EFFECTS OF DEPOLARIZATION EVOKED NA-ENDINGS( INFLUX ON INTRACELLULAR NA+ CONCENTRATION AT NEUROSECRETORY NERVE)

Electrophysiological measurements of voltage-dependent Na+ influx usin g patch-clamp methodology were combined with optical monitoring of the free intracellular Na+ concentration in isolated rat neurohypophysial nerve endings to determine the relationship between Na+ influx genera ted by repetitive stimulation and change in [Na+](i). Application of s tep depolarizations under voltage-clamp-evoked tetrodotoxin-sensitive inward currents that were dependent upon extracellular Na+ and that ex hibited rapid activation and inactivation properties. These characteri stics substantiated the evoked current as a voltage-dependent Na+ curr ent. Application of stimulus trains consisting of step depolarizations that mimick in frequency and duration those of action potentials were found to result in increases in [Na+](i). The induced change in [Na+] (i) was found to be related to the frequency and period of stimulation . Changes in [Na+](i) were greatest at frequencies of 40 Hz and gave m aximal changes with 30 s of continuous stimulation of approximately 2. 4 mM. Sodium influx expressed as a molar quantity resulted in a nearly directly proportional increase in [Na+](i) during the initial period of stimulation at low Na+ loads. When expressed as a charge density (p C/mu m(2)) Na+ influx was found to increase with smaller diameter nerv e endings as did the rate of change in [Na+](i) in response to applied repetitive step depolarizations. Repetitive step depolarizations whic h simulate impulse activity that invade neuroendocrine nerve endings i n vivo in response to physiological demand for hormone secretion resul ted in an increased [Na+](i). It is postulated that this increased [Na +](i) may provide a modulatory influence on the secretory response ind irectly via alteration of intracellular calcium regulation or, perhaps , via a direct action on the secretory mechanism. (C) 1998 IBRO. Publi shed by Elsevier Science Ltd.