AJP - Endocrinology and Metabolism, Vol 246, Issue 2 198-E201, Copyright © 1984 by American Physiological Society
Changes in cytosolic ionized calcium induced by activators of secretion in GH3 cells
K. W. Snowdowne and A. B. Borle
Cytosolic ionized calcium (Cai) was measured in GH3 cells with aequorin
incorporated by hypoosmotic shock treatment. Cai rose from 0.4 to 1.3
microM after the administration of 10(-7) M thyrotropin-releasing hormone
(TRH) and from 0.3 to 1.1 microM after depolarization caused by high-K
concentration (60 mM). Repeated TRH stimulations decreased the magnitude of
the Cai response. This magnitude was directly related to the interval
between stimuli. Incubations in low-calcium media (0.13 mM Ca) for 0.5 h
did not alter the Cai response to TRH, but incubations in Ca-free media
reduced the response. These results suggest that TRH mobilized calcium from
an endogenous pool of Ca that can be depleted and replenished. The rise in
Cai induced by K depolarization was slower than that evoked by TRH (time
constant, t = 9 s for TRH and t = 111 s for K depolarization). Even when
Cai was elevated by K depolarization, it could still be further increased
by TRH, suggesting that these two stimuli mobilize calcium from two
different pools and by different mechanisms.
