Two distinct functional effects of protein phosphatase inhibitors on guinea-pig cardiac L-type Ca2+ channels

• Published in: The Journal of physiology Vol. 484; no. Pt 3; pp. 583 - 592
• Main Authors: Wiechen, K, Yue, D T, Herzig, S
• Format: Journal Article
• Language: English
• Published: England The Physiological Society 01-05-1995
• Subjects: Animals; Calcium Channels - drug effects; Calcium Channels - physiology; Electric Conductivity; Guinea Pigs; Ion Channel Gating; Myocardium - metabolism; Phosphoprotein Phosphatases - antagonists & inhibitors; Time Factors
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.1995.sp020688

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.