Similar vascular pathological conditions are observed in diabetic animals and those with diet-induced hypergalactosemia. Both diabetes and hypergalactosemia are believed to cause vascular dysfunction via a common biochemical mechanism. In this study, we have found that both diabetes and hypergalactosemia in the short term (2-4 months) can increase total diacylglycerol (DAG) levels by 52 +/- 9 and 74 +/- 13% in the retina and aorta, respectively, of diabetic dogs, and by 94 +/- 9 and 78 +/- 11% in the retina and aorta, respectively, in dogs with hypergalactosemia as compared with normal control animals (P < 0.01). The elevation of DAG levels was maintained for 5 years in the aortas of diabetic and hypergalactosemic dogs. To characterize the mechanism of the DAG increases, we have determined that total DAG levels were significantly increased in cultured macro- and microvascular cells exposed to elevated glucose (22 mM) and galactose (16.5 mM) levels. These increased levels were not prevented by sorbinil, an aldose reductase inhibitor. One of the sources of the increased DAG levels was probably derived from de novo synthesis from both hexoses as determined by radiolabeling studies. Intracellularly, the DAG elevation activated protein kinase C (PKC) activity with increases of 58 +/- 12% (P < 0.05) and 66 +/- 8% (P < 0.01) in the membrane fraction of cultured aortic smooth muscle cells exposed to elevated glucose and galactose levels, respectively. These findings have clearly demonstrated a possible common biochemical mechanism by which hyperglycemia and hypergalactosemia can chronically activate the DAG-PKC pathway in the vasculature and could be a possible explanation for the development of diabetic vascular complications.