Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Chemical Signals in Animals:  A Case Study

Introduction:
    Bayliss and Starling conducted some of the first experiments that demonstrated the existence of chemical signals in animals.  These experiments were performed on January 16, 1902 at University College London and presented to the Royal Society shortly after.  The results were published later that same year (Bayliss & Starling, 1902b). A witness to the experiments, C.J. Martin wrote:

�I happened to be present at their discovery. In an anaesthetized dog, a loop of jejunum was tied at both ends and the nerves supplying it dissected out and divided so that it was connected with the rest of the body only by its blood vessels. On the introduction of some weak HCl into the duodenum, secretion from the pancreas occurred and continued for some minutes. After this had subsided a few cubic centimetres of acid were introduced into the enervated loop of jejunum. To our surprise a similarly marked secretion was produced. I remember Starling saying: "Then it must be a chemical reflex." Rapidly cutting off a further piece of jejunum he rubbed its mucous membrane with sand in weak HCl, filtered, and injected it into the jugular vein of the animal. After a few moments the pancreas responded by a much greater secretion then had occurred before. It was a great afternoon.�

Analysis:

1. One major advantage that we have over Bayliss and Starling is that we now know how this system.  Describe the role of secretin (recall Square Dude?).  For more information about this experiment, check your text (p 883).

2. What is the jejunum?  Where is it located? 

3. Why was the jejunum tied off and the nerves cut?

4. The four criteria to definitively establish a chemical as a hormone are listed below.  For each, indicate if the Bayliss and Starling experiment addressed the criterion and if so how?  If they did not, describe an experiment that could be used to address this criterion.

• Exogenous application of the chemical to the target tissue brings about the change.
  
   

• Lowering the endogenous level of the target chemical prevents the change from occurring.
  
   

• Lowering endogenous levels and adding the chemical exogenously will cause the change.
  
   

• During normal activity, the endogenous level of the chemical will rise in the target tissue before the change is observed.