RELATIONSHIP BETWEEN LATENCY AND PERIOD FOR 5-HYDROXYTRYPTAMINE-INDUCED MEMBRANE RESPONSES IN THE CALLIPHORA SALIVARY-GLAND

Following stimulation with a calcium-mobilizing agonist there is often a distinct latency (L) preceding the onset of the first calcium spike . In the continued presence of the agonist, repetitive spikes appear s eparated by a variable period (P). The relationship between L and P ha s been investigated in an insect salivary gland responding to 5-hydrox ytryptamine (5-HT). Both L and P were found to decrease as the concent ration of 5-HT was increased over its physiological range of 1-10 nM. Lowering the concentration of external calcium from 1 x 10(-3) M to 1 x 10(-5) M increased both P and L. However, the effect on L was appare nt only at low levels of 5-HT. Reducing the content of the internal st ores by repeated stimulation in a calcium-free medium resulted in a pr ogressive prolongation of L. On the other hand, the effect on L decrea sed when glands were stimulated repetitively in normal calcium-contain ing medium. All these results are consistent with a hypothesis that ca lcium plays a critical role in determining the kinetics of calcium rel ease during both L and P. An important component seems to be the entry of external calcium, which sets the stage for calcium release by load ing up the internal stores. As these stores fill up with calcium, the Ins(1,4,5)P-3 receptors will initiate a calcium spike once they become sensitized to the ambient level of Ins(1,4,5)P-3.