Interactions Of Responding Systems

Maintaining the integrity of the internal environment or successfully meeting an external challenge typically involves the coordinated interplay of several physiologic systems and the integration of multiple hormonal signals. Solutions to physiologic problems require integration of a large variety of simultaneous events that together may produce results that are greater or less than the simple algebraic sum of the individual hormonal responses. In the following section, we consider some of the ways in which endocrine regulatory systems may interact.

Reinforcement

Although a hormone may trigger an overall cellular response by affecting some fundamental rate-determining reaction, several cellular processes may be affected simultaneously. Hormonal effects exerted at several locales within a single cell reinforce each other and sum to produce the overall response. Let us consider, for example, just some of the ways insulin acts on the fat cell to promote storage of triglycerides:

• It acts at the cell membrane to increase availability of substrate for triglyceride synthesis.

• It activates several cytosolic and mitochondrial enzymes critical for fatty acid synthesis.

Interactions of Responding Systems

• It inhibits breakdown of already formed triglycerides.

• It induces synthesis of the extracellular enzyme lipoprotein lipase needed to take up lipids from the circulation.

Any one of these actions might accomplish the end of increasing fat storage, but collectively, these different effects make possible an enormously broader range of response in a shorter time frame. These effects of insulin will be considered in detail in Chapter 41.

Reinforcement can also be observed at the level of the whole organism, where a hormone may act in different ways in different tissues to produce complementary effects. A good example of this is the action of cortisol to promote hepatic synthesis of glucose. It acts in extrahepatic tissues to mobilize substrate and in the liver to increase conversion of these substrates to glucose (see Chapter 40). Either the extrahepatic action or the hepatic action would increase glucose synthesis, but together, these complementary actions reinforce each other and increase both the magnitude and rapidity of the overall response.

Redundancy

Fail-safe mechanisms govern crucial functions. Just as each organ system has built-in excess capacity, giving it the potential to function at levels beyond the usual day-to-day demands, so too is excess regulatory capacity provided in the form of seemingly duplicative or overlapping controls. Simply put, the body has more than one way to achieve a given end. For example (Fig. 12), conversion of liver glycogen to blood glucose can be signaled by at least two hormones, glucagon from the alpha cells of the pancreas and epinephrine from the adrenal medulla (see Chapters 40 and 41). Both of these hormones increase cAMP production in the liver, and

Glucose

Liver

Liver

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