Acute or chronic immunoneutralization of somatostatin does not affect growth hormone or thyroid hormone secretion in sheep

Abstract

The effect of acute or chronic immunoneutralization of somatostatin (SRIF) on plasma GH, thyrotrophin (TSH) and thyroid hormones was examined. Acute responses to SRIF immunoneutralization were examined using 30 intact male lambs (19.8 +/- 0.6 kg) assigned to one of five treatment groups such that control (C) lambs received no anti-SRIF immunoglobulin and SRIF-immunized (SI) lambs received 2 mg (SI2), 10 mg (SI10), 20 mg (SI20) or 100 mg (SI100) anti-SRIF immunoglobulin/kg body weight (BW). Control immunoglobulin was administered such that all lambs received 100 mg total immunoglobulin protein/kg BW. Effects of chronic SRIF immunoneutralization were examined using C and SI100 lambs which received additional (40 mg/kg BW) control and anti-SRIF immunoglobulin respectively, 4 and 8 days following the initial dose. Blood samples were collected from all lambs, at 10-min intervals, for 5 h immediately following initial immunoglobulin infusion and, from C and SI100 lambs, at 10-min intervals, for 5 h at 11 days following initial immunoglobulin infusion. At the end of each 5-h sampling period, pituitary and thyroid function was examined by i.v. challenge with thyrotrophin-releasing hormone (TRH; 0.33 microgram/kg BW). Basal plasma GH and thyroxine (T4) and the GH, TSH, T4 and tri-iodothyronine (T3) responses to TRH were not influenced by acute or chronic immunoneutralization of SRIF. Acute, but not chronic, immunoneutralization of SRIF elevated basal plasma T3 in SI100 lambs only. The results suggest that SRIF, under physiological conditions, does not influence GH or thyroid hormone secretion in sheep but may influence thyroid hormone metabolism acutely.

Hypophyseal-Portal Somatostatin (SRIH) and Jugular Venous Growth Hormone Secretion in the Conscious Unrestrained Ewe

“…The lack of a uniform decrement in SRIH secretion prior to GH release (present data) extends earlier experimental insights on SRIH/GH relationships, wherein immunization against SRIH failed to consistently elevate the amplitude of pulsatile GH secretion in the ruminant [15, 16, 17, 38]and baboon [18]. In aggregate, such data could indicate that a high frequency of low-amplitude portal SRIH pulses and intervening basal SRIH release jointly ensure continuing responsiveness of somatotrope cells to recurrent hypothalamic stimuli in the female [8, 10, 23].…”

“…In another analysis of 23 rams, only 50% of GH secretory episodes coincided with simultaneous portal-venous GHRH release and SRIH withdrawal [14]. More, in the male ruminant, immunoneutralization of SRIH altered pulsatile GH release only minimally [15, 16, 17]. In the male baboon, administration of antiserum to SRIH elevated interpulse GH release, but suppressed GH peak height [18].…”

Pulsatile and nocturnal growth hormone secretions in men do not require periodic declines of somatostatin

“…Studies in animals suggest that there is both sex and species variability in the role of SRIH in the regulation of GH pulsatility (8,21,37,41).…”

“…In contrast, in female rats, SRIH appears to be secreted in a more continuous fashion and is unlikely to be important in the generation of GH pulses (30). In other species such as sheep, hypothalamic GHRH appears to be primarily responsible for the generation of GH pulses (8,23,39,41).In humans, endogenous GHRH is required for GH pulsatility (29). However, it is not clear whether periodic hypothalamic GHRH release is responsible for the initiation of GH pulses or whether GHRH is required for the action of other factors.…”

“…In contrast, in female rats, SRIH appears to be secreted in a more continuous fashion and is unlikely to be important in the generation of GH pulses (30). In other species such as sheep, hypothalamic GHRH appears to be primarily responsible for the generation of GH pulses (8,23,39,41).…”