Voltage-dependent calcium channels in smooth muscle cells.

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.