Cell Calcium 36 (2004) 89–97
Recovery of Ins(1,4,5)-trisphosphate-dependent calcium
signaling in neonatal gonadotrophs
Hana Zemkova
a,∗
, Ales Balik
a
, Karla Kretschmannova
a
, Petr Mazna
a
,
Stanko S. Stojilkovic
b
a
Institute of Physiology, Academy of Sciences of the Czech Republic, Videnska 1083, 142 20 Prague 4, Czech Republic
b
Section on Cellular Signaling, ERRB, NICHD, NIH, Bethesda, MD, USA
Received 17 November 2003; received in revised form 16 December 2003; accepted 17 December 2003
Abstract
Pituitary gonadotrophs express non-desensitizing gonadotropin-releasing hormone (GnRH) receptors and their activations leads to inos-
itol 1,4,5-trisphosphate (InsP
3
)-dependent Ca
2+
mobilization. When added in physiological concentration range GnRH induces baseline
Ca
2+
oscillations, whereas in higher concentrations it induces a prolonged spike response accompanied with non-oscillatory or oscil-
latory plateau response. Here, we studied the recovery of calcium signaling during repetitive stimulation with short (10–30 s) GnRH
pulses and variable interpulse intervals in neonatal gonadotrophs perfused with Ca
2+
/Na
+
-containing, Ca
2+
-deficient/Na
+
-containing,
and Ca
2+
-containing/Na
+
-deficient media. In Ca
2+
/Na
+
-containing medium, baseline Ca
2+
oscillations recovered without refractory pe-
riod and with a time constant of ∼20 s, whereas the recovery of spike response occurred after 25–35 s refractory period and with a time
constant of ∼30 s. During repetitive GnRH stimulation, removal of Ca
2+
had only a minor effect on baseline oscillations but abolished spike
response, whereas removal of Na
+
slightly extended duration of baseline oscillations and considerably prolonged spike response. These
results indicate that two calcium handling mechanisms are operative in gonadotrophs: redistribution of calcium within InsP
3
-sensitive and
-insensitive pools and a sodium-dependent calcium efflux followed by calcium influx. Redistribution of Ca
2+
within the cell leads to rapid
recovery of InsP
3
-dependent pool, whereas the Na
+
-dependent Ca
2+
efflux pathway is activated by spike response and limits the time of
exposure to elevated cytosolic Ca
2+
concentrations.
© 2004 Elsevier Ltd. All rights reserved.
1. Introduction
Differentiation of pituitary secretory cells arises during
embryonic life. In rats this process occurs between 15 and
18 days of embryonic age [1]. In newborn animals, the
neonatal population of gonadotrophs is gradually substi-
tuted with the postnatal generation of cells during pre- and
peripubertal period. Both types of gonadotrophs express
G protein-coupled gonadotropin-releasing hormone (GnRH)
receptor [2,3] and their activation leads to increase in in-
ositol 1,4,5-trisphosphate (InsP
3
) production and release of
calcium through InsP
3
receptor-channel expressed in the en-
doplasmic reticulum membrane [4–6]. On the other hand,
neonatal but not postnatal gonadotrophs express melatonin
MT1 receptor [7], which inhibits GnRH-induced signaling
and secretion [8].
∗
Corresponding author. Tel.: +420-2-4106-2574;
fax: +420-2-4106-2488.
E-mail address: zemkova@biomed.cas.cz (H. Zemkova).
In contrast to postnatal gonadotrophs, the intracellular
signaling by GnRH in neonatal gonadotrophs has been
incompletely characterized. In general, GnRH-induced cal-
cium signals differ in the shape, frequency, and amplitude.
Low to intermediate GnRH concentrations induce periodic
calcium release with the [Ca
2+
]
i
oscillating around its rest-
ing level, with a cycle frequency of 10–30 min
-1
. These
high-frequency baseline calcium oscillations are not unique
for gonadotrophs, but were also observed in other cell
types [9,10]. Higher GnRH doses induce biphasic response
composed of an initial large non-oscillatory spike response
and sustained baseline oscillations [11–16]. Both patterns
of calcium signals are able to stimulate hormone secretion
via exocytosis, as assessed by measurements of capacitance
[17]. In neonatal gonadotrophs GnRH also generates an
initial spike response not accompanied with sustained os-
cillations [6], and such monophasic pattern of signaling is
comparable to that observed in GnRH-stimulated immor-
talized T3-1 neonatal gonadotrophs [18]. The dual control
of InsP
3
receptor-channels, by InsP
3
and calcium [19,20],
0143-4160/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ceca.2003.12.003