EXAMPLE 14 What Does Growth Hormone Do in Adults?

In most species, adults stop growing but still release growth hormone from the pituitary gland to regulate metabolism. Physiologists subjected groups of adult rats to various conditions that activated muscle tissue that was either fast-twitch (as sprinters use) or slow-twitch (as distance runners use). They then measured levels of a form of growth hormone (BGH) in the blood and in pituitary tissue. Units are hundreds of nanograms per milliliter of blood and micrograms per milligram of tissue, respectively.

Blood 15.8 20.0 26.7 25.0 23.0 23.8 24.7 16.3 0.8 0.8
Tissue 38.0 36.7 27.8 28.3 34.9 34.1 33.2 32.7 38.1 39.1
Blood 0.6 10.8 37.6 41.3 39.0 57.5 84.8 82.8 28.8 16.5
Tissue 43.9 42.8 19.3 13.7 11.2 14.2 9.7 9.5 31.7 32.8
Table 6.9: Data from G. E. McCall, K. L. Gosselink, et al., Muscle afferent-pituitary axis: A novel pathway for modulating the secretion of a pituitary growth factor, Exercise and Sport Science Reviews, 29 (2001): 1642–1649.

Figure 6.18 is a scatterplot of these data. The plot shows a strong negative straight-line association with correlation . Here’s the physiological mechanism for this association: When there is a higher BGH level in the blood, we can assume that means BGH must have been recently secreted by the pituitary gland so that less BGH now remains in pituitary tissue. The least-squares regression line is

or

The slope is negative, which reflects how blood and pituitary tissue levels of BGH move in opposite directions. The -intercept is the estimated amount of BGH that the pituitary gland has if it does not release any into the blood.

Next, we dig deeper into the data from this experiment. Each data point represents the mean BGH levels in the blood and in pituitary tissue for a group of rats undergoing the same treatment (Figure 6.19). The two highest points of the scatterplot represent groups of rats whose slow-twitch muscles were activated, while the five lowest points on the scatterplot involved the activation of fast-twitch muscles in separate groups of rats. The point (37.6, 19.3) comes from a group of rats that were exercised on a treadmill to activate fast- and slow-twitch muscles simultaneously. The remaining points represent groups that were untreated.

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Figure 6.18: Figure 6.18 This scatterplot of BGH level in pituitary tissue versus BGH level in the blood shows a strong negative association.
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Figure 6.19: Figure 6.19 Scatterplot of BGH level in pituitary tissue versus BGH level in the blood with specific groups of rats identified by treatment.

These data come from an experiment that assigned rats randomly to treatment (or no treatment) conditions. Random assignment makes us reasonably confident that slow-twitch muscle activation causes a decrease in BGH secretion (and hence lower levels of blood BGH) and that fast-twitch muscle activation causes an increase. We will discuss experiments in detail in Chapter 7.