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Forskolin stimulation of naphthylamidase in guinea pig thyroid sections detected with a cytochemical bioassay.

Forskolin, from the roots of the Indian medicinal plant Coleus forskohlii, has recently been shown to be a potent stimulator of adenylate cyclase in many systems, including endocrine tissues such as the thyroid gland. We describe forskolin activation of beta-naphthylamidase activity in guinea pig thyroid tissue using the cytochemical bioassay (CBA) for thyroid stimulators. This CBA is the most sensitive bioassay for TSH and LATS-B currently available, being able to detect stimulation by doses as low as 10(-5) mU TSH/l and 10(-9) mU LATS-B/l. The dose-response curve to forskolin was bell-shaped (as is seen with TSH and LATS-B) with the ascending limb of the curve produced by 10(-13) M to 10(-12) M forskolin after a 3 min exposure time. Maximal stimulation was observed with 10(-12) M forskolin. However, the dose-response curve to forskolin was not parallel to that given by TSH, the slope of the ascending limb being much greater. It has been suggested that stimulation of beta-naphthylamidase activity in the CBA is via cAMP. We report that dibutyryl cAMP at doses from 10(-16) M to 10(-11) M produces a bell-shaped dose-response curve with a very broad peak response, again not parallel to that produced by TSH. Forskolin activation of beta-naphthylamidase in the CBA is unaffected by a 1:10(6) dilution of 11E8, a monoclonal antibody raised against solubilised TSH receptors, which binds to the TSH receptor and inhibits TSH stimulation. Although the precise location of forskolin action is not known, this is further evidence that forskolin acts at a post-surface receptor site.