Sexually Dimorphic Characteristics of Clonidine-Induced Growth Hormone Release and Autofeedback*

Abstract

The mechanism underlying the sexually dimorphic pattern of GH secretion in the rat has not been clearly elucidated. In this study differences in the regulation of GH secretion in males and females were analyzed by examining their sensitivity to alpha 2-adrenergic (alpha 2) stimulation and GH autofeedback. The model used in testing this system was the capacity of ovine (o) GH, injected into the third ventricle, to suppress the endogenous GH surge induced by clonidine (CLON), an alpha 2-agonist. In morning experiments, CLON (30 micrograms/kg BW, iv) was injected between 1000-1100 h in males. oGH (20 micrograms in 2 microliters vehicle) injected 3 h earlier inhibited the GH surge, which peaked 15 min after CLON injection in control animals receiving CLON alone. In females there was both no change in plasma GH levels and no difference between treatment groups in animals receiving 30, 50, 100, or 150 micrograms/kg CLON and saline controls. Preinjection of 20 micrograms oGH in proestrous or diestrous animals or 30 micrograms oGH in diestrous animals 3 h before CLON treatment did not depress plasma GH levels below those found in animals receiving CLON alone. Further, neither reducing the preinjection time to 2 h nor injecting 7, 13, or 20 micrograms oGH at this preinjection period suppressed basal or CLON-induced GH release. To facilitate comparisons in subsequent experiments the male protocol was followed. In evening experiments, CLON (injected between 2210-2240 h) stimulated a significant GH surge in diestrous females. However, preinjected oGH lowered basal GH levels but failed to suppress the CLON-induced GH surge. In somatostatin (SRIF) antiserum (anti-SRIF; 0.5 ml, iv)-treated adult diestrous animals, CLON induced a significant GH surge that was suppressed by oGH preinjection. In normal sheep serum-treated animals there was no significant response to either CLON alone or in combination with oGH. To determine the influence of ovarian hormones on GH release females were ovariectomized (OVXed) either pre- or postpubertally. After adult OVX, CLON induced an increase in GH release that was not suppressed by oGH pretreatment. In prepubertally OVXed animals CLON induced a substantial GH surge that was suppressed in animals preinjected with oGH. In prepubertally OVXed animals implanted with testosterone capsules the response to CLON and oGH was not significantly different from that after prepubertal OVX alone. Sham-operated animals responded to treatment in a manner similar to unoperated cycling females.(ABSTRACT TRUNCATED AT 400 WORDS)