Inhibition by cyclic AMP of basal and induced inositol phosphate production in cultured aortic smooth muscle cells from Wistar-Kyoto and spontaneously hypertensive rats

Objective To investigate inositol phosphate formation and its modulation by the cyclic AMP (cAMP) pathway in cultured aortic smooth muscle cells from spontaneously hypertensive rats (SHR). Methods Phenylephrine was used to stimulate inositol phosphate formation in cultured aortic smooth muscle cells from SHR and Wistar-Kyoto (WKY) rats. The smooth muscle cells from passages 6-14 were prelabelled with myo-[2-H-3]-inositol (1.9 x 10(5) Bq/ml for 24 h) and inositol phosphate formation was measured after exposure to agonist for 45 min. (-)Isoproterenol or forskolin-induced cAMP formation was also evaluated using a radioimmunoassay method. Results The basal level of inositol phosphate formation in smooth muscle cells from SHR was higher than that observed in smooth muscle cells from WKY rats. Phenylephrine increased the formation of inositol phosphates in a concentration-dependent manner (0.1-100 mu mol/l). In the presence of 100 mu mol/l phenylephrine, the increase in inositol phosphate formation was significantly greater in smooth muscle cells from SHR (214 +/- 6%) than that observed in smooth muscle cells from WKY rats (156 +/- 8%). When the cells were pretreated with 1 mmol/l 8-bromoadenosine 3':5'-cyclic monophosphate or with 10 mu mol/l forskolin for 45 min, the basal production of inositol phosphates in smooth muscle cells both from SHR and from WKY rats was significantly and similarly decreased by about 20%. In the presence of 1 mmol/l 8-bromoadenosine 3':8'-cyclic monophosphate, 100 mu mol/l phenylephrine-induced inositol phosphate formation was similarly decreased by 33 +/- 4 and 30 +/- 3% in smooth muscle cells from SHR and from WKY rats, respectively whereas, in the presence of 10 mu mol/l forskolin, inositol phosphate formation was reduced by 25 +/- 3 and 27 +/- 5%, respectively in those cells. In contrast, isoproterenol induced less inhibition of phenylephrine-induced inositol phosphate formation in smooth muscle cells from SHR (14 +/- 2%) than it did in those from WKY rats (25 +/- 4.5%). Although there was no significant difference in basal or forskolin-induced cAMP accumulation between smooth muscle cells from SHE and those from WKY rats, (-)isoproterenol-induced cAMP accumulation was significantly lower in smooth muscle cells from SHR. Conclusion A marked inhibitory effect of cAMP on the alpha(1)-adrenoceptor-mediated inositol phosphate signal transduction pathway was demonstrated in smooth muscle cells of SHR and of WKY rats, Decreased cAMP formation with beta-adrenergic stimulation and increased inositol phosphate formation with alpha-adrenergic stimulation in SHR smooth muscle cells may both contribute to the dominant alpha(1)-adrenergic activity observed in SHR.