To expel the excess
protons generated during a cellular acidification and to fully recover basal intracellular pH (pHi), cardiac cells rely on the
amiloride-sensitive Na/H antiport. We report that rat single ventricular cardiomyocytes, loaded with the fluorescent pH
indicator Snarf-1 and treated with inhibitors of the Na/H antiport,
amiloride or its analogues, partially restored their pHi through a
bicarbonate-dependent mechanism following an
acidosis (imposed by the
ammonia-pulse technique). In the presence of
ethylisopropylamiloride (10 microM) or
amiloride (1 mM) and 25 mM
bicarbonate in the extracellular
solution, the average time that cells needed to recover half of their pHi, following the removal of 20 mM NH4Cl, was 3.4 min, while the rate of
proton efflux was calculated to be 2.0 mM/min. The
stilbene derivative, 4-4'-di-isothiocyanostilbene-2,2'-disulphonate (
DIDS 200 microM), a known blocker of
anion transporters, inhibited this recovery. Both
phenylephrine (100 microM, 3 microM
propranolol present), an alpha 1-adrenoceptor agonist, and
ATP (10 microM), a
purinergic agonist, significantly enhanced the rate of
proton efflux that was due to this HCO3-dependent alkalinizing mechanism.
Phenylephrine and
ATP also shortened by three-fold the time that a myocyte needed to recover half of its initial pHi. This
bicarbonate-dependent alkalinizing mechanism could provide an additional means by which cardiac cells recover their pHi from
acidosis, especially under conditions in which the Na/H antiport is inhibited. Furthermore,
catecholamines and
ATP, which are released under various pathophysiological conditions often associated with intracellular
acidosis, could play an important role in the modulation of pHi under these conditions.