AJP - Cell Physiology, Vol 273, Issue 3 C883-C892, Copyright © 1997 by American Physiological Society
Modulation of high- and low-voltage-activated calcium currents in smooth muscle by calcium
J. G. McCarron, J. G. McGeown, J. V. Walsh Jr and F. S. Fay
Department of Biomedical Imaging, University of Massachusetts Medical School, Worcester 01605, USA.
Ca2+ currents (ICa) and cytoplasmic Ca2+ concentration ([Ca2+]c) were
measured in isolated gastric myocytes from Bufo marinus using whole cell
voltage clamp and fura 2, respectively. After a conditioning train of
depolarizing pulses, high-voltage-activated ICa (test potential of +10 mV)
was increased, returning to control values after approximately 85 s. This
enhancement was [Ca2+]c dependent, with a maximal increase at approximately
600 nM [Ca2+]c. During the conditioning train, ICa measured at 70 ms, which
provides a measure of high-voltage-activated current, initially decreased
with each successive pulse to a minimum of 56 +/- 5% of the first pulse in
the train. Thereafter, the 70-ms current showed considerable recovery.
Blockade of calmodulin activity with a peptide (RS20) or calmidazolium did
not affect the early inhibition but did abolish current recovery. A peptide
inhibitor of calmodulin-dependent protein kinase II (CK3AA) had similar
effects. Substraction of currents measured in the presence and absence of
RS20 revealed a 2-s delay between the start of the train and the onset of
current enhancement. It was also observed that low-voltage-activated
current (test potential of -17 mV) was reduced to 76 +/- 7% of control 5 s
after the conditioning train; this inhibition recovered to 92 +/- 4% after
35 s and was not dependent on [Ca2+]c elevation.
