LH secretion in response to gonadotropin releasing hormone (GnRH) by superfused pituitaries from two species of turtles.

A superfusion system was employed to study the dynamics of the responses of LH secretion to gonadotropin releasing hormone (GnRH) by anterior pituitary glands (quartered) from adult turtles, Pseudemys scripta and Chrysemys picta. Responsiveness was highly variable in both species, but in many cases, the tissues showed marked responses to relatively low doses (2-20 ng/ml) of GnRH. GnRH had no effect on LH secretion in five female P. scripta tested in the month of August, whereas it stimulated most glands tested in June and September; the few males tested were not as responsive as females in the same season. Female P. scripta invariably showed no or only very transient responses (less than 1 hr) to continuous GnRH superfusion, although glands recovered rapidly from this state of "desensitization" as evidenced by their ability to respond to subsequent stimuli. In some cases, pulsatile GnRH was effective in preventing this rapid desensitization. In contrast, glands from female C. picta were highly responsive in August. In March, they were similar to June P. scripta in showing a rapid desensitization to GnRH, whereas in August, the tissues of C. picta maintained full responsiveness to continuous GnRH for many hours. There was little evidence of GnRH "self-priming" in the turtles. The high individual variability in the magnitude of the response of LH to GnRH, the variable but often rapid desensitization to GnRH, as well as possible sexual, seasonal, and species differences in GnRH responsiveness may underlie the discrepancies previously observed among in vivo and in vitro studies in reptiles. However, the turtle is clearly not as unresponsive to mammalian GnRH as suggested by some previous studies.