A TYPE 2A PHOSPHATASE-SENSITIVE PHOSPHORYLATION SITE CONTROLS MODAL GATING OF L-TYPE CA2-MUSCLE CELLS( CHANNELS IN HUMAN VASCULAR SMOOTH)

The patch-clamp technique was employed to investigate phosphorylation/ dephosphorylation-dependent modulation of L-type Ca2+ channels in smoo th-muscle cells isolated from human umbilical vein. Okadaic acid, an i nhibitor of phosphoprotein phosphatases type 1 (PP1) and 2A (PP2A), in creased the probability of channels being in the open state (P-o) in i ntact cells. This increase in P-o was due mainly to promotion of long- lasting channel openings, i.e. promotion of 'mode 2' gating behaviour. Exposure of the cytoplasmic side of excised patches of membrane to th e purified catalytic subunit of PP2A (PP2A(c)) resulted in the opposit e modulation of channel function. PP2A(c) (0.2 unit/ml) reduced the P- o of Ca2+ channels mainly via suppression of 'mode 2' gating. This eff ect of PP2A(c) was completely prevented by 1 mu M okadaic acid. The ca talytic subunit of PP1 (0.2 unit/ml), however, barely affected channel activity. Our results provide evidence for a PP2A-sensitive regulator y site that controls modal gating of L-type Ca2+ channels in smooth mu scle.