Regional expression of phospholamban in the human heart

Several independent lines of evidence indicate that phospholamban (PLB) expression correlates positively with depression of force of contraction and duration of contraction in isolated cardiac preparations of several animal species. Here, we studied whether PLB levels correlate with attenuation of contractility and enhancement of contractile time parameters in different parts of the human heart.

Force of contraction was measured in isolated electrically driven atrial and ventricular preparations from human hearts. Ca(2+)-uptake by human atrial and ventricular homogenates was assayed at different ionized Ca(2+)-concentrations. Protein expression of PLB and the sarcoplasmic Ca(2+)-ATPase (SERCA) was measured in homogenates by quantitative immunoblotting using specific antibodies. PLB mRNA expression was quantified in human cardiac preparations by Northern blot analysis.

The duration of contraction in isolated preparations of human right ventricle (RV) was double that found in right atrial preparations (RA) (620 +/- 25 ms versus 308 +/- 15 ms). In RA, PLB expression was reduced by 44% at the protein level and by 34% at the mRNA level compared to RV. In contrast, the SERCA protein content was increased by 104% in RA compared to RV. Ca(2+)-uptake at low ionized Ca(2+)-concentration, where the inhibiting effect of PLB is maximal, amounted to 1.39 +/- 0.28 nmol Ca2+/mg protein in RA and to 0.62 +/- 0.09 nmol Ca2+/mg protein in RV (n = 6 both).

It is suggested that duration of contraction is shorter in human atrium versus ventricle due to the combined effect of decreased PLB levels (which inhibits SERCA function) and increased SERCA levels. The lower relative ratio of PLB to SERCA leads to less inhibition of SERCA and increased Ca(2+)-uptake which enhances relaxation and contraction in human atrium.