Intracellular calcium and vasopressin release of rat isolated neurohypophysial nerve endings.
1. Monitoring of [Ca2+]i and vasopressin secretion in isolated nerve endings from the rat neurohypophysis were studied to determine the relationship between the time course of vasopressin secretion and depolarization-induced changes in [Ca2+]i. 2. Membrane depolarization by increasing the extracellular [K+] led to concentration-dependent, parallel increases in the amount of vasopressin release and in peak increases in [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-bis(O-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid), loaded into the nerve endings, reduced K+ depolarization-evoked vasopressin release and efficiently antagonized K(+)-induced changes in [Ca2+]i. Moreover, dimethyl-BAPTA dramatically reduced basal [Ca2+]i without a reduction in basal secretion. 4. The duration of the vasopressin secretory response was similar regardless of applied 50 mM K+ depolarizations longer than 30 s. The t1/2 of the secretory response was 45 s. Application of repetitive K+ depolarization pulses repetitive secretory responses of similar amplitude and duration. 5. The K(+)-induced changes in [Ca2+]i remained elevated throughout the duration of the depolarizing stimulus decreasing less than 30% over 3 min. The sustained increase in [Ca2+]i resulted largely from continued enhanced Ca2+ influx, demonstrated by susceptibility to the dihydropyridine, L-type calcium channel blocker, nicardipine. 6. Vasopressin secretion could be reinitiated following its decline to a step K+ depolarization by a further step increase in K+ or by removal and readdition of extracellular [Ca2+]. Alterations in [Ca2+]i paralleled periods of secretory activity. 7. Analysis of secretory responsiveness and change in [Ca2+]i to K+ depolarization in medium of altered extracellular [Ca2+] indicates that [Ca2+]i of 20 microM is sufficient to trigger vasopressin release. K(+)-induced alterations in [Ca2+]i could be observed at [Ca2+]o as low as 5 microM. Although smaller in amplitude to that observed at 2.2 mM [Ca2+]o the duration of the K(+)-induced secretory response increased at lower [Ca2+]o. 8. Transient vasopressin secretory responses were observed to sustained levels of [Ca2+] in digitonin and streptolysin-O-permeabilized nerve endings. Secretion could be re-evoked, following its decline, by a step increase in [Ca2+] or by removal and readdition of [Ca2+]o.(ABSTRACT TRUNCATED AT 400 WORDS)
Stuenkel
E L
EL
Department of Physiology, University of Michigan, Ann Arbor 48109-0622.
Nordmann
J J
JJ
D000818:Animals; D001127:Arginine Vasopressin / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005089:Exocytosis; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism; D009529:Nicardipine / Q000494:pharmacology; D010904:Pituitary Gland, Posterior / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar
D002614:Chelating Agents; D001127:Arginine Vasopressin; D004533:Egtazic Acid; C058180:5,5'-dimethyl-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate; D009529:Nicardipine; D011188:Potassium; D002118:Calcium