Glucagon a Islet Cells
FIGURE 12 Redundant mechanisms to stimulate hepatic glucose production.
thereby activate the enzyme phosphorylase, which catalyzes glycogenolysis. Two hormones secreted from two different tissues, sometimes in response to different conditions, thus produce the same end result.
Redundancy can also be seen at the molecular level. Using the same example of conversion of liver glycogen to blood glucose, there are even two ways that epineph-rine can activate phosphorylase (Fig. 13). By stimulating ^-adrenergic receptors, epinephrine increases cAMP formation as already mentioned. By stimulating a-adrenergic receptors, epinephrine also activates phos-phorylase, but these receptors operate through the agency of increased intracellular calcium concentrations produced by the release of inositol trisphosphate (see Chapter 2).
Redundant mechanisms not only assure that a critical process will take place, but they also offer opportunity for flexibility and subtle fine-tuning of a process. Although redundant in the respect that two different
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