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Voltage-dependent calcium channels in smooth muscle cells
T B Bolton1, I MacKenzie, P I Aaronson
1Department of Pharmacology, St. George's Hospital Medical School, London, England.
Abstract:
Channels highly selective for calcium, which open when the membrane is depolarized, have been found in all smooth muscle cells examined, whether or not they can generate action potentials. The current through these channels can also be carried by barium and is blocked by cadmium, cobalt, or manganese. If single smooth muscle cells of rabbit ear artery are held at -80 mV under voltage clamp, stepping to more positive potentials elicits an inward current which appears at about -60 mV and is well sustained; at more positive potentials, peak current is achieved within 10-20 ms and current then inactivates substantially. Maximum current in solutions with 1.5 mM calcium is found at -10 mV and in solutions with high calcium or barium concentrations the characteristics of the inward current are considerably altered. Low- and high-threshold components of current, possibly due to different populations of calcium channels, can be distinguished. Nifedipine reduces inward current at more positive test and holding potentials but potentiates inward current if the test and holding potentials are in a more negative range.
Insights
This study identifies and characterizes calcium channels in smooth muscle cells, revealing distinct low- and high-threshold components. These findings are crucial for understanding vascular smooth muscle function and drug interactions.
Area of Science:
- Cardiovascular Physiology
- Molecular Physiology
- Pharmacology
Background:
- Smooth muscle cells possess voltage-gated calcium channels crucial for contraction.
- These channels are present in smooth muscle cells regardless of their ability to generate action potentials.
Purpose of the Study:
- To characterize the properties of voltage-gated calcium channels in rabbit ear artery smooth muscle cells.
- To investigate the influence of different calcium concentrations and nifedipine on calcium channel currents.
Main Methods:
- Voltage clamp electrophysiology was used on single smooth muscle cells from rabbit ear artery.
- Inward currents were measured by stepping membrane potentials to various positive values.
Main Results:
- A sustained inward current, carried by calcium or barium and blocked by cadmium, cobalt, or manganese, was observed.
- Distinct low- and high-threshold current components were identified, suggesting different calcium channel populations.
- Nifedipine exhibited differential effects on current amplitude based on test and holding potentials.
Conclusions:
- Rabbit ear artery smooth muscle cells express voltage-gated calcium channels with distinct properties.
- These channels play a significant role in regulating vascular tone.
- Nifedipine's action is dependent on the membrane potential, impacting calcium influx differently.