We have studied why rat pineal N-acetyltransferase (NAT) activity is relatively insensitive to isoproterenol in young rats when compared with adult rats. We report that activation by isoproterenol of pineal cyclic AMP production and NAT activity is higher in adult than in 2-week-old rats. However, the effect of dibutyryl cyclic AMP, which enters the pinealocyte and duplicates the effect of cyclic AMP, on NAT activity was similar at both ages. Moreover, we found that both α and β-adrenergic receptors are highly specific at both ages, since the binding of the specific radioligands used to their receptors could be displaced only by their corresponding agonists and antagonists. However, we observed differences between pineals from young and adult rats when several families of the α subunit of G-proteins were studied in cell membranes. ADP-ribosylation and immunoblot studies have shown clear differences in both 42 and 45 kDa forms of the G(sα). Both forms exhibit low values in pineals from 2-week-old animals when compared with 6-week-old. We also show that the later appearance of both G(sα) forms is roughly similar to the potent activation of cyclic AMP production and NAT activity in adult rats when compared with young rats. In conclusion, the results presented suggest that the relative lack of sensitivity of rat pineal gland to β- adrenergic receptor agonists early in the postnatal development may be explained by the low levels of membrane G(sα), rather than postreceptor- mediated mechanisms or changes in the characteristics of the β-adrenergic receptors on the pinealocyte membrane.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism