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.
Jackson
M B
MB
Max-Planck-Institut für Biophysikalische Chemie, Gottingen, Federal Republic of Germany.
Konnerth
A
A
Augustine
G J
GJ
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000502:physiology*; D004558:Electric Stimulation; D016257:Fura-2; D066298:In Vitro Techniques; D008297:Male; D009411:Nerve Endings / Q000502:physiology; D010904:Pituitary Gland, Posterior / Q000502:physiology*; D015221:Potassium Channels / Q000502:physiology; D051381:Rats
D015221:Potassium Channels; D002118:Calcium; D016257:Fura-2