Dependence of transient and residual calcium dynamics on action-potential patterning during neuropeptide secretion

Secretion of the neuropeptide arginine vasopressin (AVP) from the neurohypo physis is optimized by short phasic bursts of action potentials with a mean intraburst frequency around 10 Hz. Several hypotheses, most prominently ac tion-potential broadening and buildup of residual calcium, have been propos ed to explain this frequency dependence of AVP release. However, how either of these mechanisms would optimize release at any given frequency remains an open question. We have addressed this issue by correlating the frequency -dependence of intraterminal calcium dynamics and AVP release during action -potential stimulation. By monitoring the intraterminal calcium changes with low-affinity indicator dyes and millisecond time resolution, the signal could be dissected into t hree separate components: rapid Ca2+ rises (Delta[Ca2+](tr)) related to act ion-potential depolarization, Ca2+ extrusion and/or uptake, and a gradual i ncrease in residual calcium (Delta D[Ca2+](res)) throughout the stimulus tr ain. Action-potential stimulation modulated all three components in a manne r dependent on both the stimulation frequency and number of stimuli. Overal l, the cumulative Delta[Ca2+](tr) amplitude initially increased with f(Stim ) and then rapidly deteriorated, with a maximum around f(Stim) less than or equal to 5 Hz. Residual calcium levels, in contrast, increased monotonical ly with stimulation frequency. Simultaneously with the calcium measurements we determined the amount of AV P release evoked by each stimulus train. Hormone release increased with fSt im beyond the peak in Delta[Ca2+](tr) amplitudes, reaching its maximum betw een 5 and 10 Hz before returning to its 1 Hz level. Thus, AVP release respo nds to the temporal patterning of stimulation, is sensitive to both Delta[C a2+](tr) and Delta[Ca2+](res), and is optimized at a frequency intermediate between the frequency-dependent maxima in Delta[Ca2+](tr) and Delta[Ca2+]( res).