As you review your notes each night before sleep, you probably become more and more aware of the importance of endocrinology in insect physiology. We saw, for example, that maintenance of appropriate titres of hemolymph fuel supplies involves action of the peptide hormone, adipokinetic hormone. Problems in water and osmotic balance are also addressed by hormone action. Insects often elaborate a diuretic hormone and an antidiuretic hormone that are involved in homeostasis of salt and water balance and also in increased fluid cycling to enhance filtration.

All of these hormones, and many others in general animal physiology, are peptide hormones. Study of peptide hormones is one of the most active areas of insect physiology. One reason for this activity is that peptide hormones appear to relate to more and more areas of physiology. We also discussed the roles of prostaglandins and other eicosanoids in regulating basal fluid secretion rates in mosquito Malpighian tubules and mediating cellular immune reactions to bacterial infections. At the cellular level, the physiological actions of prostaglandins are mediated in a manner similar to the mediation of peptide hormones.

One of the most contemporary arenas of insect and mammalian endocrinology is cellular signal transduction mechanisms. Just to get a grip on terminology, we want to recognize four forms of signaling by secreted molecules. Endocrine signaling refers to the secretion of a molecule into general circulation. The secreted species will eventually interact with target cells in one or more tissues quite distant from the endocrine gland. We also recognize paracrine signaling, in which a secreted molecule is very rapidly taken into or inactivated by neighboring cells. In this case the secreted molecule influences just a few cells in the immediate neighborhood of the secreting cells. Paracrine signaling is related to autocoid signaling. In this case, a cell may release a molecule which interacts with receptors on the surface of the same cell. Finally, we recognize synaptic signaling, which generally takes place in nerve-nerve or nerve-muscle synapses. In all of these systems entail a general mechanism. A signal molecule is released from a cell, and the action of the signal molecule follows interaction of the molecule with a specific receptor protein.

Peptide hormones are usually very short chains of amino acids. The shortest in length is 3 amino acids in some vertebrate brain hormones (the discovery of which earned a Nobel prize for a French and an American laboratory. These two lab groups were in fierce competition to be first in learning these structures). Others are commonly a dozen or so amino acids in many insect and vertebrate peptide hormones. Insulin is a protein hormone composed of over 100 amino acids linked cysteine residues in two chains. Prostaglandins are oxygenated metabolites of certain polyunsaturated fatty acids. They feature polar substitutions on carbons 1, 9, 11 and 15. PGE₂, for example, has a carboxylic acid at C-1, at keto substitution at C-9, and hydroxyls at C-11 and C-15. Peptides, prostaglandins and a few other classes of signal molecules share the physiological feature that their actions are mediated by way of specific receptor cites located on cell surfaces.

We procede by framing two questions: [1] When any particular hormone is released in circulating hemolymph, how does it know which tissue is the appropriate target? For example, DH is released from neurosecretory cells into hemolymph, and it acts only on Malpighian tubules and on hindgut. This hormone does not act on brains or on reproductive tissues or on flight muscles. Again, how does the hormone know where to go? [2] How is a hormone signal floating around in hemolymph translated into some intracellular response? What these questions amount is wondering how a hormone signal is transduced into cellular events. The whole business is known as signal transduction mechanisms.

The first question has a rather straight-forward reply. Hormones actually "go" everywhere: they enter general circulation in very small concentrations. While in circulation they come into contact with every cell in an insect body. A general concept in endocrinology is that hormones, including locally hormone-like regulators like prostaglandins, combine with specific receptors on the surface of or are inside the cells of their target tissues. I mention inside their target tissues because most steriodal hormones, such as 20-hydroxyecdysteroid, can permeate cell membranes. Receptors for steriod hormones are usually found in the cytosol of cells. Cell surface receptors interact with peptide and other hormones and regulators that can not cross membranes. In this model one imagines that a hormone is released in general circulation and can potentially come into contact with all body tissues, but interacts physiologically only with those cells that possess receptors specific for the given hormone. Hormone receptors, often called receptor sites, are mainly large proteins. These proteins confer specificity of hormone recognition upon individual cells within tissues.

