Generation of repetitive Ca2+ transients by bombesin requires intracellular release and influx of Ca2+ through voltage-dependent and voltage independent channels in single HIT cells.
In the present study, the bombesin-induced changes in cytosolic free Ca2+ ([Ca2+]i) were investigated in single Fura-2 loaded SV-40 transformed hamster beta-cells (HIT). Bombesin (50-500 pM) caused frequency-modulated repetitive Ca2+ transients. The average frequency of the Ca2+ transients induced by bombesin (200 pM) was 0.58 +/- 0.02 min-1 (n = 121 cells). High concentrations of bombesin (> or = 2 nM) triggered a large initial Ca2+ transient followed by a sustained plateau or by a decrease to basal levels. In Ca(2+)- free medium, bombesin caused only one or two Ca2+ transients and withdrawal of extracellular Ca2+ abolished the Ca2+ transients. The voltage-dependent Ca2+ channel (VDCC) blockers, verapamil (50 microM) and nifedipine (10 microM), reduced amplitude and frequency of the Ca2+ transients and stopped the Ca2+ transients in some cells. Thapsigargin caused a sustained rise in [Ca2+]i in the presence of extracellular Ca2+ while in its absence the rise in [Ca2+]i was transient. Verapamil (50 microM) inhibited the thapsigargin-induced increase in [Ca2+]i by about 50%. Depletion of intracellular Ca2+ stores by repetitive stimulation with increasing concentrations of bombesin or thapsigargin in Ca(2+)-free medium caused an agonist-independent increase in [Ca2+]i when extracellular Ca2+ was restored, which was larger than in control cells that had been incubated in Ca(2+)-free medium for the same period of time. This rise in [Ca2+]i and the thapsigargin-induced increase in [Ca2+]i were only partly inhibited by VDCC-blockers. Thus, depletion of the agonist-sensitive Ca2+ pool enhances Ca2+ influx through VDCC and voltage-independent Ca2+ channels (VICC). In conclusion, the bombesin-induced Ca2+ response in single HIT cells is periodic in nature with frequency-modulated repetitive Ca2+ transients. Intracellular Ca2+ is mobilized during each Ca2+ transient, but Ca2+ influx through VDCC and VICC is required for maintaining the sustained nature of the Ca2+ response. Ca2+ influx in whole or part is activated by a capacitative Ca2+ entry mechanism.
Schöfl
C
C
Abteilung Klinische Endokrinologie, Medizinische Hochschule Hannover, Germany.
Rössig
L
L
Leitolf
H
H
Mader
T
T
von zur Mühlen
A
A
Brabant
G
G
D000818:Animals; D001839:Bombesin / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002461:Cell Line, Transformed; D002472:Cell Transformation, Viral; D002614:Chelating Agents / Q000494:pharmacology; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007424:Intracellular Fluid / Q000187:drug effects / Q000378:metabolism*; D007515:Islets of Langerhans / Q000166:cytology / Q000187:drug effects / Q000821:virology; D009543:Nifedipine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D007328:Insulin; D004533:Egtazic Acid; D019284:Thapsigargin; D014700:Verapamil; D009543:Nifedipine; D001839:Bombesin; D002118:Calcium