Incubation of cultured rat aortic smooth muscle cells (A-10, ATCC CRL 1476) with [8-arginine]vasopressin (AVP) or thrombin increased the amount of DNA strand breakage induced by camptothecin, an inhibitor of topoisomerase I (DNA topoisomerase; EC 5.99.1.2) and transiently stimulated the extractable activity of this enzyme. Both topoisomerase-related responses were prevented by treatment of the cells with AVP or thrombin plus the appropriate receptor antagonist. The increase in strand breakage mediated by AVP and thrombin depended on the concentration of hormone. Neither AVP nor thrombin had any effect on strand breaks obtained with the epipodophyllotoxin VM-26, an inhibitor of topoisomerase II [DNA topoisomerase (ATP-hydrolysing); EC 5.99.1.3]. Pretreatment of the cells with pertussis toxin partially inhibited thrombin-mediated increases in camptothecin-induced strand breakage whereas AVP-mediated increases were unaffected. These results are consistent with the notion that AVP and thrombin induce a transient increase in intracellular topoisomerase I activity via interactions with their respective cell surface receptors and that the effects of the activation of these receptors are mediated by different G-proteins.