Two 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. As 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.
Wiechen
K
K
Department of Pharmacology, University of Kiel, Germany.
Yue
D T
DT
Herzig
S
S
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology; D004553:Electric Conductivity; D006168:Guinea Pigs; D015640:Ion Channel Gating; D009206:Myocardium / Q000378:metabolism*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D013997:Time Factors
D015220:Calcium Channels; D010749:Phosphoprotein Phosphatases