Mechanisms of plumbagin action on guinea pig isolated atria.
In electrically driven guinea pig left atria, plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone; 0.5-10 microM) produced a marked positive inotropic effect that was about 65% that caused by isoprenaline in the same experimental conditions. The effect was mainly not dependent on catecholamine release from adrenergic stores. An EC50 of 3 microM was calculated from the concentration-response curves. The increase in force of contraction was followed by a nonreversible contracture. Plumbagin was reduced by cardiac mitochondrial and soluble reductases with consequent generation of large amounts of superoxide anion. The assay of reduced glutathione/oxidized glutathione content in atria, treated with 10 microM plumbagin and frozen at the appearance of increase in diastolic tension, showed a significant decrease in reduced glutathione (-52% with respect to control atria) and a 5-fold increase in oxidized glutathione levels. Moreover, in the same experimental conditions a significant decrease in adenosine triphosphate (-55% with respect to the controls) and in adenylate energy charge (from 0.92-0.64) was observed. Of the enzymes and transport systems involved in the control of the cardiac contractility, the sarcoplasmic reticulum Ca2+ pump seemed to be a specific target for plumbagin. After 30 min of incubation with cardiac sarcoplasmic reticulum membrane vesicles, plumbagin inhibited Ca2+ uptake by the pump in a concentration-dependent manner (IC50 = 3 microM). On the basis of these results, the increase in diastolic tension caused by plumbagin appears to be related to intracellular Ca2+ accumulation, due both to the low availability of adenosine triphosphate for ionic pumps and direct inhibition of Ca2+ reuptake in sarcoplasmic reticulum.
Floreani
M
M
Department of Pharmacology, University of Padova, Italy.
Forlin
A
A
Pandolfo
L
L
Petrone
M
M
Bellin
S
S
D000818:Animals; D002316:Cardiotonic Agents / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006168:Guinea Pigs; D006325:Heart Atria / Q000187:drug effects*; D007545:Isoproterenol / Q000494:pharmacology; D009285:Naphthoquinones / Q000494:pharmacology*
D002316:Cardiotonic Agents; D009285:Naphthoquinones; D007545:Isoproterenol; C014758:plumbagin