AJP - Cell Physiology, Vol 262, Issue 5 C1258-C1265, Copyright © 1992 by American Physiological Society
Thapsigargin stimulates Ca2+ entry in vascular smooth muscle cells: nicardipine-sensitive and -insensitive pathways
Y. T. Xuan, O. L. Wang and A. R. Whorton
Department of Pharmacology, Duke University Medical Center, Durham, North Carolina 27710.
We have investigated the role of the sarcoplasmic reticulum Ca2+ pool in
regulating Ca2+ entry in vascular smooth muscle cells using a
receptor-independent means of mobilizing the intracellular Ca2+ pool.
Thapsigargin (TG) has been shown to inhibit the endoplasmic reticulum
Ca(2+)-ATPase, mobilize intracellular Ca2+, and activate Ca2+ entry in
nonmuscle tissues. When smooth muscle cells were treated with 0.2 microM
TG, cytosolic Ca2+ concentrations rose gradually over 8 min to a peak value
of 365 +/- 18 nM. Cytosolic Ca2+ remained elevated for at least 20 min and
was supported by continued entry of extracellular Ca2+. TG also stimulated
entry of Mn2+ and 45Ca2+ from outside the cell. Importantly, TG-induced
Ca2+ entry and Mn2+ entry were found to occur through mechanisms that were
independent of L-type Ca2+ channel activation because influx was not
inhibited by concentrations of nicardipine that were found to block either
endothelin- or 100 mM extracellular K(+)-induced cation influx. The
mechanism through which TG activates cation entry appears to involve
mobilization of the inositol 1,4,5-trisphosphate-responsive intracellular
Ca2+ pool. In permeabilized cells, TG prevented ATP-stimulated Ca2+ uptake
into the sarcoplasmic reticulum and slowly released sequestered Ca2+. The
Ca2+ pool involved was responsive to inositol 1,4,5-trisphosphate. However,
TG did not initiate the formation of inositol polyphosphates. Thus TG
mobilizes the sarcoplasmic reticulum Ca2+ pool and activates Ca2+ entry
through a nicardipine-insensitive Ca2+ channel in vascular smooth muscle.
The mechanism is independent of inositol polyphosphate formation.
