Phospholamban expressed in slow-twitch and chronically stimulated fast-twitch muscles minimally affects calcium affinity of sarcoplasmic reticulum Ca(2+)-ATPase.

Chronic excitation, at 2 Hz for 6-7 weeks, of the predominantly fast-twitch canine latissimus dorsi muscle promoted the expression of phospholamban, a protein found in sarcoplasmic reticulum (SR) from slow-twitch and cardiac muscle but not in fast-twitch muscle. At the same time that phospholamban was expressed, there was a switch from the fast-twitch (SERCA1) to the slow-twitch (SERCA2a) Ca(2+)-ATPase isoform. Antibodies against Ca(2+)-ATPase (SERCA2a) and phospholamban were used to assess the relative amounts of the slow-twitch/cardiac isoform of the Ca(2+)-ATPase and phospholamban, which were found to be virtually the same in SR vesicles from the slow-twitch muscle, vastus intermedius; cardiac muscle; and the chronically stimulated fast-twitch muscle, latissimus dorsi. The phospholamban monoclonal antibody 2D12 was added to SR vesicles to evaluate the regulatory effect of phospholamban on calcium uptake. The antibody produced a strong stimulation of calcium uptake into cardiac SR vesicles, by increasing the apparent affinity of the Ca2+ pump for calcium by 2.8-fold. In the SR from the conditioned latissimus dorsi, however, the phospholamban antibody produced only a marginal effect on Ca2+ pump calcium affinity. These different effects of phospholamban on calcium uptake suggest that phospholamban is not tightly coupled to the Ca(2+)-ATPase in SR vesicles from slow-twitch muscles and that phospholamban may have some other function in slow-twitch and chronically stimulated fast-twitch muscle.