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Involvement of phosphoinositide turnover in ouabain inotropism
H Gotoh1, A Kamiyama, R Shibayama
1Department of Physiology, Iwate Medical University School of Medicine, Japan.
Abstract:
We examined whether ouabain activities phospholipases and reinforces contraction force of papillary muscles through resultant second messengers. 2-Nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC), the inhibitor of phospholipase C (PLC), abolished the ouabain inotropy in rabbit papillary muscles. Calphostin C, the specific inhibitor of protein kinase C (PKC), also depressed the ouabain inotropy. 12-O-Tetradecanoylphorbor-13-acetate (TPA), the specific activator of PKC, enhanced the beat-to-beat phasic contractility at low concentrations. Radioenzymatic assay revealed that ouabain treatment doubled diacylglycerol (DG) content in excised papillary muscles. We concluded that ouabain activates PLC, and the resultant second messenger, DG, augments the cardiac contraction force through activation of PKC.
Insights
Ouabain enhances cardiac muscle contraction by activating phospholipase C (PLC). This action increases diacylglycerol (DG) levels, which in turn activates protein kinase C (PKC), augmenting heart muscle force.
Area of Science:
- Cardiovascular Physiology
- Cell Signaling
- Pharmacology
Background:
- Cardiac glycosides like ouabain are known to affect heart contractility.
- The precise intracellular mechanisms by which ouabain enhances cardiac contraction are not fully elucidated.
- Second messenger pathways involving phospholipases and kinases are critical in regulating cardiac function.
Purpose of the Study:
- To investigate the role of phospholipase C (PLC) and protein kinase C (PKC) in mediating ouabain's positive inotropic effects.
- To determine if ouabain alters intracellular levels of second messengers like diacylglycerol (DG).
Main Methods:
- Experiments were conducted using isolated rabbit papillary muscles.
- Pharmacological inhibitors (NCDC for PLC, Calphostin C for PKC) and an activator (TPA for PKC) were used.
- Radioenzymatic assays were employed to measure diacylglycerol (DG) content.
Main Results:
- Inhibition of PLC with NCDC abolished ouabain-induced inotropy.
- Inhibition of PKC with Calphostin C also reduced ouabain's inotropic effect.
- Activation of PKC with TPA enhanced contractility.
- Ouabain treatment significantly increased diacylglycerol (DG) levels in papillary muscles.
Conclusions:
- Ouabain activates phospholipase C (PLC) in cardiac muscle.
- The resulting increase in diacylglycerol (DG) activates protein kinase C (PKC).
- This PLC/DG/PKC signaling pathway is essential for ouabain's positive inotropic action, reinforcing cardiac contraction force.