It has been suggested that a result of streptozotocin (Sz)-initiated damage to beta-cell DNA is activation of the repair enzyme nuclear poly-ADP-ribose (p-ADPR) synthetase and subsequent depletion of cellular NAD. This reduction of NAD is presumed responsible for the impaired islet function. Using freshly isolated rat pancreatic islets, we have examined the ability of p-ADPR synthetase inhibitors, in particular the previously not studied thymidine, to block Sz-induced damage to the B cell. Islets were incubated in 2 mM glucose and 2 mM Sz with or without p-ADPR synthetase inhibitor and then challenged for 1 h with 25 mM glucose plus 25 microM forskolin or 100 nM phorbol ester without glucose. Sz treatment inhibited insulin secretion about 90% even when stimulated by the non-glucose-containing challenge, indicating that Sz caused a rapid and generalized lesion past glucose-specific signals. Thymidine maintained the insulin secretory response to glucose plus forskolin from Sz-treated islets as, or more, effectively than nicotinamide. In a dose-dependent manner, thymidine protected Sz-treated islets to 87% of normal islet secretion at the highest thymidine dose (20 mM). However, even protected islets could not sustain normal secretion; insulin secretion was significantly diminished in thymidine-protected islets challenged with a second consecutive 1-h glucose plus forskolin stimulus. As expected, the acute depletion of islet NAD content (approximately 50%, 1 h after Sz) was reversed to 90% of normal by thymidine. However, even in unprotected Sz-treated islets, NAD content gradually recovered over 48 h of culture in standard RPMI, although insulin secretion remained suppressed. Thus, thymidine may be a useful protective agent against chemically induced islet cell damage. Furthermore, a sustained suppression of NAD content does not explain Sz's permanent inhibition of islet secretory response. Other aspects of inhibited nuclear p-ADP-ribosylation are considered to explain protection against Sz damage to the beta-cell.