Integration of calcium signals by calmodulin in rat sensory neurons

We have used the fluorescently labelled calmodulin TA-CaM to follow calmodulin activation during depolarization of adult rat sensory neurons. Calcium concentration was measured simultaneously using the low affinity indicator Oregon Green BAPTA 5N. TA-CaM fluorescence increased during a 200-ms depolarization but then continued to increase during the subsequent 500 ms, even though total cell calcium was falling at this time. In the next few seconds TA-CaM fluorescence fell, but to a new elevated level that was then maintained for several tens of seconds. During a train of depolarizations that evoked a series of largely independent calcium changes TA-CaM fluorescence was in contrast raised for the duration of the train and for many tens of seconds afterwards. The presence of a peptide corresponding to the calmodulin binding domain of myosin light chain kinase significantly increased the depolarization-induced TA-CaM fluorescence increase and slowed the subsequent fall of fluorescence. We interpret the slow recovery component of the TA-CaM signal as reflecting the slow dissociation of calcium--calmodulin--calmodulin binding protein complexes. Our results show that after brief electrical activity calmodulin's interaction with calmodulin binding proteins persists for approximately one minute.