Extracellular potassium changes in the rat neurohypophysis during activation of the magnocellular neurosecretory system.

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Abstract
1. Potassium-sensitive microelectrodes were used to measure extracellular [K+] in the isolated rat neurohypophysis maintained in vitro. Electrical stimulation of the neurohypophysial stalk (20 Hz, 5 s) increased the inferred extracellular [K+] by 9.2 .+-. 0.4 mM (mean .+-. S.E. of mean; n = 21). 2. Veratridine (10 .mu.M) enhanced the response to stalk stimulation, and at a higher concentration (50 .mu.M) increased extracellular [K+] in the absence of stimulation. By contrast, tetrodotoxin (1 .mu.M) blocked the [K+] increase completely and reversibly in each of five experiments, indicating that the increase was a consequence of action potential generation. 3. At the end of brief periods of stimulation, the raised extracellular [K+] returned to pre-stimulation levels within 30 s. In the presence of ouabain (100 .mu.M), the recovery was slower: the half-decay time was extended by 150-300% in each of three experiments. 4. Replacement of calcium in the medium with cobalt, cadmium or magnesium reduced the amplitude of the [K+] increase by 26-30%, indicating that the [K+] increase was largely independent of events subsequent to evoked release of hormone and/or transmitters. 5. Potassium-sensitive microelectrodes were placed in the neurohypophysis of rats anaesthetized with urethane. Electrical stimulation of the pituitary stalk (50 Hz, 5 s) produced transient voltage increase of 7.6 .+-. 0.9 mV (mean .+-. S.E. of mean of seven experiments). These voltage increases were similar in magnitude to the response of the electrodes to the addition of 7.6 .+-. 1.0 mM-K+ to rat plasma. 6. In seven lactating rats, the suckling of a litter of hunger pups evoked periodic reflex milk ejections, as detected by increases in intramammary pressure. Potassium-sensitive microelectrodes in the neurohypophysis recorded transient voltage increases prior to each milk ejection (0.4-5.5 mV). Each increase preceded an increase in intramammary pressure by 12-30 s. 7. Thus synchronized high-frequency activation of magnocellular neurones can produce large changes in extracellular [K+]. The implications of these findings for stimulus-secretion coupling in the neurohypophysis are discussed in the light for previous reports that hormone release from the neurohypophysis is highly dependent on the frequency and pattern of electrical stimulation.