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Biology of Reproduction, Vol 21, 735-747, Copyright © 1979 by Society for the Study of Reproduction
1 Department of Physiology and Biophysics,
Colorado State University,
Fort Collins, Colorado 80523 Suspensions of ovine anterior pituitary cells were prepared by sequential incubation with hyaluronidase-collagenase and trypsin. After an 18 h culture in suspension, cells were used 1) as a
bioassay system to evaluate the LH releasing potency of GnRH and analogs of GnRH and 2) to
investigate the role of cAMP as an intracellular second messenger for GnRH-induced release of LH. GnRH stimulated release of LH from ovine anterior pituitary cells was dose-dependent, with an
ED50 of 0.35 nM GnRH. Release of cAMP was below the limits of detection at all concentrations
of GnRH. Therefore, a phosphodiesterase inhibitor (0.5 mM MIX) and dopamine (0.5 µM) were
added to the incubation medium to inhibit degradation of cAMP and to reduce secretion of cAMP
by lactotrophs, respectively. Under these conditions, both basal and GnRH stimulated release of
LH were potentiated. GnRH-induced release of LH and cAMP was dose-dependent and the log-linear portion of the response curves was parallel. The minimum effective dose was 0.032 nM
GnRH for release of both cAMP and LH. Maximal release of LH was effected by 2 nM GnRH,
whereas 125 nM GnRH was required to induce maximal release of cAMP. D-Ala6-GnRH ethylamide also induced release of LH and cAMP in a dose-dependent manner and the response curves
were parallel to those for GnRH. Although the LH releasing potency of D-Ala6-GnRH ethylamide
was 10-fold greater than was that of GnRH, its cAMP releasing potency was more than 100-fold
greater. A GnRH-induced increase in total LH (medium plus cellular) was not demonstrable during
the 120 min incubation. GnRH specific release of cAMP was noted within 5 min and was maximal by 180 min, while
GnRH specific release of LH was first apparent at 10 min and was maximal by 180 min. Although
total LH increased with time of incubation, no GnRH specific changes in total LH were observed
at incubation times to 240 min. Dibutyryl-cAMP (1 mM) stimulated release of LH to an extent
greater than 200 nM GnRH but had no effect on total LH. The results suggest that cAMP may be an intracellular second messenger for release of LH in
the GnRH activated gonadotroph.
Accepted on May 30, 1979
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