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Calmodulin antagonists depress calcium and potassium currents in ventricular and vascular myocytes
1Department of Applied Physiology, University of Cologne, Federal Republic of Germany.
Abstract:
Myocytes isolated from guinea pig ventricles or bovine portal veins were voltage clamped with a single patch electrode. The calmodulin antagonists (CaM-A) calmidazolium, trifluoperazine (TFP), and chlorpromazine acted as Ca antagonists; they reduced the calcium inward current ICa in a voltage- and use-dependent way. For ventricular myocytes, 50% effective concentration (EC50) of calmidazolium was 1 microM, and the EC50 for TFP was 2.5 microM. For vascular myocytes, these numbers were 0.3 and 1 microM, respectively. CaM-A moderately retarded the inactivation time course and shifted the ICa availability curve to more negative potentials. CaM-A were not selective Ca antagonists; other membrane currents such as sodium currents and inwardly and delayed potassium currents were reduced as well (EC50 between 5 and 10 microM). It is unlikely that the above effects require binding of CaM-A to Ca-calmodulin, since reduction of ICa or potassium current (IK) was not modified when 1) the cells were loaded with 100 microM exogenous calmodulin or 2) Ca ions were removed from the extra- and intracellular space. Instead, the unspecific reduction of membrane currents may result from a change in the lipids of the sarcolemma into which CaM-A partition and accumulate.
Insights
Calmodulin antagonists (CaM-A) reduce calcium and potassium currents in heart and blood vessel cells. These CaM-A are not selective and may affect cell membranes directly, not calmodulin.
Area of Science:
- Cardiovascular Physiology
- Molecular Pharmacology
Background:
- Calmodulin antagonists (CaM-A) are known to affect calcium signaling.
- Their precise mechanisms of action on cardiac and vascular myocytes require further elucidation.
Purpose of the Study:
- To investigate the effects of CaM-A on calcium inward current (ICa) and other membrane currents in ventricular and vascular myocytes.
- To determine if these effects are mediated by calmodulin binding or other mechanisms.
Main Methods:
- Isolation of myocytes from guinea pig ventricles and bovine portal veins.
- Voltage clamp technique with a single patch electrode.
- Application of calmodulin antagonists (calmidazolium, trifluoperazine, chlorpromazine) and assessment of their effects on ionic currents.
Main Results:
- CaM-A reduced ICa in a voltage- and use-dependent manner in both cell types, with varying EC50 values.
- CaM-A also reduced sodium and potassium currents, indicating a lack of selectivity.
- Effects on ICa and potassium current were not altered by exogenous calmodulin or calcium removal, suggesting a non-calmodulin-dependent mechanism.
Conclusions:
- Calmodulin antagonists exhibit non-selective inhibition of various membrane currents in cardiac and vascular myocytes.
- The observed effects are likely due to direct interaction of CaM-A with sarcolemmal lipids rather than calmodulin antagonism.