1. How does ADH act on the kidney?

• ADH uses the inositol phosphate pathway (phospholipase C activation). ADH binds to protein receptors on the surface of target cells, like kidney tubule cells, and causes phospholipase C to break inositol phospholipids found on the inner surface of cell membranes. (look) or .. (Eckert, Fig. 9-24) The inositol pathway is involved with release of Ca⁺⁺ from the ER. This release is involved in many important biological processes, including fertilization. Look at this beautiful picture to see what happens when a starfish egg is fertilized. A Ca⁺⁺ sensitive fluorescent dye records rising concentrations of Ca⁺⁺, (look). As we shall see, it is Ca⁺⁺ release from sarcoplasmic reticulum in muscle which activates skeletal muscle contraction. WARNING - your textbook claims that ADH acts through the cyclic AMP (cAMP) pathway. This assertion seems to be incorrect.
• Ca⁺⁺ concentrations in all cells, including skeletal muscle cells are kept extremely low (around 10^-7 M) by active transport pumps which pump from the cytosol into the ER or to the outside of the cell.
• Ca⁺⁺ release affects target proteins by first binding to a Ca⁺⁺ binding protein, the most common of which is calmodulin

2. Many other hormones bind to receptors on cells of the target organs and cause intracellular changes in cAMP concentrations.

• Cyclic AMP is an important intracellular messenger. Epinephrine, glucagon, and ACTH are important hormones that cause changes in cyclic AMP concentrations within target cells. (look) As you can see, one of the effects of increased concentrations of cAMP is activation of a kinase. This kinase starts a cascade (Eckert, Fig. 9-22) of enzyme activity which results in an increase in the rate at which glucose is released from glycogen. One target is the liver, where glucose units are stored in a chain called glycogen. Binding of epinephrine or glucagon, say, to liver cells, causes increased release of glucose units and decreased binding of glucose units to form longer glycogen chains. Look at the following diagram that shows how the kinase chain causes glycogen, a polymer chain of glucose units, to add or release individual glucose units - (glucose release). NOTE: Enzymes called kinases add phosphate groups and enzymes called phosphatases remove phosphate groups.

3. The final chapter of our sick patient CASE STUDY.

• We are really coming to believe that our patient isn't sick. He is careful about alcohol consumption, he isn't suffering from bipolar disorder, nobody has hit him on the head recently, and ... the results of a MRI scan are just in ... he doesn't have a brain tumor. But, we finally determine that he does have high blood glucose levels. We might suspect an overproduction of glucagon. After all, low blood glucose is sensed by receptors of the surface of certain cells in the pancreas. The pancreas would normally respond by making glucagon, which travels throughout the body in the bloodstream and, when it binds to receptors on liver cells, causes release of glucose into the bloodstream. (Eckert, Fig. 9-43). But, alas, our patient has normal glucagon levels!
• Both the phosphoinositol and cAMP signalling pathways use G-proteins. Receptors that activate G proteins have multiple segments of protein chain which pass through the membrane. These receptors can change conformation (shape) in response to binding an external messenger and transmit this change across the membrane. However, receptor proteins with a single pass through the membrane apparently can't transmit a shape change across membranes. As a result, these types of receptor proteins work by permitting linking of of receptor subuits by a signal molecule.
  • Receptor tyrosine kinases, which are single-pass membrane proteins, bind a variety of extracellular signals, including insulin and growth hormones. (look)

  • These kinases activate an important GTP binding protein called Ras. (look) Ras activation leads to a cascade of activation of kinases and eventual changes in gene regulatory proteins. Gene transcription and thus gene expression are changed, causing changes in cell proliferation and cell differentiation. (look) About 30% of human cancers have mutation in the ras genes. Since cancer is usually characterized by changes in cell proliferation and loss of the ability to differentiate, ras and related genes are significant factors in the search for a cure for cancer. But, fortunately, our patient doesn't have cancer ......
  • The insulin receptor is also a tyrosine kinase. Ras isn't involved as it is for growth hormones but the receptor is similar. When insulin, produced in the pancreatic beta cells, binds to receptor tyrosine kinases on the membranes of liver cells, phosphorylation activates a kinase chain but WITH THE OPPOSITE EFFECT TO THAT OF GLUCAGON. Now we are finding out something about our patient. He has a problem with insulin signalling high glucose levels to his liver cells. High glucose in the bloodstream should trigger insulin production by the pancreas. Insulin should signal liver cells to add more glucose units to glycogen and reduce the rate of release of glucose from stored glycogen. This isn't happening. So, it seems that we have discovered that our patient has diabetes mellitus. It remains to be discovered if this is type I diabetes (formerly called juvenile diabetes, or insulin-dependent diabetes mellitus, or IDDM) associated with a failure of the pancreas to produce insulin. Or, our patient may have type 2 diabetes (formerly called adult-onset diabetes, or noninsulin-dependent diabetes mellitus, or NIDDM), involving the failure of target organs to respond properly to insulin, i.e., a missing or defective receptor or component of the intracellular signalling pathway.

• So, what does diabetes have to do with his excessive urination. First of all, our patient isn't producing 20 l of urine a day. But, he does urinate more than is normal. Insulin, like many hormones, affects more than one type of target organ. Insulin binding to normal liver cells reduces glucose released. Insulin causes the kidney to return filtered glucose to the bloodstream. A lack of insulin will cause excess glucose to be excreted in the urine. Water will follow the glucose through the process of osmosis and urine flow will be excessive (diuresis). Failure of either the pancreas to produce insulin or BOTH liver and kidney cells to have the proper receptor for insulin will cause the high blood glucose and frequent urination observed in this patient.

4. Further study:

• We won't have time to cover steroid sex hormones in class. But, steroids, such as testosterone and progesterone, are oily (hydrophobic) molecules derived from cholesterol. Because so much of the surface of the molecule is oily, they can pass through cell membranes. Once inside a cell, they can bind to DNA binding proteins and affect protein manufacture.


All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2009, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.