The frequency-dependent acceleration of relaxation (FDAR) increases with the decrease in temperature, but is independent of the phosphorylation of Thr17 of phospholamban (PLB)

VALVERDE CA; SAID M; MUNDIÑA-WEILENMANN C; MATTIAZZI A

Cataratas del Iguazú, Argentina

Congreso; XIII Meeting of the International Society for Heart Research; 2004

International Society for Heart Research. Latin American Section

The increase in stimulation frequency (ISF) is associated with a frequency-dependent acceleration of relaxation (FDAR) of unknown mechanism. A possible mechanism involved, is a CaMKII-dependent phosphorylation of Thr17 site of phospholamban (PLB), due to the sustained increase in Cai2+ produced by ISF, which would enhance SERCA2 activity, sarcoplasmic reticulum Ca2+ uptake rate and the velocity of relaxation. Whereas a correlation between FDAR and the phosphorylation of Thr17 site was described in isolated myocytes at room temperature (JBC (2000)), experiments in the intact heart at 37 °C, failed to show such a correlation (Circulation (2003)). The present experiments, were performed in Langendorff-perfused rat hearts, at two different temperatures, 37 and 29 °C, to study the possibility that the cause of these different results resides in the different temperatures used. At 29 °C, the ISF from 2 to 6 Hz decreased half relaxation time (t1/2), from 85.9 ± 6.8 to 41.5 ± 2.5 ms, and significantly increased the phosphorylation of Thr17 from 12.8 ± 5.4% to 81.1 ± 14.6% and of Ser16 from 6.9 ± 1.8% to 49.3 ± 3.1% (expressed as % of the maximal isoproterenol-induced phosphorylation). In the presence of b-blockade, the ISF evoked a decrease in t1/2 from 114.2 ± 1.9 to 59.8 ± 5.1 ms, without any significant phosphorylation of PLB residues. The ISF at 37 °C, produced a reduction in t1/2 from 55.6 ± 1.6 to 38.2 ± 1.8 ms and from 55.7 ± 0.9 to 38.8 ± 2.4 ms, in the absence and presence of b-blockade respectively, without any significant change in PLB phosphorylation. A similar ISF at 37 °C, produced a reduction in t1/2 from 55.6 ± 1.6 to 38.2 ± 1.8 ms and from 55.7 ± 0.9 to 38.8 ± 2.4 ms in the absence and presence of propranolol, respectively. The decrease in t1/2 was not associated with any significant change in PLB phosphorylation residues. The results indicate that: (1) The ISF at low temperature was associated with a release of endogenous catecholamines that can explain the increase in PLB phosphorylation residues; FDAR cannot be explained by the phosphorylation of Thr17 of PLB either at 29 or at 37 °C.