CLINICAL CHEMISTRY Study Guide - Final Guide: Immunoassay, Radioimmunoassay, Iodine

The levels of hormones circulating in blood are tightly regulated This is especially true for most peptide hormones which have a short half-life in blood. The blood level of the peptide hormone ghrelin, an appetite stimulant, changes rapidly throughout the day. Ghrelin levels peak right before a meal and drop sharply after a meal. Select the statements that describe mechanisms for rapidly altering the amount of ghrelin circulating in blood. Upon stimulation, large amounts of ghrelin are rapidly synthesized and immediately secreted into the bloodstream. Ghrelin is stored at a high concentration in the secretory vesicles of cells lining the stomach Upon stimulation, ghrelin is rapidly released from the secretory vesicles into the bloodstream Ghrelin in circulation is rapidly removed from the bloodstream by carrier proteins that bind hormones and transport them to appropriate tissues for degradation and excretion. Circulating ghrelin is rapidly removed from the bloodstream by enzymatic degradation and excretion by the liver and kidneys. After receptor binding, ghrelin is quickly cleared by adsorption to insoluble fiber moving through the stomach.

When blood glucose is low, the pancreas releases glucagon, a peptide hormone which stimulates the liver to produce and excrete glucose. Binding of the hormone to its receptor triggers a "second messenger" cascade pathway that results in a change in the concentration or activity of many enzymes and signaling intermediates. The activity of some pathway enzymes is altered, as is the direction of net flow through some pathways. For each of the following, indicate whether glucagon stimulation results in an increase or decrease in concentration, activity or flow.

Using radioisotope to determine the blood volume A 0.20 mL sample a saline solution containing^59Fe is injected into an animal. Before the injection the sample has an activity of 1.0 times 10^4 cps. After a sufficient time for the blood to be thoroughly mixed, 0.20 mL of the blood is removed and its activity measured. If the blood sample shows an activity of 95 cps, calculate the blood volume of the animal. A blood-volume determination was carried out on a patient by injection with 20.0 mL of blood that had been radioactively labeled with Cr-51 to an activity of 4.10 mu Ci/mL. After a brief Period to allow for mixing in the body, blood was drawn from the patient for analysis. Unfortunately, a mixup in the laboratory prevented an immediate analysis, and it was not until 17.0 days later that a scintillation measurement on the blood was made. The radiation level was then determined to be 0.00935 mu Ci/mL. If^51Cr has t_1/2 = 17.7 days, what is the volume of blood in the patient?