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Department of Physiology and Biophysics, University of Nebraska Medical Center, Omaha, Nebraska 68198-4575
In mesangial cells (MC), the response of intracellular
Ca2+ concentration
([Ca2+]i) to a
contractile agonist is biphasic with a large, transient increase in
[Ca2+]i followed by a smaller but
sustained elevation as Ca2+ flows into the cell from the
extracellular fluid. It has been postulated that membrane
depolarization precedes opening of Ca2+ channels in the
plasmalemmal membrane. However, a role for voltage-gated Ca2+ channels (VGCC) in human MC has been controversial,
and their existence has not been verified with single-channel analysis. We used fura 2 fluorescence and patch-clamp techniques to determine the
properties of the Ca2+ entry pathway responsible for the
sustained response of [Ca2+]i in
human MC. We found that ANG II at 10 nM, 100 nM, and 1 µM increased
[Ca2+]i to sustained levels of
22%, 35%, and 49%, respectively, above baseline. The sustained
response to 1 µM ANG II was attenuated by diltiazem and was reduced
to a value less than baseline in the absence of external
Ca2+. None of the peak responses (due to release of
intracellular stores of Ca2+) were affected by removal of
external Ca2+ or addition of diltiazem. Upon elevating the
extracellular [K+] from 5 mM to 75 mM,
[Ca2+]i reached a sustained level
of 48% greater than baseline. This effect of high K+ was
attenuated by either Ca2+ removal or addition of diltiazem.
In the presence of 75 or 140 mM K+, the dihydropyridine
agonist BAY K 8644 (1 µM and 10 µM) initiated sustained
[Ca2+]i responses averaging 18%
and 25%, respectively, greater than baseline. With <10 nM
Ca2+ in the external solution, BAY K 8644 did not
significantly affect [Ca2+]i. In
separate patch-clamp experiments, barium-selective channels were found
in cell-attached patches with 90 mM BaCl2 and 10 µM BAY K
8644 in the pipette solution. The single-channel conductance was 11.2 pS, and the open probability increased steeply at membrane potentials
between
30 mV and 0 mV. It is concluded that human glomerular MC
contain dihydropyridine-sensitive Ca2+ channels responsible
for the voltage-regulated entry of Ca2+ into the cell
during an agonist-induced contraction.
diltiazem; BAY K 8644; voltage-gated calcium channel; fura 2; patch clamp
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