Roles of somatostatin and growth hormone-releasing factor in ether stress inhibition of growth hormone release.

In order to evaluate the release of somatostatin (SRIF) and growth hormone-releasing factor (GRF) into the pituitary gland in response to ether stress, a push-pull perifusion (PPP) technique has been used in freely moving rats. Push-pull cannulae (PPC) were implanted into the anterior pituitary (AP) glands of male rats. After a 7-day recovery period the rats were fitted with indwelling jugular catheters. The next day the animals were subjected to PPP of the AP during 1 h followed by ether stress (2 min) and another hour of perifusion. The perifusion flow was 20 microliters/min and 10-min fractions were collected and assayed for SRIF and GRF by RIA. Plasma growth hormone (GH) levels were assayed every 10 min. At the end of the experiments, the accuracy of PPC tip placements was ascertained with a dissecting microscope. Under basal conditions there were 2.9 pulses/h of SRIF with an amplitude of 12.76 +/- 0.46 pg. The output of SRIF and GRF in the 10-min period beginning with application of ether was increased 2-fold (p less than 0.005 and p less than 0.01, respectively). Interestingly, the increased release of SRIF continued for an additional 10 min, whereas GRF output decreased and was almost undetectable. The release of both GRF and SRIF had returned to basal values 20-30 min after stress. Mean plasma GH levels were significantly lowered 10 min after stress. Each of the 9 animals showed a restoration of pulsatile GH release to basal levels within 20-30 min after stress. Our findings provide compelling evidence that SRIF plays a prominent role in stress-induced inhibition of GH release in the rat by blocking the response to the transient elevation of GRF and continuing to suppress GH release for 20 min.

In vivo models for the study of gonadotropin and LHRH secretion.

The objective of this paper is to describe recent data from my laboratory dealing with the in vivo control of LHRH secretion in male rabbits and rats using the technique of push-pull perfusion (PPP) that allows repetitive determinations of this neuropeptide in quasi-normal physiological conditions. In addition, we have applied this method to simultaneously measure LHRH and LH in freely behaving male rats bearing a push-pull cannula (PPC) in the anterior pituitary. A description of the validation of this technique and its potential use will be discussed as well as data indicating that castration in the male rat induces a significant increase in the LHRH and LH signals; however, following testosterone treatment, in spite of a clear return of LH output to intact levels, even higher levels of LHRH reaching the anterior pituitary were detected. Curiously, in the rabbit no changes in LHRH release were noticed with castration, but following testosterone treatment, a transient but robust 5-8-fold increase in LHRH release was noticed. In short, these studies have demonstrated the existence of apparently opposite rather than similar responses in the testicular control of the hypothalamic-hypophysial axis of the male rat as compared with those of the male rabbit.

Simultaneous measurement of gonadotropin-releasing hormone, luteinizing hormone, and follicle-stimulating hormone in the orchidectomized rat.

In the present study two recently developed techniques have been combined to enable the simultaneous in vivo determination of pulsatile release of GnRH, LH, and FSH in the orchidectomized rat. The first of these techniques involves the implantation of two vascular catheters and collecting serial blood samples through one while simultaneously infusing a replacement blood mixture through the other; consequently, blood samples can be collected for an extended period of time, and detailed plasma LH and FSH release profiles can be established for individual animals. The second technique involves push-pull perfusion of the pituitary gland to determine changes in GnRH concentration as might be perceived by the gonadotropes. For each animal (n = 6), blood (150 microliters) and push-pull perfusate (200 microliters) samples were collected at 5- and 10-min intervals, respectively, for approximately 6 h, and the hormone release profiles were determined by RIA. All of the rats showed a clear pulsatile release pattern for GnRH, LH, and FSH. Moreover, the interpulse interval was remarkably similar for each of these hormones (36.9, 41.5, and 43.5 min, respectively, as determined by PULSAR). The percentage of GnRH pulses associated with a gonadotropin pulse was 72% for LH and 76% for FSH; only 14% of the pulses were silent for both gonadotropins. These results demonstrate that in the orchidectomized rat the pulsatile pattern of GnRH release is reflected in the pulsatile pattern of not only LH but also FSH. They may, therefore, be construed to support the concept that the pulsatile secretion of both gonadotropins is primarily orchestrated by a single hypothalamic releasing hormone. Alternatively, if two separate hypothalamic releasing hormones do indeed exist (LHRH and FSH-releasing hormone), it would appear that in the orchidectomized rat their episodic release is tightly coupled to the same hypothalamic pulse generator.