Cross talk between angiotensin AT1 and alpha 1-adrenergic receptors: angiotensin II downregulates alpha 1a-adrenergic receptor subtype mRNA and density in neonatal rat cardiac myocytes.

Signaling mediated by the angiotensin (Ang) II and alpha1-adrenergic receptor (alpha1-AR) pathways is important for cardiovascular homeostasis. However, it is unknown whether Ang II has any direct effect on alpha1-AR expression and signaling in cardiac myocytes. In the present study, we determined alpha1-AR subtype mRNA levels by RNase protection; receptor density by competition binding with 5-methylurapidil; and alpha1-AR-mediated c-fos expression by Northern blot analysis. We found that Ang II had no effect on alpha1b- and alpha1d-AR mRNA levels but decreased the alpha1a-AR mRNA level in a time- and dose-dependent manner. The maximal effect occurred at 6 hours with 100 nmol/L Ang II (40.0+/-8.2% reduction, n=4, P<.01). The decrease in alpha1a-AR mRNA level induced by Ang II is mediated by the Ang II AT1 receptor subtype and is associated with decreased stability of alpha1a-AR mRNA. Corresponding to the changes in the alpha1a-AR mRNA level, Ang II (100 nmol/L, 24 hours) reduced the density of high-affinity sites for 5-methylurapidil (alpha1A-AR) by 29% (56.5+/-6.4 versus 79.0+/-11.6 fmol/mg protein, n=4, P<.05). Alpha1-AR-stimulated c-fos induction, which could be blocked by 5-methylurapidil but not by chloroethylclonidine, was attenuated by Ang II preincubation (100 nmol/L, 24 hours). We conclude that there is previously undescribed cross talk between AT1 receptors and alpha1-ARs. Ang II selectively downregulates alpha1a-AR subtype mRNA and its corresponding receptor as well as alpha1a-AR-mediated expression of the immediate-early gene c-fos in cardiac myocytes.