Effects of hyposmotic shock on taurine transport in isolated trout hepatocytes

Abstract

SUMMARYIsolated trout hepatocytes exposed to hypotonic medium undergo a regulatory volume decrease (RVD) that occurs via two separate routes, K+-Cl-cotransport and amino acid release, the ion efflux accounting for 70 % of the total osmolyte loss. Taurine, glutamine and glutamic acid are the most important and represent 73 % of the total amino acid content (53 mmol (1 cell water)-').The osmolarity-sensitive release of amino acids was studied using [3H]taurine. Kinetic studies indicated two components for taurine influx: a linear Na+-independent transport and a saturable Na+-dependent system with a Michaelis-Menten constant (K.) of 122 fSM and a maximum velocity (Vmax) of 31-2 pmol (mg protein)-1 min-'. This second way of uptake was also chloride dependent and indicated an apparent coupling ratio Na+: Cl-: taurine of 2:1:1. The latter component and the taurine efflux were stimulated during RVD, leading to intracellular amino acid loss. Taurine efflux activation during volume recovery was transient and also dependent on the presence of both Na+ and Cl-in the extracellular medium. Furthernore, taurine release and RVD were slowed down when Ca2l was omitted from the medium. These results suggested two distinct and complementary mechanisms for volume regulation in trout hepatocytes during hypotonic conditions.

Metabolic and ionic responses of trout hepatocytes to anisosmotic exposure

“…Available reports on the transport mechanisms involved in RVD of trout hepatocytes indicate that, upon hyposmotic swelling, KCl cotransport and taurine release are the main routes mediating the loss of osmolytes (Bianchini et al, 1988(Bianchini et al, , 1991Michel et al, 1994). In principle, neither of these transport pathways should directly modify pHi, but our observations here show that a significant and reversible acidification was induced by hyposmolarity (Fig.·7).…”

“…As in this previous study, the alterations of [Ca 2+ ]i did not seem to be closely related to the changes of cell volume in the trout hepatocytes. Furthermore, in previous investigations it has been shown that the increase of K + efflux induced by hyposmotic conditions was not altered by the Ca 2+ ionophore A23187 or inhibitors of Ca 2+ -dependent K + channels (Bianchini et al, 1988), whereas a significant reduction of RVD and of taurine release was seen in the absence of extracellular Ca 2+ (Michel et al, 1994). The role of [Ca 2+ ]i in the RVD response of trout hepatocytes therefore remains ambiguous at present.…”

“…Interestingly, however, the omission of Cl -from the incubation medium prevented the hyposmotic acidification. A tentative explanation for this is that, under these conditions, RVD, and in turn also the pHi changes associated with it, might be blocked, since it was shown that both the increases in K + permeability (Bianchini et al, 1988) and in taurine release (Michel et al, 1994) were diminished in Cl --free media. A partial inhibition of RVD has also been reported for trout renal tubules in the presence of either Clchannel blockers or an inhibitor of KCl cotransport (Kanli and Norderhus, 1998).…”

“…Since, over the past decade, hepatocytes from the rainbow trout Oncorhynchus mykiss have been established as a model system for the investigation of teleostean cellular physiology (Canals et al, 1992;Furimsky et al, 2000;Krumschnabel et al, 1998;Mommsen et al, 1988;Schwarzbaum et al, 1996;Walsh, 1986), including the study of volume regulatory processes (Bianchini et al, 1988;Fossat et al, 1997;Michel et al, 1994), we chose to use these cells for our purpose. In the present study, we first examined how anisosmotic exposure of trout hepatocytes affects their cell volume.…”

Regulatory volume response following hypotonic stress in Atlantic salmon erythrocytes

“…Additionally, efflux of K ? during RVD occurs in trout hepatocytes (Michel et al 1994;Espelt et al 2008), turbot hepatocytes (Ollivier et al 2006), catfish perfused liver (Goswami and Saha 2006), trout RBCs (Egee et al 2000;Guizouarn et al 2000), and mussel gill cells (Silva and Wright 1994).…”