Modulation of BK channel calcium affinity by differential phosphorylation in developing ovine basilar artery myocytes

Large-conductance Ca²⁺-sensitive K⁺ (BK) channel activity is greater in basilar artery smooth muscle cells (SMCs) of the fetus than the adult, and this increased activity is associated with a lower BK channel Ca²⁺ set point (Ca₀). Associated PKG activity is three times greater in BK channels from fetal than adult myocytes, whereas associated PKA activity is three times greater in channels from adult than fetal myocytes. We hypothesized that the change in Ca₀ during development results from different levels of channel phosphorylation. In inside-out membrane patch preparations of basilar artery SMCs from adult and fetal sheep, we measured BK channel activity in four states of phosphorylation: native, dephosphorylated, PKA phosphorylated, and PKG phosphorylated. BK channels from adult and fetus exhibited similar voltage-activation curves, Ca₀ values, and Ca²⁺ dissociation constants (K_d) for the dephosphorylated, PKA phosphorylated, and PKG phosphorylated states. However, voltage-activation curves of native fetal BK channels shifted significantly to the left of those of the adult, with Ca₀ and K_d values half those of the adult. For the two age groups at each of the phosphorylation states, Ca₀ and K_d produced linear relations when plotted against voltage at half-maximal channel activation. We conclude that the Ca ₀ and K_d values of the BK channel can be modulated by differential channel phosphorylation. Lower Ca₀ and K_d values in BK channels of fetal myocytes can be explained by a greater extent of channel phosphorylation of fetal than adult myocytes.