Relationship between duration of urethane or pentobarbitone anaesthesia in male and female rats and adenohypophyseal response to luteinising hormone-releasing hormone.

1 Luteinizing hormone releasing hormone (50 ng/100 g body weight) was injected into male and female rats which had been anaesthetized with either urethane (1.2 g/kg) or pentobarbitone (45 g/kg) for 15, 100 or 240 min and serial blood samples were taken for estimation of plasma luteinizing hormone concentrations. 2 The rats anaesthetized with urethane tended to show a greater response to the releasing hormone than those given pentobarbitone. 3 The magnitude of the responses observed in male rats and in female rats anaesthetized with pentobarbitone did not change as the period of anaesthesia prior to injection of releasing hormone was lengthened from 15 min to 4 h. By contrast, in the groups of female rats anaesthetized with urethane the magnitude of the response to luteinizing hormone releasing hormone was related to the length of pretreatment anaesthesia. Thus both dioestrous and pro-oestrous rats given releasing hormone only 15 min after the onset of urethane anaesthesia had significantly (P less than 0.001 and less than 0.05 respectively) higher concentrations of luteinizing hormone (LH) in the plasma than rats treated with the releasing hormone after 240 min of anaesthesia. 4 These effects were not due to a differential action of the anaesthetics on the mechanism for clearing LH from the plasma.