Rhodnius prolixus doesn't dine often, but when it does, it really goes to town. Consuming fresh blood meals in excess of ten times its unfed mass, the insects really gorge. But their extravagant tastes come at an enormous metabolic cost; the insect must rid itself of huge volumes of fluid before benefiting from its diet. To counteract the dilute meal, Mike O'Donnell explains that Rhodnius produces hyposmotic urine by secreting sodium,potassium and chloride ions into the Malpighian tubule through a cotransporter. Apparently most cotransporters in this class only function if all three ions are present. But O'Donnell adds that Rhodniusreabsorbs most of the valuable potassium ions further along the Malpighian tubule, and the Rhodnius cotransporter appears to function even when potassium is absent. Puzzled, O'Donnell, Juan Ianowski and Robert Christensen were curious to resolve the apparent contradiction().
Measuring the tubule's secretion rates in the presence of potassium while they raised the sodium levels, the team found that potassium transport became inhibited as the sodium levels rose; sodium transport was replacing potassium transport. So instead of continually losing potassium, the insect retained the precious ion while increasing its sodium and chloride losses. The team suspect that this unusual cotransporter behaviour allows Rhodnius to produce large volumes of dilute salty urine, while retaining potassium, as it processes a liquid lunch.
