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Abstract: Mechanisms of ion uptake by freshwater (FW) fish have received considerable attention over the past 80 years. Through an assortment of techniques incorporating whole animal physiology, electrophysiology and molecular biological approaches, three models have been proposed to account for Na+ uptake. (1) Direct exchange of Na+ and H+ via one or more types of Na+/H+ exchanger (slc9), (2) uptake of Na+ through epithelial Na+ channels energized by an electrical gradient created by H+-ATPase and (3) Na+/Cl− co-transport (slc12). While each mechanism is supported at least in part by theoretical or experimental data, there are several outstanding questions that have not yet been fully resolved. Furthermore, there are few details concerning how these Na+ uptake mechanisms are fine tuned in response to the fluctuating FW environments. In this review, we summarize the current understanding of these three Na+ uptake mechanisms and discuss their regulation by endocrine (cortisol and prolactin) and neurohumoral (catecholamines) factors. [Copyright &y& Elsevier]

Abstract: This study examined the effect of acute hypoxic and hypercapnic cardiorespiratory stimuli, superimposed on existing cardiorespiratory disturbances in tambaqui. In their natural habitat, these fish often encounter periods of hypoxic hypercapnia that can be acutely exacerbated by water turnover. Tambaqui were exposed to periods of normoxia, hypoxia, hyperoxia and hypercapnia during which, externally oriented O2 and CO2 chemoreceptors were further stimulated, by administration into the inspired water of sodium cyanide and CO2-equilibrated water, respectively. Hyperoxic water increased the sensitivity of the NaCN-evoked increase in breathing frequency (fR) and decrease in heart rate. Hypoxia and hypercapnia attenuated the increase in fR but, aside from blood pressure, did not influence the magnitude of NaCN-evoked cardiovascular changes. Water PO2 influenced the magnitude of the CO2-evoked cardiorespiratory changes and the sensitivity of CO2-evoked changes in heart rate and blood flow. The results indicate that existing respiratory disturbances modulate cardiorespiratory responses to further respiratory challenges reflecting both changes in chemosensitivity and the capacity for further change. [Copyright &y& Elsevier]