![]() ![]() FRESHWATER LEECH IDENTIFICATION SKINVarious studies have shown that ammonia transport occurs often via tissues also responsible for gas exchange and/or osmoregulation such as skin ( 14, 51, 63), intestine ( 49, 68, 72), nephridial systems ( 29, 32), gills ( 66, 75), kidneys ( 59), and anal papillae ( 18, 64). Typically, nonmammalian aquatic organisms excrete the majority of their nitrogenous waste as ammonia ( 30, 76). Because of the toxicity of ammonia, all animals must have an efficient mechanism by which ammonia is detoxified into less toxic nitrogenous waste products or rapidly excreted to maintain body fluid levels within a tolerable range. Furthermore, as extensively studied in mammals, ammonia has various detrimental effects on the central nervous system, such as cerebral edema, cerebral atrophy, and disturbances of neuronal growth and signal transduction pathways ( 8). For example, ammonia is capable of affecting energy metabolism ( 13, 44, 71) and ion transport capabilities in vertebrates, as well as invertebrates ( 7, 21, 40). While inherently produced by all cells through protein metabolism, ammonia is toxic, exhibiting various detrimental effects. As the pKa of ammonia is relatively high (pKa = 9.2 to 9.8), the vast majority of ammonia present in the environment or body fluids will typically be in the ionic NH 4 + form, unless fairly alkaline environments or body fluid conditions are present. In solution, ammonia will be present in a pH-dependent equilibrium of the nonionic, membrane-permeable, NH 3 form and the ionic form, NH 4 + ( 11). The majority of ammonia (in this study, NH 3 refers to nonionic ammonia, NH 4 + refers to ionic ammonia, and ammonia refers to the sum of both) in an organism is synthesized through the metabolic process of deamination, while uricolytic or ureolytic pathways usually account for a small portion of ammonia produced ( 17, 38). To our knowledge, this is only the second comprehensive study regarding the ammonia excretion mechanisms in a freshwater invertebrate, but our results show that basic processes of ammonia excretion appear to also be comparable to those found in freshwater fish, suggesting an early evolution of ionoregulatory mechanisms in freshwater organisms. Exposure to high environmental ammonia (HEA) caused a new adjustment of body ammonia, accompanied with a decrease in NoRhp and Na +/K +-ATPase mRNA levels, but unaltered ammonia excretion rates. The leech Rh-ammonia transporter (NoRhp) is a member of the primitive Rh protein family, which is a sister group to the common ancestor of vertebrate ammonia-transporting Rh proteins. Most importantly, functional expression studies of the identified Rh protein cloned from leech skin tissue revealed an ammonia transport capability of this protein when expressed in yeast. ![]() ![]() Pharmacological experiments and enzyme assays suggested an ammonia excretion mechanism that involves the V-ATPase, Na +/K +-ATPase, and carbonic anhydrase, but not necessarily a functional microtubule system. Accordingly, compared with body tissues, the skin exhibited elevated mRNA expression levels of a newly identified Rhesus protein and at least in tendency the Na +/K +-ATPase. Exposure to high and low pH hampered and enhanced, respectively, ammonia excretion rates, indicating an acid-linked ammonia trapping mechanism across the skin epithelia. Excretion experiments showed that the ribbon leech is ammonotelic, excreting 166.0 ± 8.6 nmol In the current study, the nitrogen excretion mechanism in the carnivorous ribbon leech, Nephelopsis obscura, was investigated. Remarkably little is known about nitrogenous excretion in freshwater invertebrates. ![]()
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