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

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Abstract
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.