CPT 32 General Physiology and Toxicology - Lecture Notes PDF

CPT 32 General Physiology and Toxicology LECTURE 2.4 INSECT GROWTH, DEVELOPMENT and REPRODUCTION: EXCRETORY SYSTEM L.D. Samson, M.A. INTRODUCTION The excretory system consists of two organ systems working together: the Malpighian tubules and the hindgut. The Malpighian tubules typically arise...

CPT 32 General Physiology and Toxicology LECTURE 2.4 INSECT GROWTH, DEVELOPMENT and REPRODUCTION: EXCRETORY SYSTEM L.D. Samson, M.A. INTRODUCTION The excretory system consists of two organ systems working together: the Malpighian tubules and the hindgut. The Malpighian tubules typically arise at the junction of the mid- and hindgut. The tubules transfer the accumulated urine to the hindgut, where selective reabsorption by the ileum and rectum retains necessary substances within the body, while allowing waste products and excesses of useful substances to be voided with the fecal wastes. Specializations in hindgut epithelial cells facilitate reabsorption processes. INTRODUCTION INTRODUCTION The excretory system plays a major role in homeostasis of hemolymph, cells, and tissues by helping control levels of electrolytes, water, acid–base equivalents, and nitrogen metabolites. Homeostasis is challenged by food habits, habitat, and metabolic state of the insect. Some excretory products may be stored in the body or cuticle where they may offer protection from predation and parasitism. EXCRETIONS EXCRETIONS Excretion can be defined broadly as any process that eliminates the interaction of harmful substances with cells and tissues. Even useful substances, such as glucose, amino acids, and certain ions, can be harmful if present in excess amounts. Nitrogenous metabolites, ions, water, and ingested chemicals are substances that an insect may need to excrete. Allelochemicals from eaten plant tissues may be excreted from the body or stored in some inert location in the body. EXCRETIONS Storage excretion is defined as to include materials with future potential use such as glycogen as a storage form of glucose or amino acids stored as proteins. Deposit excretion is defined as waste material of no further use that needed to be removed from harmful interaction with the tissues. The distinction between stored and deposit excretion, however, is very subtle. EXCRETIONS In insects, both the Malpighian tubules and the hindgut function together as excretory organs. The Malpighian tubules collect a filtrate from the hemolymph and pass this primary urine to the hindgut. Additional components are secreted into the excreta by the hindgut, and some substances are reabsorbed into the hemolymph. excreta describes the material actually eliminated from the anus by insects because it is a mixture of undigested materials passing through the gut, substances acted upon and possibly modified by bacterial action in the gut, and urinary materials from the Malpighian tubules. https://youtu.be/l9gp0z2Yt4o MALPIGHIAN TUBULES MALPIGHIAN TUBULES Malpighian tubules are long, tubular structures, usually arising at the junction of the mid- and hindgut and terminating blindly in the hemocoel. vary in number from 2 to more than 100 in various insect species. Tracheal connections to Malpighian tubules are numerous and are indicative of a high metabolic demand for oxygen. MALPIGHIAN TUBULES Malpighian tubules are not only important in excretion. They have many functions in insects, including detoxification, metabolic functions, and a role in immunity, and now that the genomes of Drosophila melanogaster, Anopheles gambiae, and Apis mellifera are known, they may serve for organotypic study of human genes. MALPIGHIAN TUBULES The primary urine formed in the lumen of the Malpighian tubules is a filtrate of the hemolymph, and it contains most of the small ions and molecules (sugars, amino acids, ions, as well as other components) that occur in the hemolymph. Urine formation in Malpighian tubules relies upon a proton pump. MALPIGHIAN TUBULES The transport of a substance which depends directly on ATP, such as the pumping of the protons in the Malpighian tubule, is called active transport. The transport of the other ions and water is passive (by facilitated diffusion) in of itself, but is dependent on proton transport and so indirectly dependent on ATP. This mode of transport is called secondary active transport, e.g. the transport of potassium. FORMATION OF PRIMARY URINE IN MALPIGHIAN TUBULES FORMATION OF PRIMARY URINE IN MALPIGHIAN TUBULES The primary urine formed in the lumen of the Malpighian tubules is a filtrate of the hemolymph and it contains most of the small ions and molecules (sugars, amino acids, ions, as well as other components) that