Water and Ions Membranes surround cells Epithelia surround tissues Epithelia separate an organism from its environment At each level ion and water movement is controlled so volume and ion concentration can be maintained

Cell Blood

The Nernst Equation Chemical and charge gradients determine whether an ion is in equilibrium across a membrane The Nernst equation predicts the potential difference at equilibrium The equilibrium potential for sodium = RT/zF ln [Concentration out]/[Concentration in] R is the gas constant, T is the absolute temperature, z is the charge on the ion and F is Faradays’ constant 96,500 couloumbs/mole

Nernst Equation Vm = RT ln [Co] zF [Ci] R = 8.314 joules/Ko/mole T = Temperature F = Faraday’s constant 96,500 coulombs/mole

Frog Skin Model Developed by Ussing Tested in “Ussing” Chamber Uptake occurs against and electrical and a concentration gradient. Inhibited by specific inhibitors Temperature Dependent Ion specific Saturation Kinetics

The Frog Skin Model Hans Ussing worked on one of the first models of epithelia transport The skin of a frog is able to absorb sodium from a fresh water pond against a considerable concentration gradient Using the Na gradient provided by the active pumping of sodium at the basolateral(serosal) surface of the cell, Na is drawn across the apical(mucosal) surface Transport requires serosal K and displays saturation kinetics

Patterns of Osmoregulation Marine Invertebrates and Osmoconformers Regulation is Marine Elasmobranchs Seawater Fishes and Ion Balance

Osmoregulation in Marine Invertebrates Most marine invertebrates are osmoconformers, they do not regulate the NaCl concentration of the blood Inside the cell amino acids provide osmotic currency Proto kidneys do regulate calcium and magnesium

Once Upon a Time . . . In Biscayne Bay. . . There were three copepods. Blood p = External Medium p = 1000 mOs = Cell p Cell Blood

And then the rains came down . The osmolality of the external medium falls, and initially the organism swells until ions reach equilibrium

Recovery In recovery blood = external medium = 800 mOs, but cells cannot reduce ion concentrations -- so free amino acids are extruded

Marine Elasmobranchs Marine Elasmobranchs are in osmotic equilibrium with sea water p in blood = p medium Ion concentrations of the blood are below sea water Urea and TMAO increase blood osmolality

Gills and Kidneys retain Urea Drinking Rates are low 0.0125%body water/hr Toxic Effects of Urea are lessened by the presence of trimethylamine oxide Urea = 350 mOs; TMAO = 50 mOs The Rectal Gland of sharks actively extrudes chloride The rectal gland is a model Cl extruding system used in study of cystic fibrosis One FW elasmobranch of the Orinoco has lost the ability to produce and retain urea, salt uptake mechanisms are unknown.

Na,K,Cl Cl Na Lumen K Blood Rectal Gland Transport

Water Loss Ion Gain

Marine Teleosts Osmotic Water Loss Across Gills Drink Sea Water to Replace Osmotic Loss, approximatley 2% of body weight/hour Extrude Ions at Gills with Active Chloride Transport Kidneys Excrete Sulfate and Magnesium, but can’t produce a hypertonic urine.

Fish Gills in Seawater Fish gills extrude Na and Cl gained diffusively across the body surface and taken in via the gut The chloride cell is packed with mitochondria and uses the active Na/K ATPase and a linked Na/K/Cl channel to move Chloride from the blood to the seawater against a gradient

Migrating Fishes In salmon, the vector of transport and the anatomy of the gill chganges as the fish moves from FW to SW Cortisol and Growth Hormone mediate the movement from FW to SW; Prolactin stimulates the SW to FW alteration

Marine Birds and Turtles Nasal and lacrimal transport of salt Mechanisms Similar to Rectal Gland Model

Malpighian Tubule in Insects Act as the kidney of insects, but form a “urine” by the secretion of potassium from the hemolymph Using a counter current movement of ions and water, insects are able to reabsorb water from the hindgut and prevent water loss in dry environments