Figure 25–8. Control of adipose tissue lipolysis. (TSH, thyroid-stimulating hormone; FFA, free fatty acids.)Note the cascade sequence of reactions affording amplification at each step. The lipolytic stimulus is “switchedoff” by removal of the stimulating hormone; the action of lipase phosphatase; the inhibition of the lipase andadenylyl cyclase by high concentrations of FFA; the inhibition of adenylyl cyclase by adenosine; and the removalof cAMP by the action of phosphodiesterase. ACTH, TSH, and glucagon may not activate adenylyl cyclase in vivo,since the concentration of each hormone required in vitro is much higher than is found in the circulation. Positive(+ ) and negative (− ) regulatory effects are represented by broken lines and substrate flow by solid lines.
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