Hildebrandt J D, Kohnken R E
Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545.
J Biol Chem. 1990 Jun 15;265(17):9825-30.
S49 mouse lymphoma cells contain a beta-adrenergic receptor coupled to Gs that stimulates adenylyl cyclase and a somatostatin receptor coupled to Gi that inhibits adenylyl cyclase. Membranes from these cells were used to compare the inhibitory effects of somatostatin and G protein beta gamma complex to determine under what conditions beta gamma is likely to be a mediator of somatostatin action. Somatostatin was equally effective at inhibiting basal adenylyl cyclase activity in the presence of GTP, forskolin-stimulated activity, and hormone-stimulated activity. G protein beta gamma was more effective at inhibiting basal activity than was somatostatin, and these effects were partially additive. In the presence of forskolin, the two inhibitors were equally effective and not additive. In the presence of isoproterenol, beta gamma was much less effective than somatostatin, and the two inhibitors did not have additive or synergistic effects. At very high concentrations beta gamma did inhibit isoproterenol stimulation of adenylyl cyclase, although its effects were not saturating even at 100 micrograms/ml. Under conditions where beta gamma did inhibit hormone stimulation, beta gamma was a mixed inhibitor of isoproterenol stimulation, proportionally decreasing the maximum effect of the hormone and increasing the half-maximally effective concentration. Somatostatin, on the other hand, was a simple noncompetitive inhibitor of isoproterenol stimulation. These results indicate that beta gamma and somatostatin inhibit adenylyl cyclase by different mechanisms, at least in the presence of hormones that stimulate the enzyme. It is proposed that alpha i is the primary mediator of hormone inhibition of adenylyl cyclase when Gs is activated by a hormone, but that beta gamma may have a role as a mediator of inhibition of basal activity.
S49小鼠淋巴瘤细胞含有一种与刺激腺苷酸环化酶的Gs偶联的β-肾上腺素能受体和一种与抑制腺苷酸环化酶的Gi偶联的生长抑素受体。利用这些细胞的膜来比较生长抑素和G蛋白βγ复合物的抑制作用，以确定在何种条件下βγ可能是生长抑素作用的介质。在存在GTP、福斯高林刺激的活性和激素刺激的活性的情况下，生长抑素在抑制基础腺苷酸环化酶活性方面同样有效。G蛋白βγ在抑制基础活性方面比生长抑素更有效，且这些作用部分相加。在存在福斯高林的情况下，这两种抑制剂同样有效且无相加作用。在存在异丙肾上腺素的情况下，βγ比生长抑素的效果差得多，且这两种抑制剂没有相加或协同作用。在非常高的浓度下，βγ确实抑制了异丙肾上腺素对腺苷酸环化酶的刺激，尽管即使在100微克/毫升时其作用也未达到饱和。在βγ确实抑制激素刺激的条件下，βγ是异丙肾上腺素刺激的混合抑制剂，按比例降低激素的最大作用并增加半最大有效浓度。另一方面，生长抑素是异丙肾上腺素刺激的简单非竞争性抑制剂。这些结果表明，βγ和生长抑素通过不同机制抑制腺苷酸环化酶，至少在存在刺激该酶的激素时如此。有人提出，当Gs被激素激活时，αi是激素抑制腺苷酸环化酶的主要介质，但βγ可能作为基础活性抑制的介质发挥作用。
