Gonadotropin-releasing hormone-stimulated calcium mobilization is altered in pituitary cultures from anestrous ewes

GnRH transiently increases intracellular calcium ([Ca2+]i) in pituitary gonadotrophs in culture. This effect is observed within 10 sec of GnRH treatment, lasts 1-5 min, and is thought to be an important intracellular signal leading to the secretion of LH. Recent studies with ovine pituitary cultures uncovered seasonal differences in this transient calcium response to GnRH. This finding led to the hypothesis that anestrus alters the pituitary response to GnRH and prompted this study on seasonal effects on LH secretion and [Ca2+]i mobilization. Inhibin was used to enhance pituitary sensitivity to GnRH. Results showed that pituitary cultures from 16 cycling ewes secreted an average of 9% +/- 2% of stored LH after treatment with 10 nM GnRH (stored LH = 470 +/- 66 ng/million cells); 9% +/- 1% of pituitary cells increased the concentration of [Ca2+]i from 75-150 nM up to 400-1400 nM. When cultures were pretreated with inhibin, GnRH released an average of 15% +/- 3% of stored LH (stored LH = same as above), and 38% +/- 4% of cells transiently increased [Ca2+]i above 400 nM. Results also showed that pituitary cultures from 11 acycling ewes secreted similar percentages of LH although the cells contained less LH to begin with (stored LH = 190 +/- 24 ng/million cells), but absolutely none of these cells (500 counted) increased [Ca2+]i after GnRH treatment, with or without inhibin present. Levels of GnRH receptor mRNA were unchanged by season. These data show that LH secretion and the rapid, transient increase in [Ca2+]i caused by GnRH are uncoupled in the unique situation of anestrus. It is possible, in fact, that these two parameters are not necessarily coupled during estrous conditions and that the transient change in [Ca2+]i is connected with LH synthesis or some other gonadotropin function. The data also show that inhibin can increase GnRH-responding cells up to 38%, a fact suggesting that inhibin may be a differentiation factor capable of transforming multipotent pituitar