Effect of Ca2+ and cAMP on capacitance-measured hormone secretion in human GH-secreting adenoma cells.
Membrane capacitance (Cm) was measured as an index of exocytosis in human growth hormone-secreting adenoma cells using the perforated whole cell, patch-clamp technique; the effects of membrane depolarization, growth hormone-releasing hormone, and 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) were examined. Cm was increased by membrane depolarization to potentials beyond the threshold necessary to open voltage-gated Ca2+ channels. These voltage-dependent changes in Cm varied as a function of both depolarization amplitude and duration and were blocked in the presence of the Ca2+ channel antagonist nitrendipine (10(-6) M). When membrane potential was clamped at the holding potential (-78 mV), voltage-gated Ca2+ channels were closed, and neither application of growth hormone-releasing hormone nor 8-BrcAMP affected Cm. However, when these agents were applied to depolarized cells, where the voltage-gated Ca2+ channels were open, the increases in Cm were augmented. From these data, it was concluded that elevation of intracellular cAMP, per se, did not stimulate exocytosis. Rather, Ca2+ influx through voltage-gated channels was a prerequisite for cAMP-induced exocytosis.
Takei
T
T
Fourth Department of Internal Medicine, Tokyo University Branch Hospital, Tokyo 112, Japan.
Yasufuku-Takano
J
J
Takano
K
K
Fujita
T
T
Yamashita
N
N
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000236:Adenoma / Q000378:metabolism* / Q000503:physiopathology*; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology; D000242:Cyclic AMP / Q000502:physiology*; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D019382:Human Growth Hormone / Q000378:metabolism*; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009568:Nitrendipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010911:Pituitary Neoplasms / Q000378:metabolism* / Q000503:physiopathology*; D014407:Tumor Cells, Cultured
D015220:Calcium Channels; D019382:Human Growth Hormone; D015124:8-Bromo Cyclic Adenosine Monophosphate; D009568:Nitrendipine; D000242:Cyclic AMP; D002118:Calcium