The relationship between the membrane potential of neurosecretory nerve endings, as measured by a voltage-sensitive dye, and the release of neurohypophysial hormones

Abstract The membrane potential of isolated rat neurohypophyses and isolated neurosecretosomes (neurosecretory nerve endings) was monitored with the voltage sensitive fluorescent probe diS-C 3 -(5). K ions, in contrast to Na or Cl ions, give rise to large changes of the fluorescent signal. The fluorescent response is linearly related to log[K + ] 0 at values higher than 10 mM, whereas at lower [K + ] 0 the permeability of the membrane for Na ions has to be taken into account. Veratridine increases the fluorescent signal only in the presence of external sodium; this effect is blocked by tetrodotoxin. After prolonged K-induced depolarisation, addition of veratridine to the medium gives a further change in fluorescence of diS-C 3 -(5) associated with release of vasopressin. Vasopressin release from isolated neurohypophyses started to increase significantly only above 25 mk [K + ] 0 , while the depolarization of the membrane was linearly related to log[K + ] 0 . The results are consistent with the view that neurosecretory nerve endings have voltage-dependent calcium channels that regulate the amount of hormone released during depolarisation.