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+ influ x through the cyclic AMP-gated cation channel in the plasma membrane o f 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 st op of the Ca2+ influx. Cyclic AMP evoked discharge of Ca2+ from inside -out sealed vesicles of carrot plasma membrane, and it was strongly in hibited when the suspension of the vesicles was supplemented with 1 mu M of free Ca2+, while Ca2+ lower than 0.1 mu M 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 a nti-calmodulin. These results suggest that Ca2+ influx initiated by th e increase in intracellular cAMP in cultured carrot cells is terminate d when the cytosolic Ca2+ concentration reaches the excitatory level i n the cells, and calmodulin located in the plasma membrane plays an im portant role in the response decay of the cyclic nucleotide-gated Ca2 channel.