Phorbol esters and norepinephrine destabilize alpha 1B-adrenergic receptor mRNA in vascular smooth muscle cells

The mechanism by which norepinephrine (NE) down-regulates alpha 1B-adrenergic receptor (alpha-AR) mRNA was studied in rabbit aortic smooth muscle cells. NE, phorbol esters, and bradykinin each decreased alpha-AR mRNA levels by 70-80%. The protein kinase C inhibitor (+)-1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) abolished the effects of phorbol esters and NE and decreased basal mRNA levels by 52 +/- 3%. Neither ryanodine nor EGTA inhibited down-regulation of alpha-AR mRNA by NE. Actinomycin D caused alpha-AR mRNA level to decrease with a half-life of 3.2 +/- 0.4 h and blocked the effect of H-7 to decrease basal alpha-AR mRNA level. Both NE and phorbol esters increased the rate of alpha-AR mRNA degradation. In NE-desensitized cells, phorbol esters and bradykinin each caused the expected down-regulation of alpha-AR mRNA. The protein phosphatase inhibitor okadaic acid prolonged the normally transient effect of NE for at least 24 h. We conclude that protein kinase C exerts two opposing effects on alpha-AR mRNA levels, 1) a decrease in the stability of the mRNA that requires the sustained phosphorylation of a protein kinase C substrate and 2) a permissive effect on alpha-AR gene transcription.