What about the diabetic dog? Why didn’t feeding the dog pancreas work? With our modern knowledge, it is easy to understand. The pancreas has at least two functions: to synthesize insulin and to supply digestive juices to the digestive system. The problem that vexed diabetes researchers at the turn of 1900’s is this dual nature of the pancreas. The pancreas secreted both insulin (a protein) and proteases (molecules that digest or break apart proteins). When the pancreas was ground up, the digestive juices (proteases) destroyed the insulin.
The problem can be stated easily in modern terms: how can you separate the insulin from the proteases? In the language of the time, insulin was contained in the external secretions, while the proteases were part of the internal secretions.
Banting’s idea that fateful night was:
by the experimental ligation of the duct and the subsequent degeneration of a portion of the pancreas, that one might obtain the internal secretion free from the external secretion.
Banting had an idea about how to do the separation surgically. However, he lacked the means to carry out the idea. The experiments he needed to do were straightforward: operate on a dog (dogs were the experimental animal of choice at the time), tie off (ligate) the main pancreatic duct, close up the wound, and wait. He would wait for a week or so, then remove the pancreas from the dog. During that week, the portion of the pancreas that produced the destructive proteases would have atrophied.
Once the pancreas was removed, the dog would begin exhibiting symptoms of diabetes. Banting could then feed the dog parts of the ligated pancreas to test whether it contained insulin. How would he determine the success of this test? Of course, he could just keep feeding the dog the ligated pancreas and observe the health of the dog, but this could take weeks. A quicker method was needed. He would need a method to measure the amount of glucose in the blood.