Liberal Studies in Engineering - Workshop Report
Support for this event was provided by the National Science Foundation’s Improving Under- graduate STEM Education program (award no.1451399 made to MIT), by the Teagle Foundation, and by the Deans of MIT’s School of Humanities, Arts, and Social Sciences and of Claremont Graduate University’s School of Educational Studies.
Discussion Session 4
G.S. Tannenbaum I. C. A . F. Robinson (London, UK): Have you treated animals with somatostatin to see if you can block the change in GHRH immunoreactivity? G. S. Tannenbaum (Montreal, Canada): We are currently extending our investigations in these depleted animals. Somatostatin treatment is one area we intend to look at. I. C.A. F. Robinson: Would you like to speculate on the oscillation observed in GHRH levels. Do you think this is a reflection of activity or somehow causal in the rhythm? G.S. Tannenbaum: These are preliminary data taken from only one trough and one peak GH time point and therefore it is premature to speculate. We are now investigating this feature further by assaying GHRH mRNA at several different times throughout the 24-hour cycle. M. Silbermann (Haifa, Israel): Would you care to comment on the central intrahypothalamic interaction in starvation? What does it mean in terms of the hypothalamus, somatostatin and sex hormones? G. S. Tannenbaum: I believe that the high somatostatin levels circulating during starvation emanate from peripheral sites, e.g. the pancreas and GI tract, rather than central, hypothalamic sites and act to suppress GH secretion at the level of the pituitary. Concerning sex hormones, there is controversy over whether the sex steroids have a direct effect at the level of the pituitary or whether they act via the CNS. No direct effect of oestradiol at the pituitary was found by Thorner's group. Chowen-Breed et al. (Endocrinology 1989; 125: 357) have shown differences in hypothalamic somatostatin mRNA in females compared with males and that testosterone can modulate somatostatin gene expression in the brain. Hence, available evidence suggests that steroidal sex hormones act via the CNS. J . 0 . Jansson (Gothenburg, Sweden): What are the effects of long-term infusion with somatostatin? G.S. Tannenbaum: We have not looked at long-term infusion; other workers are currently investigating this. However, we would assume that both long-term infusion and pretreatment with a long-acting somatostatin analogue would produce an augmentation of the response of the somatotroph to endogenous stimuli such as GHRH. 0. G. P. Isaksson (Gothenburg, Sweden): Why were male rats synchronized in Canada, but not in the London and Gothenburg experiments? G.S. Tannenbaum: Single blood samples taken at supposed peak and trough GH secretion cannot be depended on to represent true maximum and minimum values, respectively. We have measured GH secretion in this way and have not always been able to find a significant difference between the two different time points. Although there was a trend for higher GH levels in rats killed at 11.00 hours, this did not reach significance because of the huge variability in values obtained. A peak in GH secretion is an episode which lasts for about an hour, and while 11 .OO hours is taken to be the time of peak secretion, measurements taken at this time could be situated on either the upswing or the downswing of an episode. Hence, the necessity for chronic 6-hour sampling.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.