Hypoosmotic volume regulation in bivalves: Protein kinase C and amino acid release

EDCs trigger immune-neurotransmitter related gene expression, and cause histological damage in sensitive mud crab Macrophthalmus japonicus gills and hepatopancreas

Endocrine-disrupting chemicals (EDCs), distributed at various concentrations in freshwater and marine ecosystems, affect the survival, reproduction, and behavior of wide ranges organisms. Most toxicology studies on EDCs have focused on the endocrine system of invertebrates, and research on invertebrate neurotransmitters is limited. In the present study, we investigated the expression of Macrophthalmus japonicus genes encoding γ-aminobutyric acid transporter subtype 2 (GAT-2) and glutamine synthetase (GS), which play important roles as neurotransmitters at synapses. We observed differences in the mRNA expression levels of GAT-2 and GS as well as histological changes in various tissues after exposure to bisphenol-A (BPA) and di-(2-ethylhexyl) phthalate (DEHP). The amino acid sequences of M. japonicus GAT-2 and GS formed separate branches in crustaceans, fish, insects, and mammals. M. japonicus GAT-2 and GS expression levels were highest in the gills, hepatopancreas, and stomach, and showed different between DEHP or BPA treatments. In particular, hepatopancreas GS expression on Day 1, the first step in the presynaptic process, was upregulated after BPA and DEHP exposure, while GAT-2, sequential step in the presynaptic process, was significantly elevated only in DEHP. After BPA treatments, gill GS expression was increased at all concentrations, whereas GAT-2 expression was overall down regulations. In contrast, in DEHP treatment groups hepatopancreatic GS and GAT-2 expression at Day 1 was only significantly higher and all groups including gill GS and GAT-2 expression were downregulation. Histological changes in the gills and hepatopancreas were observed in a concentration-dependent manner. Accordingly, BPA and DEHP exposure in crabs could be stimulate neurotransmitter gene expression and alter the morphological structure of gill and hepatopancreas.

Osmoionic homeostasis in bivalve mollusks from different osmotic niches: Physiological patterns and evolutionary perspectives

Physiological knowledge gained from questions focused on the challenges faced and strategies recruited by organisms in their habitats assumes fundamental importance about understanding the ability to survive when subjected to unfavorable situations. In the aquatic environment, salinity is particularly recognized as one of the main abiotic factors that affects the physiology of organisms. Although the physiological patterns and challenges imposed by each occupied environment are distinct, they tend to converge to osmotic oscillations. From a comparative perspective, we aimed to characterize the osmoregulatory patterns of the bivalve mollusks Corbicula largillierti (purple Asian cockle), Erodona mactroides (lagoon cockle), and Amarilladesma mactroides (white clam) - inhabitants of different osmotic niches - when submitted to hypo- and/or hyperosmotic salinity variations. We determined the hemolymph osmotic and ionic concentrations, tissue hydration, and the intracellular isosmotic regulation (IIR) from the use of osmolytes (organic and inorganic) after exposure to species-specific salinity intervals. Additionally, we incorporated phylogenetic perspectives to infer and even broaden the understanding about the patterns that comprise the osmoionic physiology of Bivalvia representatives. According to the variables analyzed in the hemolymph, the three species presented a pattern of osmoconformation. Furthermore, both ionic regulation and conformation patterns were observed in freshwater, estuarine, and marine species. The patterns verified experimentally show greater use of inorganic osmolytes compared to the participation of organic molecules, which varied according to the osmotic niche occupied in the IIR for the mantle, adductor muscle, and gills. This finding widens the classic vision about the preferential use of certain osmolytes by animals from distinct niches. Our phylogenetic perspective also indicates that environmental salinity drives physiological trait variations, including hemolymph osmolality and the ion composition of the extracellular fluid (sodium, chloride, magnesium, and calcium). We also highlight the important role played by the shared ancestry, which influences the interspecific variability of the hemolymph K⁺ in selected representatives of Bivalvia.

