Our current idea is that the prostaglandins modulate the effects of major hormones at the tissue level. It follows that there are at least two tiers, or levels, of regulation of fluid secretion. Malpighian tubule and hindgut functions involve moving ions from place to place. We use the expression epithelial transport to describe the movement of ions from hemolymph into Malpighian tubule lumen or from rectal lumen into hemolymph. Epithelial transport entails moving ions across one cell membrane, through the cell, and finally across another cell membrane.
Epithelial transport occurs through a number of processes.
One process is movement through ion channels. An ion channel might look like a couple of large proteins assembled such that they form a tube running through a biomembrane. We also can imagine a group of proteins forming a tube or channel allowing passive movement of ions according to the forces of electrochemical gradients. Ions can be moved across epithelia against electrochemical gradients by active processes. Primary active transport processes directly the potential energy in ATP to move ions.