Invertebrates Excretory Systems

1. Invertebrates Excretory Systems • Protonephridia • Metanephridia • Malpighian Tubules

2. Protonephridia • freshwater flatworms • network of blind-ended tubes opening only to the exterior • tubes branch through the body, ending in flame bulbs • tuft of cilia that beat, forcing fluids through tubes • urine empties via a nephridiopore

4. Metanephridia • most annelids • each segment contains a pair of metanephridia • tubules bathed in coelomic fluid and encircled by capillaries • nephrostome collects fluid from coelom ( ultra filtration) in the first filterate is isosomotic • transport epithelia in lumen of tubules resorb and secrete molecules • urine exits nephridiopore

6. Osmoregulation in insects • Osmoregulatory system of insects • The main organs involved in solute and water balance are: • Malpighian tubules (MTs) • Form primary urine • Lower MTs and hindgut (ileum, colon, rectum) • Reabsorption of water and ions

7. Malpighian Tubules • insects and other terrestrial arthropods • remove wastes from hemolymph and osmoregulate • open in digestive tract, tips immersed in hemolymph • transport epithelia line tubules • solutes are secreted into tubules and some are reabsorbed by the rectum • causes the precipitation of uric acid

8. Osmoregulation in insects • Malpighian tubules • MTs empty into the alimentary canal between the midgut and hindgut • The number of MTs varies from 4-200 depending on the species • 2-100 mm in length and 30-100 μm in diameter • Walls of the MTs consist of a single layer of epithelial cells • Process ECF at high rates to regulate composition and volume of ECF • MTs are not innervated and fluid secretion is controlled by the action of hormones

9. Osmoregulatory system of insects (Eckert, Fig. 14-42)

11. Osmoregulation in insects • Malpighian tubules • MTs lie free in hemocoel and are not supplied with blood vessels • Insect circulatory system is at relatively low pressure, therefore urine is formed entirely by secretion • NaCl and KCl are transported from the hemolymph into the lumen of the MT • MTs secrete K+ in herbivorous insects and Na+ in blood-feeders • NaCl and KCl are returned to the hemolymph across the rectal wall

12. Osmoregulatory system of an insect Na+, K+, Cl- -reabsorption of water and ions Hyperosmotic or isosmotic urine/excreta K+, Cl- Na+, K+, Cl- & water -formation of primary urine (Eckert, Fig. 14-42)

13. Osmoregulation in insects • Hormonal control of fluid secretion • Diuretic hormones (DHs) • Substances that increase tubule secretion and/or inhibits fluid reabsorption in the hindgut • Antidiuretic hormones (ADHs) • Substances that inhibit tubule secretion and/or promotes reabsorption of ions and water in the hindgut

14. (or synthetic peptides, neurotransmitters) Liquid paraffin Ramsay Assay for Measuring Fluid Secretion

15. Osmoregulation in insects • Hormonal control of fluid secretion in Rhodnius prolixus • Types of DHs in Rhodnius : • Serotonin (5-hydroxytryptamine, 5-HT) • Also a cuticular plasticizing factor • Signals through cAMP pathway • Widely distributed in the nervous system and released from abdominal nerves into the hemolymph after feeding • Corticotropin-releasing factor (CRF)-like peptides • At least 15 different CRF-like peptides identified • 30-47 aa residues • Signal through a cAMP pathway • Present in the brain and mesothoracic ganglionic mass (MTGM) and released from abdominal nerves into the hemolymph after feeding

16. Central nervous system of Rhodnius -contain CRF-like peptides subesophageal ganglion prothoracic ganglion Abdominal nerves mesothoracic ganglionic mass -source of CRF& other unidentified diuretic peptides Posterior lateral neurosecretory cells

17. Osmoregulation in insects • Hormonal control of fluid secretion in Rhodnius prolixus • Rhodnius consumes >10 times its body weight during a single blood meal • The excess fluid gained after feeding severely restricts mobility, therefore excess fluid load (salt and water) must be voided rapidly • Minutes after a blood meal, the MTs increase fluid secretion 1000-fold • Rapid elimination of Na+ and water requires coordinated synergistic action of diuretic hormones

18. Unfed Rhodnius prolixus Blood-fed Rhodnius prolixus

19. Osmoregulation in insects • Hormonal control of fluid secretion in Rhodnius prolixus • H+-ATPase on the apical membrane creates EC gradient • H+ is returned to the cytoplasm in exchange for either Na+ or K+ • Na+-K+-2Cl- cotransporter on basolateral side • Cl- diffuses out on the apical side, some K+ recycled on the basolateral side • Extracts of MTGM (CRF +other peptide DHs) and 5-HT act synergistically to promote diuresis

20. Osmoregulation in insects • Hormonal control of fluid secretion in Rhodnius prolixus • Cessation of urine production must also be tightly controlled to avoid dehydration and excessive loss of NaCl • Cardioaccelatory peptide 2b (CAP2b) functions as an antidiuretic hormone • CAP2b activates a cGMP second messenger pathway to increase a cAMP phosphodiesterase thereby inhibiting cAMP-mediated diuresis

26. II. Osmoregulation in aquatic environments • Marine mammals • Do not have salt glands and do not drink seawater • Obtain water from food and metabolism • Highly efficient kidneys produce a hypertonic urine • Nursing females produce milk with high fat but low water content • Some juvenile animals can use water derived from the oxidation of body fat • Modifications in nasal passages to reduce water loss • Ability to lower metabolic rate

27. Water-salt relations in a marine mammal -obtain water from food and metabolism -conserves water by producing a hypertonic urine