El. Stuenkel et Jj. Nordmann, INTRACELLULAR CALCIUM AND VASOPRESSIN RELEASE OF RAT ISOLATED NEUROHYPOPHYSEAL NERVE-ENDINGS, Journal of physiology, 468, 1993, pp. 335-355
1. Monitoring of [Ca2+]i and vasopressin secretion in isolated nerve e
ndings from the rat neurohypophysis were studied to determine the rela
tionship between the time course of vasopressin secretion and depolari
zation-induced changes in [Ca2+]i. 2. Membrane depolarization by incre
asing the extracellular [K+] led to concentration-dependent, parallel
increases in the amount of vasopressin release and in peak increases i
n [Ca2+]i. Half-maximal activation of a change in [Ca2+]i was attained
at 40 mm extracellular K+. 3. The Ca2+ chelator dimethyl-BAPTA 1,2-bi
s(O-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid), loaded into the ne
rve endings, reduced K+ depolarization-evoked vasopressin release and
efficiently antagonized K+-induced changes in [Ca2+]i. Moreover, dimet
hyl-BAPTA dramatically reduced basal [Ca2+], without a reduction in ba
sal secretion. 4. The duration of the vasopressin secretory response w
as similar regardless of applied 50 mm K+ depolarizations longer than
30 s. The t1/2 of the secretory response was 45 s. Application of repe
titive K+ depolarization pulses produced repetitive secretory response
s of similar amplitude and duration. 5. The K+-induced changes in [Ca2
+]i remained elevated throughout the duration of the depolarizing stim
ulus decreasing less than 30 % over 3 min. The sustained increase in [
Ca2+]i resulted largely from continued enhanced Ca2+ influx, demonstra
ted by susceptibility to the dihydropyridine, L-type calcium channel b
locker, nicardipine. 6. Vasopressin secretion could be reinitiated fol
lowing its decline to a step K+ depolarization by a further step incre
ase in K+ or by removal and readdition of extracellular [Ca2+]. Altera
tions in [Ca2+]i paralleled periods of secretory activity. 7. Analysis
of secretory responsiveness and change in [Ca2+]i to K+ depolarizatio
n in medium of altered extracellular [Ca2+] indicates that [Ca2+]i of
20 muM is sufficient to trigger vasopressin release. K+-induced altera
tions in [Ca2+]i could be observed at [Ca2+]. as low as 5 muM. Althoug
h smaller in amplitude to that observed at 2.2 mm [Ca2+]. the duration
of the K+-induced secretory response increased at lower [Ca2+]o. 8. T
ransient vasopressin secretory responses were observed to sustained le
vels of [Ca2+] in digitonin and streptolysin-O-permeabilized nerve end
ings. Secretion could be re-evoked, following its decline, by a step i
ncrease in [Ca2+] or by removal and readdition of [Ca2+]o. 9. These re
sults show that the amount and duration of depolarization-induced vaso
pressin secretion from isolated nerve endings may be regulated not onl
y by the absolute increase but also by periodic changes in [Ca2+]i.