Modulation of ca 2+ current in canine colonic myocytes by cyclic
nucleotide-dependent mechanisms.
Koh, Sang Don, and Kenton M. Sanders.
Department of Physiology and Cell Biology, University of Nevada
School of Medicine, Reno, Nevada 89511, USA, (702) 784-6908 or FAX
(702) 784-6903, Email: kent@physio.unr.edu
APStracts 3:0082C, 1996.
Regulation of Ca2+ currents by cyclic nucleotide-dependent mechanisms
was studied in circular muscle cells isolated from canine proximal
colon. Whole-cell Ca2+ currents were recorded at 32o C using
amphotericin B perforated patches. The effects of several agents
known to increase levels of cAMP were tested. VIP and isoproterenol
(10-7 to 10-5 M) increased Ca2+ current in a concentration-dependent
manner. Forskolin (10-7 M) and dibutyryl cAMP (10-6 to 10-5 M) also
increased Ca2+ current. Higher concentrations of forskolin (10-6 to
10-5 M) caused inhibition of Ca2+ current. Low concentrations (10-5
to 10-7 M) of dibutyryl cAMP or 8-Br-cAMP caused concentration
-dependent enhancement in Ca2+ current, and these effects were
reversible upon washout of the cAMP analogues. When the concentration
of cAMP analogues was increased (10-3 to 10-4 M), we observed
inhibition of Ca2+ current similar to the effects of forskolin.
Membrane permeable analogues of cGMP produced exclusively inhibitory
effects. The non-specific protein kinase inhibitor H7 (up to 60
[mu]M) failed to block the effects of VIP, isoproterenol and
forskolin and it produced inhibitory effects on Ca2+ current
independent of agonist stimulation. The data suggest that low levels
of cAMP may, via phosphorylation by protein kinase A, enhance L-type
Ca2+ current, but higher concentrations of cAMP may "cross
-over" and activate protein kinase G. Phosphorylation by protein
kinase G appears to produce a dominant inhibition of Ca2+ current.
Received 31 July 1995; accepted in final form 5 March 1996.
APS Manuscript Number C467-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96