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Am J Physiol Heart Circ Physiol 273: H2415-H2422, 1997;
0363-6135/97 $5.00
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Vol. 273, Issue 5, H2415-H2422, November 1997

Protein kinase A does not alter unloaded velocity of sarcomere shortening in skinned rat cardiac trabeculae

Paul M. L. Janssen and Pieter P. De Tombe

Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, Illinois 60607-7171

Whether beta -adrenergic stimulation affects the cross-bridge cycling rate independently of its effect on Ca2+ handling by the cardiac myocyte is still unknown. An increase in cross-bridge cycling rate may result in increased unloaded velocity of sarcomere shortening (Vo). To test this hypothesis directly, skinned rat cardiac trabeculae were attached between a silicon strain gauge (~3.5 kHz resonant frequency) and a fast displacement motor. Vo was measured by a modified "Edman slack test" during a single maximal activation using seven to eight sarcomere-length step releases (measured by laser diffraction) ranging between 0.12 and 0.20 µm (15.0 ± 0.1°C). beta -Adrenergic stimulation was mimicked by exposing the trabeculae to the catalytic subunit of protein kinase A (PKA). Treatment with PKA (3 µg/ml; 45 min) caused a significant (P < 0.01) increase (41 ± 13%) in the Ca2+ concentration required for half-maximal steady-state tension development. Neither maximum tension nor Vo was affected by treatment with PKA, suggesting that beta -adrenergic stimulation does not affect the rate-limiting step of cross-bridge cycling during unloaded shortening in myocardium.

Edman slack test; contractile protein phosphorylation; beta -adrenergic stimulation; cross-bridge cycling; calcium sensitivity


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