Acid–base regulation in the Dungeness crab (Metacarcinus magister)

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Many studies have investigated ammonia excretion and acid-base regulation in aquatic arthropods, yet current knowledge of marine chelicerates is non-existent. In American horseshoe crabs (), book gills bear physiologically distinct regions: dorsal and ventral half-lamellae, a central mitochondria-rich area (CMRA) and peripheral mitochondria-poor areas (PMPAs). In the present study, the CMRA and ventral half-lamella exhibited characteristics important for ammonia excretion and/or acid-base regulation, as supported by high expression levels of Rhesus-protein 1 (LpRh-1), cytoplasmic carbonic anhydrase (CA-2) and hyperpolarization-activated cyclic nucleotide-gated K channel (HCN) compared with the PMPA and dorsal half-lamella. The half-lamellae displayed remarkable differences; the ventral epithelium was ion-leaky whereas the dorsal counterpart possessed an exceptionally tight epithelium. LpRh-1 was more abundant than Rhesus-protein 2 (LpRh-2) in all investigated tissues, but LpRh-2 was more prevalent in the PMPA than in the CMRA. Ammonia influx associated with high ambient ammonia (HAA) treatment was counteracted by intact animals and complemented by upregulation of branchial CA-2, V-type H-ATPase (HAT), HCN and LpRh-1 mRNA expression. The dorsal epithelium demonstrated characteristics of active ammonia excretion. However, an influx was observed across the ventral epithelium as a result of the tissue's high ion conductance, although the influx rate was not proportionately high considering the ∼3-fold inwardly directed ammonia gradient. These novel findings suggest a role for the coxal gland in excretion and in the maintenance of hemolymph ammonia regulation under HAA. Hypercapnic exposure induced compensatory respiratory acidosis and partial metabolic depression. Functional differences between the two halves of a branchial lamella may be physiologically beneficial in reducing the backflow of waste products into adjacent lamellae, especially in fluctuating environments where ammonia levels can increase.

Section: Hypercapniasupporting

confidence: 54%

Ammonia excretion and acid-base regulation in the American horseshoe crab,Limulus polyphemus

Hans

Quijada-Rodriguez

Allen

et al. 2018

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“…Interestingly, green crabs acclimated to brackish water also excrete near-equal amounts of H + -equivalents and ammonia, suggesting that much of the acid expulsion is excreted as NH 4 + . In comparison, upon exposure of M. magister to 330 Pa P CO2 , the animal accumulates HCO 3 − to compensate for extracellular pH disturbance whilst reducing branchial ammonia excretion rates and experiencing metabolic depression similar to that of the Norway lobster Nephrops norvegicus (Hagerman et al, 1990), which lowers hemolymph ammonia load (Hans et al, 2014). Overall, while the response of animals to hypercapnia varies, it is often linked to changes in ammonia excretion processes.…”

Section: Ammonia As a Major Acid-base Homeostatic Moleculementioning

confidence: 99%

Ammonia excretion in aquatic invertebrates: new insights and questions

Weihrauch

Allen

2018

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Invertebrates employ a variety of ammonia excretion strategies to facilitate their survival in diverse aquatic environments, including freshwater, seawater and the water film surrounding soil particles. Various environmental properties set innate challenges for an organism's ammonia excretory capacity. These include the availability of NaCl and the respective ion-permeability of the organism's transport epithelia, and the buffering capacity of their immediate surrounding medium. To this end, some transporters seem to be conserved in the excretory process. This includes the Na/K(NH)-ATPase (NKA), the NH/CO dual gas-channel Rhesus (Rh)-proteins and novel ammonia transporters (AMTs), which have been identified in several invertebrates but appear to be absent from vertebrates. In addition, recent evidence strongly suggests that the hyperpolarization-activated cyclic nucleotide-gated K channel (HCN) plays a significant role in ammonia excretion and is highly conserved throughout the animal kingdom. Furthermore, microtubule-dependent vesicular excretion pathways have been found in marine and soil-dwelling species, where, unlike freshwater systems, acid-trapping of excreted ammonia is difficult or absent owing to the high environmental buffering capacity of the surroundings. Finally, although ammonia is known to be a toxic nitrogenous waste product, certain marine species readily maintain potentially toxic hemolymph ammonia as a sort of ammonia homeostasis, which suggests that ammonia is involved in physiological processes and does not exist simply for excretion. Such findings are discussed within this Commentary and are hypothesized to be involved in acid-base regulation. We also describe excretory organs and processes that are dependent on environmental constraints and indicate gaps in the current knowledge in these topics.

“…Furthermore, fish and crustaceans exposed to high environmental TAM or elevated P CO2 attempt to actively maintain blood and haemolymph TAM concentrations and pH despite changing external conditions (Claiborne and Evans, 1988;Appelhans et al, 2012;Martin et al, 2011;Hans et al, 2014). Nevertheless, long-term exposure to high environmental TAM or elevated P CO2 can affect the performance of crustaceans, resulting in increasing extracellular TAM concentration or lowered TAM excretion (Martin et al, 2011;Hans et al, 2014). In contrast, Mytilus species tolerate substantial fluctuations of their extracellular pH, TAM and ion concentrations and do not actively regulate these parameters (Sadok et al, 1995;.…”

Section: Mechanisms Of Tam Excretion In Bivalvesmentioning

confidence: 99%

Ammonia excretion in mytilid mussels is facilitated by ciliary beating

Thomsen

Himmerkus

Holland

et al. 2016

The excretion of nitrogenous waste products in the form of ammonia (NH 3 ) and ammonium (NH 4 + ) is a fundamental process in aquatic organisms. For mytilid bivalves, little is known about the mechanisms and sites of excretion. This study investigated the localization and the mechanisms of ammonia excretion in mytilid mussels. An Rh protein was found to be abundantly expressed in the apical cell membrane of the plicate organ, which was previously described as a solely respiratory organ. The Rh protein was also expressed in the gill, although at significantly lower concentrations, but was not detectable in mussel kidney. Furthermore, NH 3 /NH 4 + was not enriched in the urine, suggesting that kidneys are not involved in active NH 3 /NH 4 + excretion. Exposure to elevated seawater pH of 8.5 transiently reduced NH 3 /NH 4 + excretion rates, but they returned to control values following 24 h acclimation. These mussels had increased abundance of V-type H + -ATPase in the apical membranes of plicate organ cells; however, NH 3 /NH 4 + excretion rates were not affected by the V-type H + -ATPase specific inhibitor concanamycin A (100 nmol l −1). In contrast, inhibition of ciliary beating with dopamine and increased seawater viscosity significantly reduced NH 3 excretion rates under control pH (8.0). These results suggest that NH 3 /NH 4 + excretion in mytilid mussels takes place by passive NH 3 diffusion across respiratory epithelia via the Rh protein, facilitated by the water current produced for filter feeding, which prevents accumulation of NH 3 in the boundary layer. This mechanism would be energy efficient for sessile organisms, as they already generate water currents for filter feeding.