The main objective of this study was to investigate the role of beta-adrenoceptor activation in the development of polyploidy in these cells. Smooth muscle cells (SMCs) were cultured from 3- to 4-, 10- to 12-, and 28- to 30-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Functional expression of beta-adrenoceptors was examined by treatment of cultured SMCs with a nonselective beta-adrenoceptor agonist, isoproterenol, or an adenylate cyclase activator, forskolin, or a membrane-permeable analog of adenosine 3',5'-cyclic monophosphate (cAMP), 8-bromo-cAMP, and the measurement of intracellular cAMP levels, using a radioimmunoassay. The effects of these treatments on polyploidy development were also studied by measuring the DNA content of SMCs, using scanning microdensitometry. Treatments with isoproterenol or forskolin increased intracellular cAMP levels in both strains of rats in all three age groups. Addition of the beta-adrenoceptor antagonist DL-propranolol inhibited the isoproterenol-stimulated response in SMCs from both SHR and WKY in all three age groups. The number of polyploid SMCs was significantly increased by treatments with isoproterenol, forskolin, or 8-bromo-cAMP in SMCs from 3- to 4-week-old WKY and SHR, but in the 10- to 12-week age group, an increase was found only in SMCs from WKY. Such treatments had no effect on the incidence of polyploid SMCs in the 28- to 30-week groups. We conclude that (i) beta-adrenoceptors expressed by the SMCs from WKY and SHR at all three ages are functional and are coupled via Gs proteins to the stimulation of adenylate cyclase, (ii) treatments that elevate intracellular cAMP levels (by activation of beta-adrenoceptors or of adenylate cyclase) lead to increased polyploid SMCs from WKY and SHR in the younger age group, confirming a role for the beta-adrenoceptor-adenylate cyclase pathway in the development of polyploidy in cultured SMCs from both of these strains of rats, and (iii) the absence of these treatment effects in the induction of polyploid SMCs in older age groups suggests that in these cells, other factors or processes may be involved in regulating the development of polyploid SMCs.