Vasopressin induces frequency-modulated repetitive calcium transients in single insulin-secreting hit cells.
Ca2+ is central to the stimulation of insulin secretion from pancreatic beta-cells. Arginine-vasopressin (AVP) may participate in the modulation of insulin release. In the present study, the AVP-induced changes in cytosolic free Ca2+ ([Ca2+]i) were investigated in single fura-2 loaded insulin-secreting HIT cells. Stimulation with AVP (0.1-5 nM) caused repetitive Ca2+ transients. The frequency but not the amplitude of the Ca2+ transients was modulated by the concentration of AVP. High concentrations of AVP (10-100 nM) triggered a biphasic rise in [Ca2+]i. In Ca(2+)-free medium AVP caused only one or two Ca2+ transients. Withdrawal of extracellular Ca2+ rapidly abolished the AVP-induced Ca2+ transients in all cells tested. The Ca2+ channel blocker, verapamil (50 microM), reduced amplitude and frequency of the Ca2+ transients by about 25% and 60%, respectively, and terminated the Ca2+ transients in 2 of 6 cells. When HIT cells were incubated in Ca(2+)-free medium, and extracellular Ca2+ was restored, there was a small increase in [Ca2+]i. If, however, the agonist-sensitive Ca2+ pool was functionally depleted by repetitive stimulation with high concentrations of AVP or thapsigargin in Ca(2+)-free medium before extracellular Ca2+ was restored, an agonist-independent increase in [Ca2+]i was observed, which was transiently larger than in the control cells, and was mainly preserved in the presence of verapamil. Thus, depletion of the agonist-sensitive Ca2+ pool enhances the influx of extracellular Ca2+ through a Ca2+ entry mechanism independent from verapamil-sensitive voltage-dependent Ca2+ channels (VDCC).(ABSTRACT TRUNCATED AT 250 WORDS)
Schöfl
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Abteilung Klinische Endokrinologie, Medizinische Hochschule Hannover, Germany.
Schulte
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Rössig
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von zur Mühlen
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Brabant
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D000818:Animals; D001127:Arginine Vasopressin / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002461:Cell Line, Transformed; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D014700:Verapamil / Q000494:pharmacology
D007328:Insulin; D013729:Terpenes; D001127:Arginine Vasopressin; D019284:Thapsigargin; D014700:Verapamil; D000252:Calcium-Transporting ATPases; D002118:Calcium