Troponin I phosphorylation plays an important role in the relaxant effect of beta-adrenergic stimulation in mouse hearts.
OBJECTIVE
The present study was designed to address the question of the contribution of cardiac troponin I (cTnI) phosphorylation to the enhanced rate of relaxation during beta-adrenergic stimulation in hearts in situ.


METHODS
In situ hemodynamic measurements were performed in mouse hearts that (1) express normal level of phospholamban (PLB) and either express cTnI (PLB/cTnI) or the slow skeletal isoform of TnI (PLB/ssTnI) that cannot be phosphorylated by protein kinase A (PKA) or (2) do not express PLB and either express cTnI (PLBKO/cTnI) or ssTnI (PLBKO/ssTnI).


RESULTS
In the basal state, there was no difference in heart rate (HR), developed pressure (DP), left ventricular end-diastolic pressure (LVEDP) or rate of contraction (+dP/dt) between PLB/cTnI and PLB/ssTnI groups. However, hearts expressing ssTnI (PLB/ssTnI) showed a significantly decreased rate of relaxation (-dP/dt) when compared with hearts expressing cTnI (PLB/cTnI). In response to beta-adrenergic agonist, isoproterenol (ISO), HR increased similarly in both groups. At the two highest doses of ISO, the rate of relaxation (-dP/dt) was significantly smaller in PLB/ssTnI than in PLB/cTnI hearts. In the basal state, there was no difference in HR, DP, LVEDP,+dP/dt and -dP/dt between PLBKO/cTnI and PLBKO/ssTnI hearts. In response to ISO, HR increased similarly in both groups and was only slightly smaller in PLBKO/ssTnI group at the lowest dose of ISO. However, during ISO perfusion, when cTnI was phosphorylated, the rate of relaxation was significantly slower in PLBKO/ssTnI compared to PLBKO/cTnI hearts.


CONCLUSIONS
Our data support the hypothesis that phosphorylation of cTnI significantly contributes to the enhanced rate of relaxation during beta-adrenergic stimulation.
Program in Cardiovascular Sciences, Department of Medicine, Section of Cardiology, University of Illinois at Chicago, Chicago, IL 60612, USA.
James R
JR
Peña
Beata M
BM
Wolska
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006339:Heart Rate / Q000187:drug effects; D017403:In Situ Hybridization / Q000379:methods; D007545:Isoproterenol / Q000494:pharmacology*; D051379:Mice; D008822:Mice, Transgenic; D009206:Myocardium / Q000378:metabolism*; D010766:Phosphorylation; D013268:Stimulation, Chemical; D019210:Troponin I / Q000235:genetics / Q000378:metabolism*; D000099211:Phospholamban
D000318:Adrenergic beta-Agonists; D002135:Calcium-Binding Proteins; D007545:Isoproterenol; D019210:Troponin I; D000099211:Phospholamban