Transient extracellular volume reduction in neural lobe of rat hypophysis in response to neural stalk stimulation in vitro and its relationship to extracellular potassium.

1. Using ion-sensitive microelectrodes, a transient reduction in the local volume of neural lobe extracellular space was found to accompany the elevation in extracellular potassium induced by stimulation of the neural stalk. The volume decrease and potassium increase had similar stimulus-response curves when stimulus frequency was varied from 1 to 40 Hz, with maximal response at 20 Hz. The curves for stimulus duration diverged, as a near maximal potassium response was reached in 4-16 s with a 20-Hz stimulus, while the extracellular volume decrease was maximal at 64 s. 2. The volume decrease, but not the potassium increase, was strongly inhibited by lowering bath temperature and moderately inhibited by furosemide and by lowering extracellular chloride concentration. Both the volume and the potassium response were enhanced by ouabain. 3. In conclusion, shrinkage of the local extracellular space in neural lobe during nerve activity is mediated by a metabolically active process which is only partially dependent upon extracellular chloride concentration and anion-cation co-transport, but is relatively independent of Na(+)-K+ pump activity. A transient shrinkage in extracellular space during increased neurohypophysial nerve activity would be expected to play a role in hormone diffusion, ion buffering, and extracellular current flow.