Rate-dependent abbreviation of Ca2+ transient in rat heart is independent of phospholamban phosphorylation

The mechanisms underlying the accelerated decline of the intracellular Ca2+ transient that occurs in cardiac muscle when stimulation rate is increased have been investigated in ventricular myocytes from rat hearts. Increasing stimulation rate from 0.1 to 0.5 and 1 Hz decreased the time taken for the Ca2+ transient to decline from its peak to 50% of its peak value in cells generating action potentials, when the duration of depolarization was held constant by voltage clamp, and when Na/Ca exchange was inhibited. The sarcoplasmic reticulum Ca2+ adenosinetriphosphatase inhibitor thapsigargin inhibited rate-dependent abbreviation of the Ca2+ transient. However, neither a chemical inhibitor of Ca(2+)-calmodulin-dependent protein kinase II (KN62) nor a peptide inhibitor of this enzyme (calmodulin-binding domain peptide) had a significant effect on rate-dependent abbreviation of the Ca2+ transient. Analysis of the phosphorylation of the regulatory sites Ser16 and Thr17 of phospholamban showed no significant change in phosphorylation with changes of stimulation rate. These data suggest that rate-dependent shortening of the Ca2+ transient is due predominantly to enhanced Ca2+ uptake by the sarcoplasmic reticulum without changes in phospholamban phosphorylation.