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 activities of chicken LH-RH-I and -II in laying, incubating and out-of-lay turkey hens. The highest plasma concentrations of LH were measured in laying turkey hens, whereas hypophyseal concentrations were highest in incubating hens (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) using 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. Although both Peptides elicited comparable responses in I hens, cLHRH II was more effective in OL hens. This difference was attributable to a greater amplitude of the response, whose duration was unchanged. Hypophyseal desensitization 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 intervals. In vivo, the injection of cLHRH I or II, at doses of 10(-8) and 10(-10) M/kg B.W. stimulated increases in the plasma concentrations of LH, which were initiated within 1 min of injection in OL and I hens but from 5 to 20 min postinjection in L hens. The responses were dose-related and greater 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 sustained phase during which LH concentrations were either stable (I hens) or continued to increase (L and OL hens) from 20 to 60 min after injection. In contrast, the injection of cLHRH I at doses of 10(-8) or 10(-10) M/kg or cLHRH II at a dose of 10(-10) M/kg in I and OL hens, produced a peak of LH concentrations in plasma within 5 min and thereafter declined gradually. The difference 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 summary, the responses to both forms of chicken LH-RH varies markedly with the stage of the reproductive cycle (L, I, and OL) and differs between the in vivo and in vitro situations. Although cLHRH II may be more active than cLHRH I, controversy still surrounds its precise physiological role.