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The relationship between the number of β-adrenoceptors and polyploidy in cultured aortic smooth muscle cells derived from different age groups of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were examined.The number of β-adrenoceptors, the percentage of multinucleated cells and the incidence of polyploidy from cultured smooth muscle cells derived from SHR and WKY rats aged 3-4, 10-12 and 28-30 weeks were measured. The effect of passaging of the cells on the expression of β-adrenoceptors and polyploidy on cultured smooth muscle cells from both SHR and WKY rats was also investigated.Receptor binding experiments were carried out using -monoiodocyanopindolol with osmotically lysed cultured aortic smooth muscle cells to investigate the properties of vascular β-adrenoceptors in SHR and WKY rats. The proportion of polyploid smooth muscle cells was determined by frequency distribution analyses of Feulgen DNA microdensitometric measurements.The incidence of polyploid smooth muscle cells was consistently higher in cells cultured from SHR than in those from WKY rats in all three age groups, with a positive correlation between polyploidy and age in SHR. Furthermore, in all three age groups the number of β-adrenoceptor binding sites was also higher in cultured smooth muscle cells from SHR than in those from WKY rats. There was no significant difference in the receptor affinity. The increase in β-adrenoceptor number was associated with an increase in polyploidy, and both of these changes were positively correlated both with the age of the rats from which these cells were derived and with the number of passages.Under cell culture conditions the expression of P-adrenoceptor density increases with the number of passages in both SHR and WKY rats. Smooth muscle cells derived from older SHR and WKY rats have a greater propensity to develop polyploidy. This trend is significantly accelerated in cultured smooth muscle cells derived from SHR compared with those from WKY rats, suggesting a premature ageing process. These findings suggest that, in cultured smooth muscle cells from SHR and WKY rats, β-adrenoceptors may influence the expression of polyploidy.