[Calcium channels in smooth muscle cells]

In smooth muscle cells, the electrophysiological properties of potential-dependent calcium channels are similar to those described in other excitable cells. The calcium current is dependent on the extracellular calcium concentration; it is insensitive to external sodium removal and tetrodotoxin application. Other ions (Ba2+, Sr2+, Na+) can flow through the calcium channel. This channel is blocked by Mn2+, Co2+, Cd2+ and by organic inhibitors. The inactivation mechanism is mediated by both the membrane potential and the calcium influx. Ca2+ ions can also penetrate into the cell through receptor-operated channels. These channels show a low ionic selectivity and are generally less sensitive to organic Ca-blockers than the potential-dependent calcium channels. The finding of specific channel inhibitors as well as the study of the biochemical pathways between receptor activation and channel opening are prerequisites to further characterization of receptor-operated channels.