Having dispatched that issue, the next general question is, what is the biochemical basis for hormone action? This is, how is a hemolymph-borne signal translated into intracellular activity? This is a slightly more complicated area that depends upon a hormone's structure. Here, we will limit our discussion to peptide hormones and a word or two about prostagladins.

Peptides mostly adjust or regulate metabolic events. An interesting feature of peptides is that they are not able to permeate cell membranes. The hormone itself can not move from extracellular to intracellular compartments. The first issue in the physiology of peptide hormone action is that their specific receptor proteins are located on the outside of target cell membranes.

We want to make three points today:

[1] This is a general sort of hormone mechanism, called a second messenger system.

[2] In this system, a hormone signal, but not the hormone, gets into a cell.

[3] Within cells, a low-signal hormone is rapidly amplified.

Turning attention to the figure, the transduction process begins when a blood-borne hormone interacts with a specific receptor site.

ATP is converted into cAMP by release of free pyrophosphate, PPi. cAMP is a biologically ubiquitous intracellular messenger molecule. This is called a second messenger sysem beause the first messenger - the hormone - stimulates production of a second, intracellular, messenger, namely cAMP. The main point is that a message is carried across a cell membrane by changes in membrane protein conformations.

It is reasonable to ask "What does cAMP have to do with regulation of metabolic events?" The figure shows the most general reaction. There is an intracellular enzyme that goes under the name c-AMP dependent protein kinase, commonly called A-kinase. We might wonder what kinase means. This refers to an enzyme that activates another enzyme. A-kinase exists as two linked subunits, a catalytic subunit that actually performs the chemistry of activting enzymes, and a regulatory subunit that inhibits action of the catalytic subunit. cAMP acts by combining with the regulatory subunit of protein kinase. This causes a conformation change, which results in separation of the subunits, thereby yielding free, and now active, protein kinases.

The active protein kinase links a phosphate group onto another enzyme, thereby changing an inactive enzyme into an active form of the same enzyme. The phosphorylated enzyme leads to the cellular response. The figure shows several arrows that are meant to indicate a number of steps leading to the final response. Now imagine this: inactive phosphorylase kinase becomes active phosphorylase kinase after A-kinase links a phosphate group onto it. Now let's ask what does kinase do? The kinase hooks a phosphate group onto still another enzyme, thereby changing it from inactive to an active form. In the example, the phosphorylase kinase activates glycogen phosphorylase. Looks like a whole lot of activating going on. The glycogen phosphorylase produces the final cell response to the hormone: release of free sugar from glycogen.

Why are there so many steps in this process? The answer lies in realization that this is correctly called an amplification cascade, which tells us how the incoming signal can be amplified to produce peak cellular activity level in seconds. The key is that each step in the cascade is catalytic. By way of example, imagine that each step in a cascade operates only ten times. Ten is a completely arbitrary number I selected to make the point of amplification:

Step 1 gives 10 molecules of cAMP

Step 2 gives 100 active protein kinase molecules

Step 3 gives 1,000 active kinase molecules

Step 4 gives 10,000 enzyme molecules

Step 5 gives 100,000 metabolic steps

Again, these numers are not data. I made them up to illustrate rapid amplification of a hormone signal.

Now, the next question to ask is "How do we turn this cascade off?" cAMP production depends upon continued secretion of hormone. Once hormone secretion stops, the hormone-receptor complex dissociates, usually with deactivating metabolism of the hormone. All cells capable of making cAMP express an enzyme called phosphodiesterase. This is a continuously running enzyme that rapidly hydrolyzes any cAMP it can find. When cAMP formation is stopped, intracellular cAMP levels rapidly approach zero. Then protein kinase subunits reassemble, whereupon the phosphorylated enzymes are dephosphorylated to inactive forms by another set of enzymes called phosphatases. Cells are thereby returned to their unstimulated state, tiny enzymic-sized beers are opened and things sort of chill out until the next episode.

To summarize, a hormone interacts with a specific receptor cite, and the hormone-receptor interaction leads to increased intracellular cAMP concentrations by activation of an effector protein, in this case, adenylate cyclase. There are other effector proteins. The receptors are coupled to the effector proteins through intermediates known as G proteins.

 It has been my experience that once this system is revealed to people they generally change majors to physiology and biochemistry.