occur in the hemolymph. FORMATION OF PRIMARY URINE IN MALPIGHIAN TUBULES Urine formation in Malpighian tubules relies upon a proton pump. Proton pumps are ubiquitous in all eukaryotic cells. Such pumps are particularly important not only in the Malpighian tubules but also in the midgut, salivary glands and rectum. The pump causes the tubule lumen to become positive to the hemolymph and creates highly variable gradients in pH across the apical membrane of principal cells. The proton gradient provides the energy for an antiporter mechanism that exchanges K+ for H+ across the apical membrane. SELECTIVE REABSORPTION IN THE HINDGUT Anatomical Specialization of Hindgut Epithelial Cells The hindgut is the second system that completes the excretion process by selectively reabsorbing some substances into the hemolymph, leaving others in the lumen, and actively secreting some substances into the hindgut lumen. In the rectum, small groups of cells are variously called the rectal cells, rectal pad cells, or rectal papillae cells in different insects. Secretion and Reabsorption in the Ileum The ileum is the most anterior part of the hindgut, occurring just posterior to the origin of the Malpighian tubules in most insects. The ileum plays a major role in acid–base balance by secretion of H+ into the lumen, formation of NH4 + , and reabsorption of HCO3 −. Reabsorption in the Rectum The rectum is the final and major site for reabsorption of ions, water, and nutrients, and it is capable of reabsorbing fluid against strong osmotic gradients, ultimately producing in many insects a very concentrated, hyperosmotic excreta. The driving mechanism for cation and water reabsorption, as in the ileum, is an electrogenic Cl− pump under the influence of a neuropeptide hormone, chloride transport stimulating hormone, from the corpora cardiaca (CC). It acts on the rectal epithelium to promote active Cl− absorption. The pump provides the energy for K+ reabsorption. ROLE OF THE EXCRETORY SYSTEM IN MAINTAINING HOMEOSTASIS Role Of The Excretory System In Maintaining Homeostasis Maintenance of constancy of the internal environment of cells, tissues, and organisms is the process of homeostasis. The excretory system plays a major role in eliminating metabolic wastes and toxins acquired with or from the food, typically by increasing the basal rate of fluid secretion, and transport of the toxin, into the lumen of Malpighian tubules. Role Of The Excretory System In Maintaining Homeostasis Dynamic changes in salt, water, acid–base, and nitrogen amounts occur from time to time in all organisms as a result of food ingested, environmental conditions, and metabolism. Regulatory mechanisms that respond rapidly to these changes are necessary to preserve the integrity of cells and tissues. Ingestion of plant phloem or xylem sap results in an excessive intake of water, and usually more sugar and some amino acids than needed. Nitrogen metabolites from proteins, amino acids, and purines must be disposed of by all cells. Role Of The Excretory System In Maintaining Homeostasis Electrolyte homeostasis - the necessity of ion and solute transport across cell membranes Water homeostasis - very important to insects because they have a high surface-to-volume ratio and their food often has variable water content. Diuretic hormones promote fluid formation and rapid excretion by the Malpighian tubules Antidiuretic hormones act upon the hindgut (with one exception) and promote water reabsorption. Role Of The Excretory System In Maintaining Homeostasis Acid-base homeostasis - Acidosis or alkalosis may be experienced by an insect depending on various foods, the presence of certain types of chemical compounds in plants eaten, type of proteins metabolized (whether proteins yield a high proportion of acidic, basic, or neutral amino acids), and metabolic conditions such as exercise that produce acids in the tissues Role Of The Excretory System In Maintaining Homeostasis Nitrogen homeostasis - nitrogen, although known to be a growth-limiting nutrient for some insects, is nevertheless excreted in several forms by insects. Excess nitrogen that must be excreted may come from ingestion of proteins leading to an imbalance of amino acids; they save essential amino acids and may metabolize the nonessential ones as energy sources. Insects also ingest nitrogen in the nucleic acids of their food. Excess protein nitrogen is excreted as uric acid (a purine), as purines related to uric acid, as ammonia or ammonium salts, and in several other (usually minor) forms. Thank You! NEXT TOPIC: REPRODUCTIVE SYSTEM REFERENCES Nation, James L Sr. Insect Physiology and Biochemistry. 3rd Edition. CRC Press. USA. pp. 33-58.