Phospholamban (PLB) inhibits the sarcoplasmic reticulum (SR) Ca(2+)-ATPase 
(SERCA), and this inhibition is relieved by Ca(2+) calmodulin-dependent protein 
kinase II (CaM kinase II) phosphorylation. We previously reported significant 
differences in contractility, SR Ca(2+) release, and CaM kinase II activity in 
gastric fundus smooth muscles as a result of PLB phosphorylation by CaM kinase 
II. In this study, we used PLB-knockout (PLB-KO) mice to directly examine the 
effect of PLB absence on contractility, CaM kinase II activity, and 
intracellular Ca(2+) waves in gastric antrum smooth muscles. The frequencies and 
amplitudes of spontaneous phasic contractions were elevated in antrum smooth 
muscle strips from PLB-KO mice. Bethanecol increased the amplitudes of phasic 
contractions in antrum smooth muscles from both control and PLB-KO mice. 
Caffeine decreased and cyclopiazonic acid (CPA) increased the basal tone of 
antrum smooth muscle strips from PLB-KO mice, but the effects were less 
pronounced compared with control strips. The CaM kinase II inhibitor KN-93 was 
less effective at inhibiting caffeine-induced relaxation in antrum smooth muscle 
strips from PLB-KO mice. CaM kinase II autonomous activity was elevated, and not 
further increased by caffeine, in antrum smooth muscles from PLB-KO mice. 
Similarly, the intracellular Ca(2+) wave frequency was elevated, and not further 
increased by caffeine, in antrum smooth muscles from PLB-KO mice. These findings 
suggest that PLB is an important modulator of gastric antrum smooth muscle 
contractility by modulation of SR Ca(2+) release and CaM kinase II activity.