VPT 421: Veterinary Toxicology (2+0)

ABSORPTION AND FATE

• Excess iron is not readily excreted. Normally biological levels of iron are maintained by selective absorption using an energy dependent carrier mechanism.
• Absorbed ferrous iron is oxidized to ferric iron and bound to transferrin. In this form iron is transported throughout the body.
• Iron is primarily used for oxygen transport by haemoglobin and myoglobin.
• Serum transferrin concentration greatly exceeds that necessary to bind iron under normal conditions. This capacity provides protection against iron becoming free in the systemic circulation.
• But, in intoxication, transferrin gets saturated. So the free iron interacts with the cellular constituents.
• In the cells, iron that is not needed for production of protein is bound to ferritin an iron storage protein in the tissues.
• Excess iron absorbed over a period of time is stored as haemosiderin or ferritin.
• In chronic exposure to iron, production of additional trasferrin and ferritin are induced. But, in acute cases this does not occur.
• Kinetics of iron is more complex and does not follow normal pattern.
• Overall body load of iron is regulated at the point of absorption. But there is no mechanism to actively eliminate iron.
• So animals with enough iron in the body are susceptible than those actually requiring iron.
• Absorption of iron from the gastric lumen follows two steps.
  • In the first step the iron from the lumen is transferred into the gastric mucosal cells.
  • In the second step this iron is either transferred to the systemic circulation or it is lost when the cells are sloughed off during normal cellular turnover.
  • Absorption of iron is affected by the extraluminal factors like level of erythropoitic activity, body iron stores and anaemic status and intraluminal factors like level of ascorbic acid, dicarboxylic acids, sugars and amino acids which increase the absorption and phosphates, calcium, phytates, oxalates, bicarbonates, tannins and fibre which decrease the absorption.
• Haemosiderosis is a localized process of abnormal iron pigmentation caused by increased amounts of haemosiderin in tissues.
• Haemochromatosis is a systemic disease characterized by widespread haemosiderosis and micronodular cirrhosis (inherited disease in humans and Saless cattle).