[Correlation between myocardial relaxation and phosphorylation of phospholamban]

The relationship between myocardial contractility and relaxation and phospholamban phosphorylation was studied in the isolated rat heart beating at a constant rate and perfused at constant coronary flow with a Ringer solution with 32Pi. At the end of the experiments, membrane vesicles, composed primarily of sarcoplasmic reticulum (SR), were isolated by differential centrifugation. Electrophoresis was performed on acrylamide-SDS gels. Perfusion with isoproterenol (ISO) significantly increased developed tension (T) by 40 +/- 8% and maximal rate of rise of tension (+T) by 76 +/- 12%. The ratio between +T and maximal velocity of relaxation (+T/-T) was significantly decreased from 1.65 +/- 0.04 to 1.23 +/- 0.04. Time to half relaxation (t 1/2) and the rate constant of relaxation (Tau) were significantly decreased by 27 +/- 2 y 6 +/- 1 msec respectively. When the increase in contractility produced by ISO was restored to control levels by addition of nifedipine (ISO-NIFE) or perfusion with low calcium (ISO-Ca+2), +T/-T decreased from 1.63 +/- 0.07 to 1.47 +/- 0.07 and 1.66 +/- 0.06 to 1.41 +/- 0.06 respectively, t 1/2 and Tau significantly decreased by 16 +/- 2 and 3 +/- 1 msec with ISO-NIFE and 19 +/- 2 and 5 +/- 1 msec with ISO-Ca+2 respectively. These decreases were significantly lower than those produced by ISO. Perfusion with high calcium significantly increased T and +T, without significant alterations in relaxation parameters. Phospholamban phosphorylation, in pmoles/mg of SR protein, increased from control values, 25 +/- 6 to 110 +/- 11 (ISO-NI-FE), 117 +/- 17 (ISO-Ca+2) and 197 +/- 21 (ISO) and did not change with high calcium. Results showed a striking correlation between relaxation parameters, +T/-T, t 1/2 and Tau, and phospholamban phosphorylation (r = -0.98, -0.99 and 0.96, respectively). This correlation was much lower between T and +T and phospholamban phosphorylation.