Endothelin ETA receptors mediate the signaling and secretory actions of endothelins in pituitary gonadotrophs.
Specific receptors for endothelin (ET), localized by autoradiographic studies with [125I]ET in frozen sections of the rat pituitary gland, were abundant in the adenohypophysis, but not in the neurohypophysis. Specific binding of [125I]ET-1 and [125I]ET-3 was also demonstrable in 3-day-old primary cultures of anterior pituitary cells. The binding of [125I]ET-1 to its receptors was time and temperature dependent and was followed by rapid internalization of the receptor-ligand complex. Binding of [125I]ET-1 and [125I]ET-3 to pituitary tissues and cells was more effectively displaced by ET-1 and ET-2 than by ET-3. In cultured pituitary cells, ET-1 caused a rapid increase in polyphosphoinositide hydrolysis, and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] production, with a prompt rise in the cytoplasmic calcium concentration ([Ca2+]i) and LH secretion. The Ins(1,4,5)P3 response to 100 nM ET-1 was transient, with a spike at 10 sec followed by an exponential decrease toward the low steady state level. Ins(1,3,4)P3 and inositol bisphosphate (InsP2) increased more slowly, reaching peak values 30-40 sec after stimulation. The kinetics of the [Ca2+]i response to ET-1 were similar to those of the Ins(1,4,5)P3 response and more rapid than those of the Ins(1,3,4)P3 and InsP2 responses. In perifused cells, ET-stimulated increases in LH release showed the same biphasic patterns as the Ins(1,4,5)P3 and [Ca2+]i responses. ET-1 was more potent than ET-3 in stimulating [Ca2+]i and LH responses, consistent with its higher affinity for the pituitary ET receptors. The initial activation of Ca2+ signaling and LH exocytosis by ETs was followed by prolonged refractoriness to both ET-1 and ET-3. The development of desensitization occurred more rapidly in ET-1- than ET-3-stimulated cells and correlated temporally with endocytosis of the receptor-ligand complex. These findings indicate that stimulation of gonadotropin release by ETs occurs via activation of ETA-type receptors, which are coupled to polyphosphoinositide hydrolysis and [Ca2+]i mobilization, and undergo rapid internalization and profound desensitization.
Stojilković
S S
SS
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Balla
T
T
Fukuda
S
S
Cesnjaj
M
M
Merelli
F
F
Krsmanović
L Z
LZ
Catt
K J
KJ
D000818:Animals; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004705:Endocytosis; D016232:Endothelins / Q000378:metabolism / Q000494:pharmacology*; D005260:Female; D007295:Inositol Phosphates / Q000096:biosynthesis; D007986:Luteinizing Hormone / Q000378:metabolism*; D010903:Pituitary Gland, Anterior / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000502:physiology*; D017466:Receptors, Endothelin
D016232:Endothelins; D007295:Inositol Phosphates; D011956:Receptors, Cell Surface; D017466:Receptors, Endothelin; D007986:Luteinizing Hormone; D002118:Calcium