Abstract Hypotonic swelling of teleost erythrocytes ac-
tivates multiple transport systems leading to the regula-
tory decrease of cell volume. We have examined using
pharmacological manipulation the swelling-induced tau-
rine flux pathway in red blood cells of the rainbow trout
and its relationship to swelling-induced K flux pathways.
We show that the activation and deactivation of taurine
flux is rapid and that the flux is a sigmoidal function of
cell volume. N-ethylmaleimide (NEM) and the non-spe-
cific protein kinase inhibitor, staurosporine, both inacti-
vated the hypotonically-induced taurine flux with con-
centrations eliciting half-maximal inhibition (IC
50
s) of
212 and 17 µmol l
–1
, respectively. The low taurine fluxes
under isotonic conditions were unaffected. By contrast,
the tyrosine kinase inhibitor, genistein, partially inhibited
taurine flux under both isotonic and hypotonic condi-
tions. The specific phosphatase inhibitor, calyculin A,
had no inhibitory or stimulatory effect under either con-
dition whilst the less-specific phosphatase inhibitor,
ortho-vanadate, reduced taurine flux only under hypo-
tonic conditions. In these respects the regulatory control
of the taurine pathway differs from the Cl-dependent K
flux. However, NEM and staurosporine also inhibited the
Cl-independent K flux, both with similar IC
50
s to those
observed for taurine fluxes. This supports the idea of the
hypotonically-induced taurine flux and the Cl-indepen-
dent K flux sharing the same transport pathway.
Key words Volume regulation · Volume sensitivity ·
Staurosporine · Calyculin A
Introduction
Most types of animal cell defend themselves against
swelling by losing solute together with osmotically
obliged water, a process known as regulatory volume de-
crease (RVD) [42]. The principal solutes are K and Cl, as
well as a variety of intracellular organic osmolytes, such
as taurine, amino acids and myo-inositol. KCl can cross
the membrane either via the electroneutral KCl cotrans-
porter [10] or via separate K and anion channels [35].
Recently, much attention has been focused on this latter
volume-sensitive anion channel, since it may show a
broad substrate selectivity for different anions, organic
osmolytes and even cations, meaning that a fraction of
the uncoupled transport of Cl, taurine and even K may
share the same transport route [2, 26].
Fish red blood cells have a particularly active RVD
response, involving both ions and non-electrolytes [19].
The cotransport of KCl in trout red blood cells has been
confirmed by kinetic analysis and inhibitor studies [3, 9],
and the presence of a K
+
/Cl
–
cotransporter-1 (KCC1) ho-
mologue in trout red blood cells has been confirmed by
the reverse transcription-polymerase chain reaction (RT-
PCR) technique and sequencing [21]. In addition, a sig-
nificant hypotonic swelling-activated taurine flux has
been demonstrated in flounder, eel and trout red blood
cells [18, 19, 44], which accounts for 30–50% of the vol-
ume regulatory response. This taurine transport pathway
bears a striking similarity with anion-selective channels
observed in some mammalian cells [43], since they are
swelling-activated and carry a range of other small elec-
troneutral organic solutes including choline, nucleotides,
γ-aminobutyric acid (GABA) and glycine.
Fluxes of K and anions, via pathways distinct from
KCC1 have also been characterised [3, 9, 19, 22], con-
firming the involvement of multiple pathways for RVD.
Motais and colleagues [31] have proposed that this Cl-
independent route may still represent transport via the
KCC1, but with an altered pattern of anion dependence
that is induced by hypotonic swelling. On the other hand,
trout erythrocytes can show activation of K flux without
K. Kiessling · J.C. Ellory
University Laboratory of Physiology, University of Oxford,
Parks Road, Oxford OX1 3PT, UK
A.R. Cossins (
✉
)
Integrative Biology Research Division,
School of Biological Sciences, University of Liverpool,
P.O. Box 147, Liverpool L69 3BX, UK
e-mail: cossins@liverpool.ac.uk
K. Kiessling
Winklerstrasse 7, D-12623 Berlin, Germany
Pflügers Arch – Eur J Physiol (2000) 440:467–475
Digital Object Identifier (DOI) 10.1007/s004240000296
ORIGINAL ARTICLE
K. Kiessling · J.C. Ellory · A.R. Cossins
The relationship between hypotonically-induced taurine and K fluxes
in trout red blood cells
Received: 11 February 1999 / Received after revision: 25 February 2000 / Accepted: 28 February 2000 / Published online: 21 April 2000
© Springer-Verlag 2000