Question

Many hormones use cAMP as the secondary messenger to carry out their function. But different hormones have different functions, then how can they use the same messenger to get different ends?

Answer 1

This is a superb question.

The problem you describe - that of the same second messenger acting in different pathways - is called cross talk. I will preface this by saying that science does not yet know all there is to about this; this is still an area of much current work. That said,

Cells deal with it a number of different ways and actually exploit it for a variety of different purposes.:

1. The general cellular effects of the different pathways involving a single second messenger tend to be the same, or are different aspects of the same response to a certain environmental condition. For example, cAMP tends to promote glycogen breakdown and glycolysis. It also regulates hypoxia inducible factor (HIF), which in turn up-regulates a set of genes involved in glycolysis, including several glycolytic enzymes.

It provides a way for info from all sorts of different first messengers are integrated: there is a lot of cAMP about from all these different sources; the cell decides to respond by increasing cellular respiration; and it uses that cAMP to make that response in a number of different ways, through phosphorylation of enzymes, through transcription of more enzymes and a variety of other ways.

2. Cross talk provides noise protection: all of these events, like a signalling molecule binding to its receptor, or a downstream protein binding with its target and the message being passed on - are governed by chance. Multiple pathways acting through the same effector make sure that responses which are supposed to happen do, that is, they are not much affected by random chance.

3. Localisation. Many of the proteins in a specific signalling pathway are physically close to each other inside the cell. Instead of second messengers having to diffuse all the way across the cell to act on their targets, they only have to diffuse a few Angstroms from one neighbouring protein to the next. That physical isolation lets different pathways use the same second messenger without interfering with each other.

4. Cells are fundamentally indeterminate systems. That one is more controversial - but it turns out that all the little oscillations from random (often called 'stochastic') processes themselves encode useful information which determines how the cells react and adapt. The ability to adapt is very important for cell survival. It is likely that some cells do 'get it wrong' as a result of cross talk, but that in cases where the environment has changed suddenly, they will actually have a survival advantage over the cells which 'got it right.' So what is quite bad for a few individual cells is, in general, good for a lot more of them.

Very good question.