Norepinephrine in neonatal rat ventricular myocytes: association with the cell nucleus and binding to nuclear alpha 1- and beta-adrenergic receptors.

Chronic exposure of neonatal rat ventricular myocytes to norepinephrine (NE) has been demonstrated to induce fetal cardiac gene expression and hypertrophy. The precise signaling mechanism of NE induction, as well as the long delay for the onset of NE effect, are not well understood. To examine the possibility that the hormone may be transported into the cell and exerts its effect through an intracellular site, ventricular myocytes from neonatal rats were incubated with [3H]-labeled NE and the cytosolic and nuclear fractions of the cell were measured for radioactivity. The presence of intracellular adrenergic binding sites was also explored. Following incubation of neonatal rat ventricular myocytes with [3H]NE for different time intervals (from 30 min to 22 h), the highest proportion (more than 80%) of NE taken up by the cell was recovered in the nuclear fraction. The nuclear accumulation was slow and time-dependent, being non-detectable in the first 60 min. Furthermore, isolated nuclei from the ventricular myocytes contain binding sites for [3H]prazosin and dihydroalprenolol, suggesting the presence of alpha 1 and beta 1 adrenergic receptors. The apparent KD and Bmax were 0.6 nM and 0.6 fmol/mg protein for alpha 1-adrenergic receptors, while beta-adrenergic nuclear receptors exhibited an apparent KD of 12 nM and a Bmax of 61 fmol/mg protein. Thus, neonatal rat ventricular myocytes exposed to NE accumulate the hormone in the cell nucleus where it can bind to high affinity alpha 1- and beta-adrenergic receptors.