All creatures regulate their ionic balance by excreting water and ions to carefully maintain their delicate osmotic balance. Most insects do this through the Malpighian tubule, which transports ions and fluid from the haemolymph into the lumen of the tubule via the principal cells that make up the structure. Geoff Coast from Birkbeck University of London, UK, explains that one class of insect diuretic hormones, diuretic kinins, binds to stellate cells. However, the Malpighian tubules of house crickets lack this cell type, so Coast decided to investigate the function of one diuretic kinin, Achdo-KII, in the transport of ions from the haemolymph across the cricket principal cell and into the tubule lumen (p. 2774).
Measuring Na+, K+ and Cl− activity (effective concentration) in principal cells before and after the addition of Achdo-KII, Coast also measured the voltage across the principal cell basolateral membrane (the principal cell membrane that faces the haemolymph) and the tubule fluid secretion rate. Coast admits that building the double-barrelled ion-selective microelectrodes that he used to measure the intracellular ion activity was particularly challenging,s and says, ‘I spent months struggling with this and was only successful when I switched to a different type of double-barrelled “piggyback glass” on the recommendation of Mike O'Donnell and Juan Ianowski.’ Then, after calculating the electrochemical gradients across the basolateral membrane and estimating the gradients for Na+ and K+ across the apical membrane (the lumen side of the principal cell), Coast constructed a model for ion transport across the cricket Malpighian tubule in the presence and absence of the diuretic hormone Achdo-KII.
Outlining the ion transport model for Malpighian tubule principal cells that have not been stimulated with diuretic hormone, Coast suggests that a Na+ gradient drives the transport of 1 Na+ and 1 K+ ion for every 2 Cl− ions from the haemolymph into the principal cell while Na+ and K+ ions are transported in turn out of the cell and into the tubule lumen through cation/H+ antiporters driven by a proton gradient. Transport is also restricted: Coast explains that the Cl− gradient across the apical membrane in unstimulated principal cells is more than five times greater than the gradient across the basolateral membrane, resulting in low Cl− transport into the tubule lumen, raising the lumen voltage and limiting Na+ and K+ transport in turn. However, Achdo-KII increases the Cl− conductance across the apical cell membrane into the lumen, resulting in increased Na+ and K+ transport across the principal cell and into the tubule lumen.
Comparing the cricket Malpighian tubule with the Malpighian tubule of the blood-sucking insect Rhodnius prolixus, Coast points out that like the cricket, R. prolixus Malpighian tubules also lack stellate cells and that the transport mechanisms in both Malpighian tubules are very similar. He adds, ‘This work could readily be expanded upon to investigate the mode of action of another important family of insect diuretic hormones, the corticotrophin releasing factor (CRF)-related neuropeptides.’
