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 calyculi n 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 b y the phosphatase inhibitors to a varying extent. Only okadaic acid pr omotes the high-activity gating mode ('mode 2'), while calyculin A inc reases 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 o kadaic acid, and mode 2 lifetime is prolonged. Calyculin A fails to af fect these parameters. Thus, voltage-facilitated mode 2 gating reveals the same pharmacological properties as the mode 2 sweeps observed usi ng conventional pulse protocols.4. The results are interpreted in term s of the different sensitivity of protein phosphatase subtypes towards the inhibitors: channel availability appears to be controlled by a ph osphorylation site dephosphorylated by a type 1-like phosphatase, whil e mode 2 gating is coupled to a distinct site, dephosphorylated by a t ype 2A-like phosphatase.