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1Burdon Sanderson Cardiac Science Centre, University Laboratory of Physiology, Oxford OX1 3PT, United Kingdom; 2Dipartimento di Biologia Evolutiva e Funzionale, University of Parma, Parma 43100, Italy; and 3Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, Salt Lake City, Utah 84112-5000
Submitted 27 March 2003 ; accepted in final form 7 May 2003
Intracellular pH (pHi) is an important modulator of cardiac
function. The spatial regulation of pH within the cytoplasm depends, in part,
on intracellular H+
(
) mobility. The apparent
diffusion coefficient for
,
was estimated in single ventricular myocytes isolated from the rat, guinea
pig, and rabbit. 
was
derived by best-fitting predictions of a two-dimensional model of
H+ diffusion to the local rise of intracellular [H+],
recorded confocally (ratiometric seminaphthorhodafluor fluorescence)
downstream from an acid-filled, whole cell patch pipette. Under
conditions, was similar
in all three species (mean values: 8–12.5 x
10–7 cm2/s) and was over 200-fold lower
than that for H+ in water. In guinea pig myocytes,
was increased 2.5-fold in
the presence of
buffer, in agreement with previous observations in rabbit myocytes.
mobility is therefore low
in cardiac cells, a feature that may predispose them to the generation of
pHi gradients in response to sarcolemmal acid/base transport or
local cytoplasmic acid production. Low
mobility most likely
results from H+ shuttling among cytoplasmic mobile and fixed
buffers. This hypothesis was explored by comparing the pHi
dependence of intrinsic, intracellular buffering capacity, measured for all
three species, and subdividing buffering into mobile and fixed fractions. The
proportion of buffer that is mobile will be the main determinant of
. At a given
pHi, this proportion appeared to be similar in all three species,
consistent with a common value for
. Over the pHi
range of 6.0–8.0, the proportion is expected to change, predicting that
may display some
pHi sensitivity.
intracellular pH; H+ diffusion
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