Volume changes in cardiac ventricles from Aplysia brasiliana upon exposure to hyposmotic shock

We investigated the possible role of ion channels and transporters in cell volume control using Aplysia brasiliana ventricular tissues exposed to a 26% hyposmotic shock, by assessing changes in wet weight, intracellular water and ionic contents. Thirty minutes after the shock, the wet weight of isolated ventricles increase about 20% above control levels and then attain near original weight within 60 min after the shock. At the time when the wet weight returned to control values, intracellular water and KCl contents are decreased by 22 and 20%, respectively. The K(+) channel blockers, 4-AP and TEA, but not the cotransport blockers, hydrochlorothiazide and furosemide, greatly affect the magnitude of wet weight gain and the time course of weight recovery, indicating that KCl loss occur through conductive pathways. Intracellular recordings performed on ventricular myocytes during exposure to the osmotic shock showed an immediate membrane hyperpolarization and blockade of spontaneous electrical activity; diastolic membrane potential recover over time and spontaneous action potentials are completely restored 60 min after the hyposmotic shock. Because significant weight loss is observed during the exposure of ventricular tissues to 26% hypo-ionic, but isosmotic saline, it is suggested that ventricular volume restoration is accomplished by two distinct but simultaneously occurring processes: a volume-dependent and a volume-independent mechanism. Because wet weight restoration is completely prevented by exposing ventricular tissue to a Ca(2+)-free hyposmotic solution, we postulate that both processes involved in A. brasiliana ventricular weight restoration are Ca(2+)-dependent mechanisms.

Modulation of the swelling-activated amino acid channel of Leishmania major promastigotes by protein kinases

Leishmania promastigotes respond to hypotonic challenges by a mechanism of regulatory volume decrease (RVD), whereby anionic amino acid channels (HAAC) are hypotonically-activated and intracellular amino acids are released from the cells. Irrespective of the experimental conditions, restoration of isotonicity triggered an immediate blockage of the amino acid release. Both the speed and amplitude of the response depended on the hypotonic stimulus and on the operation of intracellular signaling mechanisms. The initial (5 s) hypotonic-induced release of amino acids (ri) and the steady state levels of amino acids attained (5 min) or amplitude (A), were markedly affected by modulators of protein kinase C: phorbol 12-myristate 13-acetate, 1-oleoyl-2-acetylglycerol and phorbol 12,13-diacetate whereas staurosporine and the related analog, bis-indolylmaleimide I (GF-109203.X) inhibited the RVD response. Agonists of cAMP-dependent protein kinase A such as forskolin or (8-(4-chlorophenylthio))-adenosine-3',5'cyclic-monophosphate enhanced the speed of the response but had little effect on its amplitude. Neither 4alpha-phorbol 12,13-didecanoate,1,9-dideoxyforskolin nor genistein, tamoxifen or thapsigargin had any apparent effect on either parameter tested. The most striking stimulation of hypotonic-induced amino acid release was exerted by arachidonic acid or by its non-metabolizable analog, 5,8,11,14-eicosatetraynoic acid (ETYA). These agents caused a major increase in the initial rate of amino acid release as well as a higher amplitude of the response, both of which were markedly inhibited by an anion channel blocker. The present studies indicate not only that hypotonicity is an obligatory and dominant component in HAAC activation, but implicate specific second messengers in the modulation of the RVD response. The modes of activation or attenuation of HAAC activity apparently differ for PKC and PKA modulators as well as for arachidonic acid. The involvement of Ca2+ in HAAC was studied in hypotonic challenged cells which were treated with intracellular Ca2+-chelators or Ca2+-free medium. These cells showed a lag in AA release and a modest inhibition of the amplitude. The inhibition of HAAC was markedly increased when cells were treated with the ionophore A23187 in Ca2+-free media. The HAAC activity was accompanied by a significant increase in internal Ca2+ when performed in Ca2+-containing medium (from 88+/-9 to 179+/-22 nM) but by no significant change when measured in Ca2+-free medium. These studies indicate that although Ca2+ might be involved in the early activation phase of HAAC, it is either not absolutely required or its action might be associated with localized events.