Invertebrates Excretory Systems Protonephridia Metanephridia Malpighian Tubules

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

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

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

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

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

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

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

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)

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

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

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

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

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

Unfed Rhodnius prolixus Blood-fed Rhodnius prolixus

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

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

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

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