Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Abstract

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+/H+-exchanger in teleost fish differs from the H+/2Na+ (or Ca2+)-exchanger in crustaceans. In osmoregulation, Na+ and Cl− predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl− concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+/K+-adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+/H+ exchange; elevated HCO3− inhibits Cl− uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42− is actively transported, but most epithelia are generally impermeant to SO42−. Transporters of Ca2+ maintain homeostasis of dissolved Ca2+. More integration of physiology with toxicology is needed to fully understand freshwater ion effects.

Using extirpation to evaluate ionic tolerance of freshwater fish

“…Physiological studies have described epithelial cellular processes that affect the tolerance of freshwater fish to elevated ionic concentrations [7]. However, the effect gradients of decreasing species occurrence (Supplemental Data, Figure S3) and the extirpation of species ( Figure 5) may not be entirely values (open circles) for fish genera observed !25 times are plotted along with the species XC95 value (small solid circles) for fish species observed !25 times, although some species are obscured by plotting at the same location.…”

“…Because the water is more dilute than their blood or cytoplasm, fish must deal with absorption of water across their epithelial membranes and diffusional loss of salts through semipermeable membranes and by excretion. These ion concentration gradients facilitate the movement of solutes among these 2 compartments and the external water [5][6][7]. Water is excreted as dilute urine by their renal system.…”

“…Several anatomical and physiological mechanisms are involved. Increased concentrations of some ions in water can alter osmotic stress [8,10], but increased concentrations of specific ions, such as H þ , K þ , and HCO 3 À , alter electrochemical gradients, affecting ion transport and associated energy expenditures that can cause ionoregulatory stress and acid-base imbalances [7,10,11]. Salts are absorbed by various transporter proteins or permeases on gill and renal epithelia that transport ions against concentration gradients [6,8,9].…”

“…Water is excreted as dilute urine by their renal system. Differences among species in the capacity or affinity of the ion transporter proteins for specific ions can affect the tolerance of fish species to increased ionic concentrations in water [7,12]. All ions contribute to the osmolality of blood and cytoplasm, but the concentrations of specific ions are maintained at differing levels in both the blood and cytoplasm.…”

Protective Effects of Dietary Spirulina platensis against Cadmium-Induced Oxidative Stress in Gills of Rainbow Trout

“…Gills of bony fish are vulnerable to exposure to toxic compounds [25]. Although damage to gills is often attributed to a direct contact with contaminants, transfer of the compounds through food chain can also cause severe damages to target tissues [26].…”