Beta- and alpha-adrenergic mechanisms are involved in regulation of rat pineal type II thyroxine 5'-deiodinase activity during development

Abstract

Adrenergic regulation of type II T4 5'-deiodinase (5'-D) activity has been studied in the rat pineal gland during development. Nocturnal increases in 5'-D activity gradually rose from the second week of age until the sixth week. Isoproterenol, a beta-adrenergic agonist, stimulated 5'-D activity during the daytime; however, the pattern of stimulation during development was slightly different from nocturnal activation of the enzyme, since pineal 5'-D activity could be clearly activated by isoproterenol from the first week of age. Isoproterenol activation of the enzyme always reached more than 70% of nocturnal values at all ages studied. Phenylephrine, an alpha-adrenergic agonist, did not increase the enzyme activity. Another selective alpha-adrenergic agonist, methoxamine, as well as clonidine, an alpha 2-adrenergic agonist, were ineffective in stimulating the enzyme, while norepinephrine or terbutaline, a beta-adrenergic agonist, clearly activated it. Additional results showing a role for alpha-adrenergic receptors on pineal 5-D activity were obtained when alpha- and beta-adrenergic receptor blockers were used. Thus, propranolol, a beta-adrenergic blocker, clearly prevented pineal 5'-D activation in both 2- and 6-week-old rats when the gland was stimulated by either norepinephrine or darkness at night. However, prazosin, a selective alpha 1-adrenergic receptor blocker, also prevented the norepinephrine-induced or darkness-induced activation of the enzyme in 2-week-old rats, although it was ineffective when used in 6-week-old rats. Moreover, at night and after 3 h of exposure to darkness, phenylephrine was able to activate the enzyme. Results strongly suggest a role for alpha-adrenergic receptors, in addition to beta-adrenergic receptors, in regulating rat pineal 5'-D activity during development.