CHOLINERGIC REGULATION OF AMYLASE GENE EXPRESSION IN THE RAT PAROTID GLAND - INHIBITION BY TWO DISTINCT POST-TRANSCRIPTIONAL MECHANISMS

摘要

Stimulation of the beta-adrenergic or cholinergic muscarinic receptors are the principal mechanisms by which parotid salivary secretion is regulated in vivo. In this study we have examined the effects of cholinergic stimulation on amylase gene expression in dispersed rat parotid cells. [H-3]Leucine incorporation into amylase and total protein was inhibited by carbamylcholine. Within 5 min of its addition, 10 mu M carbamylcholine induced a 50-60 % reduction in the rate of amylase synthesis which was sustained for more than 2 h. Blockade of the muscarinic receptor with atropine 8 min after addition of 10 mu M carbamylcholine reversed the carbamylcholine-induced inhibition of amylase synthesis. When cells were exposed to carbamylcholine for 2 h before addition of atropine, there was only a slight reversal of inhibition. Carbamylcholine had no significant effect on the rate of total RNA synthesis but caused a progressive loss of amylase mRNA. After 2 h, amylase mRNA in cells treated with 10 mu M carbamylcholine was 46% of control levels. Actinomycin D (5 mu g/ml) lowered amylase mRNA by 8 %; cycloheximide and phorbol 12-myristate 13-acetate had no effect. Isoprenaline (isoproterenol; at a concentration of 10 mu M), which is an inducer of amylase gene transcription, elevated the amylase mRNA content by 30% after 2 h. The calcium ionophore A23187 mimicked the effect of carbamylcholine by inhibiting [H-3]leucine incorporation into amylase and lowering amylase mRNA content. The results suggest that acute stimulation of the muscarinic cholinergic receptor inhibits amylase biosynthesis in parotid cells not only by rapid attenuation of translation but also by causing a gradual loss of amylase mRNA, apparently by a Ca2(+) dependent destabilization of the mRNA.