LH responses to chicken luteinizing hormone-releasing hormone I and II in laying, incubating, and out of lay turkey hens

An experiment was conducted to assess the relative in vivo and in vitro act ivities of chicken LH-RH-I and -II in laying, incubating and out-of-lay tur key hens. The highest plasma concentrations of LH were measured in laying t urkey hens, whereas hypophyseal concentrations were highest in incubating h ens (I) and lowest in the laying hens at the end of the laying period (EL). Hypophyseal and plasma concentrations of LH decreased with aging in laying hens (L) and the greater decrease occurred in the hypophyses. An in vitro hypophyseal acute challenge with 2-min pulses of cLHRH I or II (10(-7) M) u sing a perifusion technique resulted in an increase in the release of LH in out-of-lay (OL) and incubating (I) hens, but not in laying (L) hens. Altho ugh both peptides elicited comparable responses in I hens, cLHRH II was mor e effective in OL hens. This difference was attributable to a greater ampli tude of the response, whose duration was unchanged. Hypophyseal desensitiza tion to a subsequent stimulation was observed in OL hens when the interval between stimulations was 30 min, but this did not occur at 60- or 120-min i ntervals. Tn vivo, the injection of cLHRH I or II, at doses of 10(-8) and 1 0(-10) M/kg B.W. stimulated increases in the plasma concentrations of LH, w hich were initiated within 1 min of injection in OL and I hens but from 5 t o 20 min postinjection in L hens. The responses were dose-related and great er immediate responses were measured with cLHRH I than with cLHRH II. Also, after the injection of cLHRH II at the 10(-8) M/kg B.W. dose, the shape of the LH response consisted of an initial increase, followed by a more susta ined phase during which LH concentrations were either stable (I hens) or co ntinued to increase (L and OL hens) from 20 to 60 min after injection. In c ontrast, the injection of cLHRH I at doses of 10(-8) or 10(-10) M/kg or cLH RH II at a dose of 10(-10) M/kg in I and OL hens, produced a peak of LH con centrations in plasma within 5 min and thereafter declined gradually. The d ifference in the in vivo responses to LHRH I and II could not be attributed to a greater potency of cLHRH II, but to a more prolonged action. In summa ry, the responses to both forms of chicken LH-RH varies markedly with the s tage of the reproductive cycle (L, I, and OL) and differs between the iri v ivo and in vitro situations. Although cLHRH II may be more active than cLHR H I, controversy still surrounds its precise physiological role. (C) 1999 E lsevier Science Inc. All rights reserved.