Influence of sarcoplasmic reticulum calcium loading on mechanical and relaxation restitution

Mechanical and relaxation restitution represent the restoration of contract ile force and relaxation, respectively, in premature beats having progressi vely longer extrasystolic intervals (EST); these phenomena are related to i ntracellular activator Ca2+ by poorly defined mechanisms. We tested the hyp othesis that the level of phospholamban [which modulates the affinity of th e sarcoplasmic reticulum (SR) Ca2+-ATPase for Ca2+, and thus the SR Ca2+ lo ad] may be an important determinant of both mechanical and relaxation resti tution. Five mice with ablation of the phospholamban (PLB) gene (PLBKO), ei ght isogenic wild-type controls (129SvJ), eleven mice with PLB overexpressi on (PLBOE), and nine isogenic wild-type (FVB/N) controls were anesthetized and instrumented with a 1.4-Fr Millar catheter in the left ventricle and a 1-Fr pacemaker in the right atrium. At a cycle length of 200 ms, extrastimu li with increasing ESI were introduced, and the peak rates of left ventricu lar isovolumic contraction (+/-dP/dt(max)) were normalized and fit to monoe xponential equations. In a subset, the protocols were repeated after ryanod ine 14 ng/g) was administered to deplete SR Ca2+ stores. The time constant of mechanical restitution in PLBKO was significantly shorter [6.3 +/- 1.2 ( SE) vs. 47.7 +/- 7.6 ms] and began earlier (50 +/- 10 vs. 70 +/- 19 ms) tha n in 129SvJ. In contrast, the time constant of mechnical restitution was si gnificantly longer (80.3 +/- 7.6 vs. 54.1 +/- 9.2 ms) in PLBOE than in FVB/ N. The time constant of relaxation restitution was less in PLBKO than in 12 9SvJ (26.2 +/- 9.9 vs. 44.6 +/- 3.3, P < 0.05) but was similar in PLBOE and FVB/N (21.1 +/- 6.3 vs. 20.5 +/- 5.7 ms), Intravenous ryanodine decreased significantly the time constants of mechanical restitution in PLBOE, 129SvJ , and FVB/N but was lethal in PLBKO. In contrast, ryanodine increased the t ime constant of relaxation restitution. Thus 1) the phospholamban level is a critical determinant of mechanical restitution and (to a lesser extent) r elaxation restitution in these transgenic models, and 2) ryanodine differen tially affects mechanical and relaxation restitution. Furthermore, our data suggest a dissociation of processes within the SR that govern contraction and relaxation.