ABSTRACT
This chapter reviews the existent literature to illustrate the particularities of Ca2+ balance in teleost fish: the tissues and mechanisms involved in uptake and excretion in freshwater (FW) and marine species and the interaction with ambient Ca2+. It focuses on the role of endocrine factors that control Ca2+ homeostasis in fish and the relevant developments. Transepithelial potential measurements in the isolated nephron segments of FW-fish produced variable results, but the epithelium seems to be impermeable to passive paracellular Ca2+ movements, indicating that the processes of reabsorption must occur transcellularly. The enterocyte cytoplasm is negatively charged when compared to the intestinal lumen, and intracellular Ca2+ concentrations are in the nanomolar range. The urine produced by the kidney is retained in the urinary bladder and its ionic composition is further modified there through transport of water and/or monovalent ions.
