Vitamin E, an antioxidant, improves insulin sensitivity through the suppression of conventional PKC in vascular smooth muscle cells. It has been reported that vitamin E reduces platelet aggregation through the suppression of PKC alpha and beta (Diabetes 47 (1998) 1494). On the other hand, 1 alpha,25-dihydroxy vitamin D3 (1,25D3) activates conventional PKC and may subsequently cause insulin resistance. Against this background, we examined the effect of vitamin E and 1,25D3 on PKC beta and PKC zeta/lambda activities in vitro and 10 nM insulin-induced glucose uptake in rat adipocytes. In vitro PKC beta activity of adipocytes was slightly decreased by the addition of 1 microM vitamin E, but not PKC zeta/lambda activity. In contrast, a 10-1000 nM 1,25D3 dose responsively activated PKC beta activity of adipocytes (ED 50%, 10 nM), but not PKC zeta/lambda activity. Pretreatment with 1 microM vitamin E for 60 min did not improve the insulin-induced glucose uptake. On the other hand, pretreatment with a 10-1000 nM 1,25D3 dose responsively suppressed insulin-induced glucose uptake. Moreover, 1,25D3 increased membrane-associated PKC beta immunoreactivity for 60 min, but no additional increase in membrane-associated PKC beta immunoreactivity during treatment with insulin was observed. These results suggest that 1,25D3 reduces insulin-induced glucose uptake via activation of PKC beta, but not vitamin E in rat adipocytes.