Browsing by Author "ter Keurs, H. E. D. J."

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    The steady-state force–Ca2+ relationship in intact lobster
    (Springer-Verlag, 2004-05-07) Cavey, Michael J.; Shinozaki, T.; Wilkens, J. L.; Yazawa, T.; ter Keurs, H. E. D. J.; Biological Sciences; Faculty of Science; University of Calgary
    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
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    Structure and contractile properties of the ostial muscle (musculus orbicularis ostii) in the heart of the American lobster
    (Springer-Verlag, 1999-08-10) Cavey, Michael J.; Yazawa, T.; Wilkens, J. L.; ter Keurs, H. E. D. J.; Biological Sciences; Faculty of Science; University of Calgary
    "Venous" blood enters the crustacean heart through bivalved ostia. Each ostium is a discrete ana- tomical unit that remains functional even when isolated from the heart. Muscle ®bers produce overshooting ac- tion potentials that have a plateau of variable duration in response to nervous drive from the cardiac ganglion or during trains of electrical stimuli. Contractions show summation and facilitation when stimulated by trains of stimuli delivered at rates greater than 0.5 s)1 and 0.2 s)1, respectively. Contraction amplitude increases with stimulating impulse frequency and train duration. Maximum force occurs at 1.2 times the slack length. The morphology of ostial ®bers resembles that of myocardial ®bers. Interconnected bundles of myo®laments occur in both the ostial ®bers and the myocardial ®bers. In ostial and myocardial ®bers, the myo®lament bundles are in- vested by perforated sheets of sarcoplasmic reticulum, and these sheets interface with a network of sarcolemmal tubules to form dyadic interior couplings at the level of the sarcomeric H-bands. The contractile apparatus originates and terminates at intermediate junctions on the transverse cellular boundaries, and the lateral sur- faces of the muscle ®bers are linked by a modest number of communicating (gap) junctions.