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On the teleost system for enhanced oxygen unloading : time course following catecholamine removal and an investigation into other teleosts Shu, Jacelyn


Recent studies suggest that teleost fishes may be able to greatly enhance the amount of oxygen (O₂) unloaded to the tissues during a generalized acidosis associated with stress. This mechanism relies on pH-sensitive hemoglobin, a red blood cell (RBC) β-adrenergic Na⁺/H⁺ exchanger (β-NHE) activated by catecholamines to protect RBC pHi, and plasma-accessible carbonic anhydrase (paCA) at the tissues, but not at the gills, to short-circuit the β-NHE, acidify the RBC, and unload O₂ from hemoglobin. This system has been shown to increase tissue PO₂ (∆PO₂) by up to 30 Torr and has been proposed to play an important role both during early teleost evolution, as well as in modern teleost physiology. To date, most studies regarding this system have been conducted in the context of high stress and circulating catecholamines; little is known about this system under low catecholamine levels, such as following stress. In addition, despite being often extrapolated to all teleosts, this system has only been studied within the salmonids. Here, I investigate the time course of β-NHE short-circuiting by paCA in rainbow trout blood following in vitro stimulation by isoproterenol, a synthetic catecholamine, as well as natural catecholamines in vivo to determine how long after adrenergic stimulation this system may remain operational. A significant increase in plasma ∆PO₂ due to β-NHE short-circuiting by paCA was found in rainbow trout blood stimulated in vitro up to one hour after removal of isoproterenol; this was reduced to less than 30 min under natural catecholamines. A significant ∆PO₂ of approximately 5 Torr was determined even 6 h after stimulation, as well as in pharmaceutically blocked blood, suggesting that enhanced O₂ unloading may be operational in resting fish. The system was also investigated in two teleost species distantly related to the salmonids, cobia (Rachycentron canadum) and mahi-mahi (Coryphaena hippurus). In cobia and mahi-mahi, evidence of a RBC β-NHE was found, and it is predicted that cobia may exhibit up to a 61% increase in enhanced O₂ unloading with no change in blood flow through short-circuiting of RBC β-NHE, a value consistent with salmonids. This second phylogenetically distant group provides support that this system may be functional in teleosts in general.

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