We have recently shown that vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit enhanced expression of Gialpha proteins, which was attributed to the enhanced oxidative stress. Since C-ANP(4-23) that specifically interacts with natriuretic peptide C (NPR-C) receptor has been shown to decrease the expression of Gialpha protein in VSMC, the present study was undertaken to examine if C-ANP(4-23) can also decrease the enhanced expression of Gialpha protein in VSMC from SHR and whether it is attributed to its ability to attenuate the enhanced oxidative stress. Aortic VSMC from 12-week-old SHR and their age-matched Wistar-Kyoto (WKY) rats were used for the present studies. VSMC from SHR exhibited enhanced expression of Gialpha-2 and Gialpha-3 proteins, different subunits of NADPH oxidase such as Nox(4) and p(47phox) proteins but not of p(22phox), enhanced production of superoxide anion as well as NADPH oxidase activity as compared to age-matched WKY rats. Treatment of VSMC from SHR with C-ANP(4-23) decreased towards control levels the enhanced expression of Gialpha proteins, enhanced superoxide anion production and enhanced NADPH oxidase activity as well as the enhanced expression of Nox(4) and p(47phox). However, C-ANP(4-23)-induced attenuation of the enhanced level of O(2)(-) and NADPH oxidase activity occurs at 4 h before the decrease in the enhanced expression of p(47phox) that occurs at 16 h of C-ANP(4-23) treatment. The decreased expression of NADPH oxidase in SHR was also associated with further decrease in O(2)(-) and NADPH oxidase activity. Furthermore, treatment of VSMC from SHR with pertussis toxin (PT) decreased the enhanced levels of superoxide anion as well as NADPH oxidase activity; however, the enhanced levels of different subunits of NADPH oxidase were not attenuated by PT treatment. These results suggest that C-ANP(4-23) decreases the enhanced oxidative stress in SHR by attenuating the enhanced expression of Gialpha proteins and also the enhanced levels of NADPH oxidase.