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.