted ions is much trickier; even mealworms that live on a diet of dry cereals produce urine!Insects have overcome the Malpighian tubule's low blood pressure problem by actively secreting ions through a variety of ion pumps and channels. Emmy Van Kerkhove and team are interested in how potassium ions cross from the blood to the urine, and found that a subset of the cells' potassium transporters in the tubules membrane were involved in extracting potassium from the blood,including the sodium/potassium ATPase (Na+/K+-ATPase)which contributes to the negative voltage difference by pumping 3 sodium ions out and 2 potassium ions in each time(p. 949).

The mealworm Malpighian tubule consists of a single layer of cells. Each cell has a membrane abutting the tubule lumen on one side and blood on the other. These two membranes have very different properties: the membrane on the blood side is designed to take up passing ions, and the luminal membrane is designed to secrete the same ions into the tubule lumen. It is already well understood that movement of potassium from inside the cells into the tubule itself is driven by proton pumps and exchangers in the luminal membrane that add an extra negative voltage difference across the blood-side membrane. But how potassium ions get into the tubule cells from the blood against a steep ion concentration gradient wasn't clear. Van Kerkhove and colleagues decided to find out how potassium ions are extracted from the blood by the tubule's cells, by subjecting the tubules to a variety of `blood' concentrations and blockers in a complex set of ion-substitution experiments.

First, the team tracked down the channels and ion pumps that were involved in ion transport by selectively blocking potassium channels, cotransporters and ATPases and by varying the levels of potassium outside of the Malpighian tubules. But then an unexpected result threw everything into turmoil: whilst trying to understand the role of the Na+/K+-ATPase,Uschi Wiehart blocked the ATPase, expecting to see the cell accumulate positive charge when the pump could no longer remove sodium from the cell. But instead of accumulating positive charge, the cell became more negative(p. 959)!

Van Kerkhove realised that the accumulation of negative charge must be caused by the proton pump, which continues pumping protons out of the cell while the blocked ATPase is unable to exchange sodium for potassium. She also noticed that a potassium channel in the blood-side membrane must be blocked,preventing potassium from entering the cell to balance the extruded protons. Blocking the Na+/K+-ATPase may also cause cellular levels of ATP to rise, so the team wondered if there might be a potassium ATP channel that closes in high levels of ATP. They tested the tubules with a potassium-ATP channel blocker and found that the mealworms had this new type of potassium channel!

But why does the cell need a further potassium channel, when the Na+/K+-ATPase pumps potassium ions perfectly well? Van Kerkhove explains that the new channel could be a potassium emergency service:when potassium in the blood is scant and the Na+/K+-ATPase is working overtime to pump potassium into the cell, the extra gates might augment the struggling Na+/K+-ATPase. Potassium-ATP channels have already been identified in vertebrate cells, but this is the first time that they have been found in an insect epithelium. Van Kerkhove is very excited by the new discovery and adds `when you find something new it stimulates people to look in other cells' and she is optimistic that the team's finding might`ultimately stimulate new research.'

References

Wiehart, U. I. M., Nicolson, S. W. and Van Kerkhove, E.(
2003
). K+ transport in Malpighian tubules of Tenebrio molitor: a study of electrochemical gradients and basal K+ uptake mechanisms.
J. Exp. Biol.
206
,
949
-957.
Wiehart, U. I. M., Klein, G., Steels, P., Nicolson S. W. and Van Kerkhove, E. (
2003
). K+ transport in Malpighian tubules of Tenebrio molitor: is a KATP channel involved?
J. Exp. Biol.
206
,
959
-965.