Biochemical and electrophysiological mechanisms of the positive inotropic effect of calyculin A, a protein phosphatase inhibitor.
Calyculin A (CyA; 1 microM) increased the force of contraction in isolated guinea pig papillary muscles to 144% of control without affecting contraction parameters. The effect of CyA on L-type calcium channels was assessed in cell-attached patches of guinea pig ventricular cardiomyocytes. Unitary Ba++ current recordings revealed that CyA at micromolar concentrations enhanced channel availability almost 2-fold, whereas the duration of individual openings and closures remained unchanged. In whole-cell recordings with Ca++ as the charge carrier, intracellular dialysis of 1 microM CyA enhanced peak current to a similar extent. In homogenates from guinea pig ventricles, 1 microM CyA completely inhibited phosphorylase phosphatase activity. In isolated [32P]-labeled guinea pig ventricular cardiomyocytes, 1 microM CyA increased the phosphorylation state of phospholamban (to 267% of control), that of the inhibitory subunit of troponin (to 182% of control) and those of various additional proteins. We conclude that the effects of CyA are likely to be mediated by increasing the phosphorylation state of several regulatory proteins.
Abteilung Allgemeine Pharmakologie, Universitäts-Krankenhaus Eppendorf, Hamburg, Germany.
J
J
Neumann
P
P
Bokník
S
S
Herzig
W
W
Schmitz
H
H
Scholz
K
K
Wiechen
N
N
Zimmermann
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects; D002135:Calcium-Binding Proteins / Q000378:metabolism; D004988:Ethers, Cyclic / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008387:Marine Toxins; D009200:Myocardial Contraction / Q000187:drug effects*; D019319:Okadaic Acid; D010080:Oxazoles / Q000494:pharmacology*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D010766:Phosphorylation; D013268:Stimulation, Chemical; D000099211:Phospholamban
D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D004988:Ethers, Cyclic; D008387:Marine Toxins; D019319:Okadaic Acid; D010080:Oxazoles; D010749:Phosphoprotein Phosphatases; D000099211:Phospholamban; C059041:calyculin A