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Am J Physiol Heart Circ Physiol 280: H786-H794, 2001;
0363-6135/01 $5.00
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Vol. 280, Issue 2, H786-H794, February 2001

Role of protein phosphatases in regulation of cardiac inotropy and relaxation

Peter Bokník1, Sascha Khorchidi1, Geza S. Bodor2, Sabine Huke1, Jörg Knapp1, Bettina Linck1, Hartmut Lüss1, Frank Ulrich Müller1, Wilhelm Schmitz1, and Joachim Neumann1

1 Institut für Pharmakologie und Toxikologie, Westfälische Wilhelms-Universität Münster, D-48129 Münster, Germany; and 2 Department of Laboratories, Denver Health Medical Center, Denver, Colorado 80204-4507

We studied the effects of the protein phosphatase (PP) inhibitor cantharidin (Cant) on time parameters and force of contraction (FOC) in isometrically contracting electrically driven guinea pig papillary muscles. We correlated the mechanical parameters of contractility with phosphorylation of the inhibitory subunit of troponin (TnI-P) and with the site-specific phosphorylation of phospholamban (PLB) at serine-16 (PLB-Ser-16) and threonine-17 (PLB-Thr-17). Cant (after 30 min) started to increase FOC (112 ± 4% of control, n = 10) and TnI-P and PLB-Thr-17 (120 ± 5 and 128 ± 7% of control) without any alteration of relaxation time. Cant (10 µM) started to increase PLB-Ser-16, but the relaxation was shortened at only 100 µM (from 140 ± 9 to 116 ± 12 ms, n = 9). Moreover, 100 µM Cant, 3 min after application, started to increase PLB-Thr-17, TnI-P, and FOC. Cant (100 µM) began to increase PLB-Ser-16 after 20 min. This was accompanied by shortening of relaxation time. Differences in protein kinase activation or different substrate specificities of PP may explain the difference in Cant-induced site-specific phosphorylation of PLB in isometrically contracting papillary muscles. Moreover, PLB-Thr-17 may be important for inotropy, whereas PLB-Ser-16 could be a major determinant of relaxation time.

cantharidin; protein phosphatase inhibitors; protein phosphorylation; phospholamban; inhibitory subunit of troponin


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