Vol. 275, Issue 1, H204-H212, July 1998
Genistein elicits biphasic effects on L-type
Ca2+ current in feline atrial
myocytes
Yong G.
Wang and
Stephen L.
Lipsius
Department of Physiology, Loyola University Chicago, Stritch School
of Medicine, Maywood, Illinois 60153
A perforated patch recording method was used to
determine the effects of genistein (Gen), a protein tyrosine kinase
(PTK) inhibitor, on basal L-type
Ca2+ current
(ICa,L) in
feline atrial myocytes. Gen (50 µM) elicited biphasic changes in
ICa,L: an initial
inhibition (
55 ± 4%; phase 1) followed by a secondary stimulation (34 ± 9%;
phase 2) of
ICa,L. Withdrawal
of Gen elicited a further potentiation of
ICa,L (152 ± 19%; phase 3) above control
(n = 46). In general,
phase 1 inhibition and
phase 3 potentiation varied directly
with Gen concentration, and phase 2 stimulation exhibited biphasic concentration-dependent changes compared
with control. When cells were dialyzed using a ruptured patch recording
method, Gen elicited only inhibition of
ICa,L;
phases 2 and
3 were abolished. Vanadate (1 mM), an
inhibitor of protein tyrosine phosphatase, abolished both Gen-induced
inhibition and stimulation of
ICa,L. Daidzein
(50 µM), a weakly active analog of Gen, exerted no significant
effects on ICa,L,
and withdrawal of daidzein failed to potentiate
ICa,L. In a few
cells, Gen elicited a prominent vanadate-sensitive stimulation of
ICa,L in the
absence of any significant inhibition of
ICa,L.
Gen-induced changes in ICa,L were
unaffected by either 100 µM
1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM) or 1 µM ryanodine, agents that
alter intracellular Ca2+; 4 µM
H-89 or 50 µM Rp diastereomer of adenosine
3',5'-monophosphothioate (RP-cAMPS), inhibitors
of protein kinase A (PKA); 0.1 µM calphostin C or 2 µM
chelerythrine, inhibitors of protein kinase C (PKC); or 100 µM
NG-monomethyl-L-arginine
(L-NMMA), an inhibitor of nitric oxide (NO) synthase. We
conclude that in feline atrial myocytes, Gen acts via membrane-bound
PTKs to inhibit
ICa,L and via
cytosolic PTKs to stimulate
ICa,L.
Gen-induced changes in
ICa,L are not related to changes in intracellular
Ca2+ or to secondary interactions
with either PKA, PKC, or NO signaling pathways. These results indicate
that in atrial myocytes
ICa,L is
regulated by two independent and competing PTK signaling mechanisms.
electrophysiology; cardiac; vanadate; daidzein; perforated patch
