Role of phospholamban in the modulation of arterial Ca2+ sparks and Ca2+-activated K+ channels by cAMP

Phospholamban (PLB) inhibits the sarcoplasmic reticulum. (SR) Ca2+-ATPase, and this inhibition is relieved by cAMP-dependent protein kinase (PKA)-mediated phosphorylation. The role of PLB in regulating Call release through ryanodine-sensitive Ca2+ release channels, measured as Ca2+ sparks, was examined using smooth muscle cells of cerebral arteries from PLB-deficient (knockout) mice (PLB-KO). Ca2+ sparks were monitored optically using the fluorescent Ca2+ indicator fluo 3 or electrically by measuring transient large-conductance Ca2+-activated K+ (BK) channel currents activated by Call sparks. Basal Ca2+ spark and transient BK current frequency were elevated in cerebral artery myocytes of PLB-KO mice. Forskolin, an activator of adenylyl cyclase, increased the frequency of Ca2+ sparks and transient BK currents in cerebral arteries from control mice. However, forskolin had little effect on the frequency of Call sparks and transient BK currents from PLB-KO cerebral arteries. Forskolin or PLB-KO increased SR Ca2+ load, as measured by caffeine-induced Ca2+ transients. This study provides the first evidence that PLB is critical for frequency modulation of Ca2+ sparks and associated BK currents by PKA in smooth muscle.