Simon Maddrell believes in the Krogh principle. So when he wanted to understand the physiology of fluid secretion, a South American blood-sucking insect called Rhodnius proved to be `the animal of choice'. Rhodnius is remarkable because it can go an entire year without feeding, shrivelling down to almost nothing. But within moments of feeding the insect's tiny Malpighian tubules, which produce the insect's liquid waste,reactivate and begin secreting fluid at enormous rates. Maddrell has identified several hormones that regulate secretion in Rhodnius and many other insects. In this issue of J. Exp. Biol., working with an international team he reports an enormous increase in the number of hormones that have a diuretic effect in the large hawkmoth, Manduca(p. 1869). But why so many hormones when one would probably work just as well? Maddrell believes that the cocktail of hormones could be working as a general broadcasting system that reports an organism's state of health to all tissues.
Nathan Tublitz led Maddrell down the path of secretary regulation in the enormous moth. He contacted Maddrell and told him about a peptide hormone that regulates the insect's heartbeat, but also activates the Malpighian tubules. Maddrell was curious why one hormone would regulate two essential life support mechanisms, and wondered if any other moth hormones affect Malpighian tubules too.
Working with Nick Skaer, they began testing the effects of eight different molecules on Malpighian tubule secretion by bathing the closed end of each tubule in drops of saline and hormone mixes. They pegged the other end of the tubule outside of the drop and made a tiny nick in it. As the tubule responded to the hormone, liquid began oozing out of the hole, and Maddrell and Skaer calculated the excretion rate by measuring the drop's volume after a given time. Every one of the eight compounds they tested boosted the tubule's excretion rates.
Most of the compounds Maddrell tested were moth signalling molecules, but one group of hormones, called the tachykinin related peptides, had only been identified in locusts. Maddrell needed to prove that these peptides had a function in moth metabolic regulation. Working with Dick Nässel in Stockholm, they probed a variety of tissues from the moth, and found that the peptides were mostly used for signalling in the brain and in some gut cells. So why were so many signalling molecules affecting excretion, when each hormone's primary function was elsewhere in the insect's body?
This problem puzzled Maddrell, until it occurred to him that the multi-hormone mixture could be working as a general broadcasting system,keeping all of the insect's tissues up to date with the entire body's status. He suggests that as the insect responds to different situations, it changes the balance of hormones in the mixture so that the appropriate tissues react to a specific demand, while others simply tune into the message to pick up the news.