Involvement of plasma membrane-located calmodulin in the response decay of cyclic nucleotide-gated cation channel of cultured carrot cells

Increase in cytoplasmic cyclic AMP concentration stimulates Ca²⁺ influx through the cyclic AMP-gated cation channel in the plasma membrane of cultured carrot cells. However, the Ca²⁺ current terminated after a few minutes even in the presence of high concentrations of cyclic AMP indicating that hydrolysis of the nucleotide is not responsible for stop of the Ca²⁺ influx. Cyclic AMP evoked discharge of Ca²⁺ from inside-out sealed vesicles of carrot plasma membrane, and it was strongly inhibited when the suspension of the vesicles was supplemented with 1 μM of free Ca²⁺, while Ca²⁺ lower than 0.1 μM did not affect the Ca²⁺-release. The Ca²⁺ flux across plasma membrane was restored from this Ca²⁺-induced inhibition by the addition of calmodulin inhibitors or anti-calmodulin. These results suggest that Ca²⁺ influx initiated by the increase in intracellular cAMP in cultured carrot cells is terminated when the cytosolic Ca²⁺ concentration reaches the excitatory level in the cells, and calmodulin located in the plasma membrane plays an important role in the response decay of the cyclic nucleotide-gated Ca²⁺ channel.