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Articles in PresS, published online ahead of print September 27, 2001
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00536.2001
Submitted on June 25, 2001
Accepted on September 19, 2001
1 Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA
2 Department of Physiology, University of Maryland School of Medicine, Baltimore, MD, USA; Medical Biotechnology Center, University of Maryland Biotechnology Institute, Baltimore, MD, USA
* To whom correspondence should be addressed. E-mail: wdubell{at}som.umaryland.edu.
Calyculin A was used to examine the importance of phosphatases in the modulation of cardiac contractile magnitude in the absence of any neural or humoral stimulation. PP1 and PP2A activity, twitch contractions, [Ca2+]i transients, action potentials, membrane currents and myofilament Ca2+-sensitivity were measured in isolated mouse ventricular myocytes. Calyculin A (125 nM) inhibited PP1 and PP2A by 50 and 85%, respectively, while it doubled the twitch magnitude and increased twitch duration by 50% in field-stimulated cells. Calyculin A-evoked increases in ICa (70%) and the resulting [Ca2+]i transient (83%) explain the positive inotropic response. However increases in twitch and action potential durations did not result from increased myofilament Ca2+-sensitivity or K+ current inhibition, respectively. Comparison of the effects of calyculin A and isoproterenol on [Ca2+]i transients and twitch contractions revealed that calyculin A had a much smaller lusitropic effect than the ß-agonist, indicating that calyculin A did not significantly increase SR Ca2+ uptake. Thus, while cardiac contractile magnitude is controlled by a steady-state kinase/phosphatase balance, this regulation is not equally operative at all of the steps in the E-C coupling cascade, and may in fact be most important to the regulation of the L-type Ca2+ channel.
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