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JB Pracyk and TA Slotkin
Department of Pharmacology, Duke University Medical Center, Durham, North Carolina.
In mature animals, thyroid hormone is permissive for beta adrenergic receptor expression and adrenergic control of adenylate cyclase. To determine if endogenous thyroid hormones play a similar role in the development of receptors and transduction mechanisms, we administered propylthiouracil perinatally to rat dams and pups from gestational day 17 through postnatal day 5. Circulating thyroid hormones were completely suppressed through postnatal day 10 and then rose to only slightly subnormal values by the 3rd to 4th postnatal week. In the heart, hypothyroidism completely suppressed the initial development of beta adrenergic receptor binding sites, with recovery paralleling the return of thyroid hormone levels. In contrast, development of basal and isoproterenol-stimulated adenylate cyclase activity showed more lasting deficiencies with a delayed onset corresponding to general growth impairment; however, forskolin-stimulated adenylate cyclase developed in a nearly normal pattern. Effects on development of renal beta receptors and adenylate cyclase were of smaller magnitude and comprised only the delayed onset phase; receptor deficiencies appeared after 10 days and adverse effects on adenylate cyclase were limited to the isoproterenol-sensitive component, consisting of a shift of the ontogenetic peak to later ages. Endogenous thyroid hormones thus contribute two distinct factors to beta receptor/adenylate cyclase development: they are obligatory for cardiac beta receptor development, but also, in parallel with general effects on growth and development, serve to program the ontogeny of transduction factors linking the receptors to adenylate cyclase. The predominance of propylthiouracil effects on isoproterenol-stimulated adenylate cyclase but not on enzymatic responses to forskolin suggests that thyroid hormones may be controlling the development of regulatory G-proteins.
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