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1 Department of Medicine and 2 Institute of Urology and Nephrology, University College London, London W1P 7PN, United Kingdom
The role of Na+/Ca2+ exchange in
regulating intracellular Ca2+ concentration
([Ca2+]i) in isolated smooth muscle cells
from the guinea pig urinary bladder was investigated. Incremental
reduction of extracellular Na+ concentration resulted in a
graded rise of [Ca2+]i; 50-100 µM
strophanthidin also increased [Ca2+]i. A
small outward current accompanied the rise of
[Ca2+]i in low-Na+ solutions
(17.1 ± 1.8 pA in 29.4 mM Na+). The quantity of
Ca2+ influx through the exchanger was estimated from the
charge carried by the outward current and was ~30 times that which is
necessary to account for the rise of [Ca2+]i,
after correction was made for intracellular Ca2+ buffering.
Ca2+ influx through the exchanger was able to load
intracellular Ca2+ stores. It is concluded that the level
of resting [Ca2+]i is not determined by the
exchanger, and under resting conditions (membrane potential 
50 to
60 mV), there is little net flux through the exchanger. However, a
small rise of intracellular Na+ concentration would be
sufficient to generate significant net Ca2+ influx.
urinary bladder; intracellular calcium; sodium/calcium exchange
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