2 DISTINCT FUNCTIONAL-EFFECTS OF PROTEIN PHOSPHATASE INHIBITORS ON GUINEA-PIG CARDIAC L-TYPE CA2+ CHANNELS

1. The effects of the phosphatase inhibitors okadaic acid and calyculin A on single guinea-pig ventricular L-type Ca2+ channels were studied. The inactive derivative norokadaone was used as a, negative control. 2. The two known effects of cAMP-dependent stimulation are mimicked by the phosphatase inhibitors to a varying extent. Only okadaic acid promotes the high-activity gating mode ('mode 2'), while calyculin A increases channel availability to a larger extent. Bs revealed by kinetic analysis of slow gating, the two phosphatase inhibitors retard a slow rate constant, which is assumed to represent exit from the available state by dephosphorylation. Norokadaone was inactive in both regards. 3. Mode 2 gating elicited by very positive prepulses is augmented by okadaic acid, and mode 2 lifetime is prolonged. Calyculin A fails to affect these parameters. Thus, voltage-facilitated mode 2 gating reveals the same pharmacological properties as the mode 2 sweeps observed using conventional pulse protocols. 4. The results are interpreted in terms of the different sensitivity of protein phosphatase subtypes towards the inhibitors: channel availability appears to be controlled by a phosphorylation site dephosphorylated by a type 1-like phosphatase, while mode 2 gating is coupled to a distinct site, dephosphorylated by a type 2A-like phosphatase.