title: Effects of raised extracellular potassium on the excitability of, and hormone release from, the isolated rat neurohypophysis.
abstract: 1. Single neurohypophyses from male rats were maintained in an in vitro perifusion chamber. Ion-sensitive microelectrodes were introduced into the tissue to measure changes in [K+]o and [Ca2+]o during electrical stimulation. 2. Electrical stimulation at 6 Hz for 1 min and 30 Hz for 12 s raised [K+]o by 5.4 +/- 0.4 and 13.5 +/- 0.5 mM (mean +/- S.E.M., n = 8) respectively. To investigate the effects of raised [K+]o on the excitability of the neurosecretory terminals, stimulations were repeated in media of altered K+ concentration. The increase in [K+]o evoked by 6 Hz stimulation was elevated in 10 mM-K+ medium (133% of that in 5 mM-K+ medium) and reduced in 0 mM-K+ medium and in 25 mM-K+ medium. Thus it appeared that stimulus-induced changes in [K+]o might enhance the excitability of the tissue during electrical activation. 3. To test this hypothesis, we measured the field potential responses evoked by 0.5 Hz stimulation in media of different K+ concentrations. The size of the field potential was enhanced in 10 mM-K+ medium and depressed in 0 mM-K+ medium and in 25 mM-K+ medium. 4. Electrical stimulation (6 Hz, 1 min) decreased [Ca2+]o by 10.9 +/- 1.8% (n = 6). This decrease was absent in the presence of 1 microM-tetrodotoxin or 1 mM-cadmium. Again, the [Ca2+] response to stimulation was enhanced in 10 mM-K+ medium and depressed in 0 mM-K+ medium or 25 mM-K+ medium. 5. The release of vasopressin and oxytocin evoked by stimulation at 6 or 30 Hz from isolated neurohypophyses was measured by radioimmunoassay in a separate series of experiments. Stimulation at 30 Hz for 1 min released 5- to 6-fold more hormone than stimulation at 6 Hz for 5 min. Release evoked by 6 Hz stimulation was enhanced in 15 mM-K+ medium and depressed in 25 mM-K+ medium. 6. We conclude that the rise in [K+]o that accompanies high-frequency activation of axons and terminals in the neurohypophysis contributes to the facilitation of hormone release with increasing frequencies of stimulation, and in particular to the efficiency of the milk-ejection burst discharge of oxytocin neurones for evoking oxytocin release.
authors.affiliation: Department of Neuroendocrinology, AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge.
authors.forename: G
authors.initials: G
authors.lastname: Leng
authors.forename: K
authors.initials: K
authors.lastname: Shibuki
authors.forename: S A
authors.initials: SA
authors.lastname: Way
mesh_terms: D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004558:Electric Stimulation; D066298:In Vitro Techniques; D008297:Male; D010121:Oxytocin / Q000378:metabolism*; D010904:Pituitary Gland, Posterior / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014667:Vasopressins / Q000378:metabolism*
chemical_list: D014667:Vasopressins; D010121:Oxytocin; D011188:Potassium; D002118:Calcium