Shigeki Miyamoto, Masatoshi Hori, Masanori Izumi, Hiroshi Ozaki* and
Hideaki Karaki
Department of Veterinary Pharmacology, Graduate School of Agriculture
and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan
*Corresponding author. FAX: +81-3-5841-8183
E-mail: aozaki@mail.ecc.u-tokyo.ac.jp
Abstract: Although b-adrenergic stimulation
has been shown in many studies to decrease myofilament Ca2+ sensitivity
in various types of cardiac muscle such as rat and rabbit ventricles, other
studies disagree with this conclusion. In the present study, we aimed to
explain these contradictory findings. We examined the effect of b-adrenoceptor
stimulation on Ca2+ sensitivity using guinea pig and rat ventricles.
We performed the experiment at two different temperatures and compared the
results. In guinea pig ventricles, isoproterenol and forskolin did not alter
the relationship between [Ca2+]i and muscle force
during the relaxation phase of tetanic contraction at either 24¡C or 30¡C.
In rat ventricles, in contrast, isoproterenol shifted the [Ca2+]i-force
curve to the right at 24¡C, but not at 30¡C. In guinea pig ventricles permeabilized
by a-toxin, in which the cAMP/PK-A system is
intact, the addition of cAMP did not decrease Ca2+ sensitivity.
These results suggest that there are species- and temperature-dependent
differences in the regulation of myofilament Ca2+ sensitivity
by b-adrenergic stimulation.
Keywords: Isoproterenol, Protein kinase A, Ca2+ transient,
Permeabilized fiber
Copyright The Japanese Pharmacological Society
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