Action potential broadening and frequency-dependent facilitation of calcium signals in pituitary nerve terminals.

Hormone release from nerve terminals in the neurohypophysis is a sensitive function of action potential frequency. We have investigated the cellular mechanisms responsible for this frequency-dependent facilitation by combining patch clamp and fluorimetric Ca2+ measurements in single neurosecretory terminals in thin slices of the rat posterior pituitary. In these terminals both action potential-induced changes in the intracellular Ca2+ concentration (Ca2+i) and action potential duration were enhanced by high-frequency stimuli, all with a frequency dependence similar to that of hormone release. Furthermore, brief voltage clamp pulses inactivated a K+ current with a very similar frequency dependence. These results support a model for frequency-dependent facilitation in which the inactivation of a K+ current broadens action potentials, leading to an enhancement of Ca2+i signals. Further experiments tested for a causal relationship between action potential broadening and facilitation of Ca2+i changes. First, increasing the duration of depolarization, either by broadening action potentials with the K(+)-channel blocker tetraethylammonium or by applying longer depolarizing voltage clamp steps, increased Ca2+i changes. Second, eliminating frequency-dependent changes in duration, by voltage clamping the terminal with constant duration pulses, substantially reduced the frequency-dependent enhancement of Ca2+i changes. These results indicate that action potential broadening contributes to frequency-dependent facilitation of Ca2+i changes. However, the small residual frequency dependence of Ca2+i changes seen with constant duration stimulation suggests that a second process, distinct from action potential broadening, also contributes to facilitation. These two frequency-dependent mechanisms may also contribute to activity-dependent plasticity in synaptic terminals.