The steady-state force–Ca2+ relationship in intact lobster
Date
2004-05-07
Journal Title
Journal ISSN
Volume Title
Publisher
Springer-Verlag
Abstract
The heart of the decapod crustacean is activated
by regular impulse bursts from the cardiac ganglion.
The cardiac pump function depends on ganglionic
burst frequency, burst duration, and burst impulse frequency.
Here, we activated isolated lobster cardiac ostial
muscle (Orbicularis ostii muscle, OOM) by stimulus
trains in vitro in order to characterize the response of the
contractile apparatus to [Ca2+]i . We employed stimulus
trains that generate a steady state between the [Ca2+]i
and force in order to estimate the Ca2+ sensitivity of
myofilaments. Force and [Ca2+]i transients were simultaneously
recorded using a silicon strain gauge and the
fluorescence of iontophoretically microinjected fura-2
salt. We examined the effects of tetanus duration (TD),
the interval between trains, and 6 lM cyclopiazonic
acid, an inhibitor of the SR Ca2+ pump, on the steadystate
force–[Ca2+]i relationship. The instantaneous
force–[Ca2+]i relationships appeared sigmoidal (EC50
and Hill coefficient, 98.8±32.7 nM and 2.47±0.20,
mean ± SD, respectively), as did the curves superimposed
after 500 ms following the start of stimulation,
indicating that the force–[Ca2+]i relationship had
reached a steady state at that time. Also, the maximum
activated force (Fmax) was estimated using the steadystate
force–[Ca2+]i relationship. Prolonged stimulus
trains, decreasing the interval between recurrent trains
from 5 to 2.5 s, and cyclopiazonic acid each increased
the measured EC50 without changing Fmax. The EC50
correlated strongly with averaged [Ca2+]i over time. We
conclude that the steady-state force–[Ca2+]i relationships
in the OOM indicate cooperation between force
generation and Ca2+ binding by the myofilaments. Our
data also suggest the existence of a novel Ca2+-dependent
mechanism which reduces Ca2+ sensitivity and
accelerates relaxation of lobster cardiac muscle myofilaments.
http://www.springerlink.com/content/n343yxe5clj0b40n/?p=7981f8215b7a455f96538d280dbad79aπ=5
Description
Keywords
Biology
Citation
T. Shinozaki, J. L. Wilkens, T. Yazawa, M. J. Cavey, H. E. D. J. ter Keurs " The steady-state force–Ca2+ relationship in intact lobster ( Homarus americanus) cardiac muscle" J Comp Physiol B (2004) 174: 407–414 DOI 10.1007/s00360-004-0427-3