AJP - Cell Physiology, Vol 259, Issue 1 C19-C25, Copyright © 1990 by American Physiological Society
Effects of low extracellular Ca2+ on cytosolic free Ca2+, Na+, and pH of MDCK cells
A. B. Borle, C. J. Borle, P. Dobransky, A. M. Gorecka-Tisera, C. Bender and K. Swain
Department of Physiology, University of Pittsburgh School of Medicine, Pennsylvania 15261.
The effects of lowering the Ca2+ electrochemical potential (delta mu Ca2+)
by removing extracellular calcium (Ca2+o) was studied in Madin-Darby canine
kidney (MDCK) cells. We measured cytosolic free calcium (Ca2+i),
intracellular Na+ (Na+i), and intracellular pH (pHi) as well as Ca2+ and
Na+ influx and efflux and H+ secretion. As soon as Ca2+o was lowered to
zero, Ca2+ efflux increased 2-fold, and Na+ influx increased 4.3-fold. As a
result, Ca2+i fell 71% from 120 to 34.7 nM, and Na+i rose 56% from 16.7 to
26.1 mM. At the same time, H+ secretion was depressed 38%, and pHi fell
from 7.32 to 7.22. When Ca2+o was restored to normal (1.3 mM), there was a
sharp rise in Ca2+i from 34.7 to 243 nM (+600%) caused by an immediate
increase in Ca2+ influx (+112%). At the same time, Na+ efflux increased
20%. Ten to fifteen minutes after restoring Ca2+o to 1.3 mM, Ca2+i, Na+i,
and pHi were back to control levels. These results suggest that reducing
delta mu Ca2+ activates the forward mode of Na(+)-Ca2+ exchange which leads
to a fall in Ca2+i and a decreased delta mu Na+ by increasing Na+i. The
fall in delta mu Na+ secondarily depresses Na(+)-H+ exchange which results
in a reduced H+ secretion and a fall in pHi. When Ca2+o is restored to 1.3
mM, the reverse mode of Na(+)-Ca2+ exchange is activated, since delta mu
Ca2+ is larger than normal because of the low Ca2+i, and delta mu Na+ is
reduced because of the high Na+i.(ABSTRACT TRUNCATED AT 250 WORDS)
