Inhibition of Ca(2+)-induced calcitonin secretion by somatostatin: roles of voltage dependent Ca2+ channels and G-proteins.
Somatostatin has recently been applied therapeutically for hypercalcitonemia in patients with calcitonin-producing tumours. Using calcitonin-secreting cells (C-cells) of the medullary thyroid carcinoma cell line rMTC 44-2, we investigated the inhibitory action of somatostatin on calcitonin release, cytosolic Ca2+ and Ca2+ channel currents. The Ca(2+)-induced rises of the cytosolic Ca2+ and calcitonin secretion were greatly inhibited by somatostatin or its stable analogue octreotide. The effects of somatostatin were pertussis toxin-sensitive. Under voltage clamp conditions, C-cells exhibited slowly inactivating Ca2+ channel currents. Bath application of 100 nM somatostatin reversibly reduced the Ca2+ channel current by about 30%. The Ca2+ channel current and its inhibition by somatostatin were not affected by intracellularly applied cyclic AMP. Moreover, pretreating the cells with pertussis toxin had no effect on the control Ca2+ channel currents but greatly abolished its inhibition by somatostatin. The data show that somatostatin suppresses the Ca(2+)-stimulated calcitonin secretion by inhibiting voltage-dependent Ca2+ channel currents and by lowering cytosolic Ca2+. These actions of somatostatin involve pertussis toxin-sensitive G-proteins and occur independently of changes in the cyclic AMP concentration.
Scherübl
H
H
Pharmakologisches Institut der Freien Universität Berlin, Germany.
Hescheler
J
J
Schultz
G
G
Kliemann
D
D
Zink
A
A
Ziegler
R
R
Raue
F
F
D000818:Animals; D002116:Calcitonin / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000502:physiology*; D008564:Membrane Potentials; D051381:Rats; D013004:Somatostatin / Q000502:physiology*
D015220:Calcium Channels; D013004:Somatostatin; D002116:Calcitonin; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D002118:Calcium