Nitrate eyed as endocrine disrupter
Janet Pelly

The nitrate in manure and fertilizer runoff is now under suspicion as a potential endocrine disrupter, according to new research from developmental endocrinologist Lou Guillette of the University of Florida. Although more work needs to be done to demonstrate cause and effect, scientists say the findings could have tremendous implications for water quality standards.

A retrospective study of seven Florida lakes shows that as nitrate-nitrogen concentrations in lake water rise above 10 parts per million (ppm), the limit for drinking water, testosterone levels in juvenile male alligators fall by 50% and the animals have smaller penises, Guillette told attendees at a water monitoring conference in Ames, Iowa on February 19.

Guillette stumbled across the association between nitrate and depressed hormone expression when he found low levels of testosterone in alligators from eutrophic lakes with only traces of pesticides but high levels of nitrogen. The testosterone levels were similar to those of alligators inhabiting lakes with high levels of pesticide contamination.

By analyzing the data statistically using linear regression, Guillette found a dramatic association between the low testosterone levels and high nitrogen concentrations in the water. “We know that exposure of young alligators to organochlorine pesticides can alter the synthesis of hormones, but could nitrate in the environment continue to depress testosterone production?” Guillette warns that “these results are very preliminary and we need to go out and measure concentrations of testosterone and nitrate in the blood and urine of individual alligators.”

“Guillette’s results fit well with my studies showing that corticosterone and testosterone levels dropped by half in rats given 50 ppm sodium nitrate in their drinking water for four weeks,” says Nirmal Panesar, a steroid endocrinologist at the Chinese University of Hong Kong. He has also published research showing that exposure of testosterone-producing cells from mouse testes to nitrate shuts down their synthesis of testosterone.

“These experiments show indirectly that mammalian cells can reduce nitrate to nitric oxide,” Panesar says. Nitric oxide acts as a hormone in the body, inhibiting hormone synthesis in the testis or causing vasodilation, he adds. For instance, nitric oxide action is the mechanism behind the anti-impotence drug, Viagara, but too much nitric oxide during early developmental stages appears to depress testosterone synthesis, which is necessary for normal development of the penis, Guillette says.

For a long time, scientists thought that nitric oxide in the body was only synthesized from the amino acid L-arginine. But in the mid-1990s, the cardiovascular literature began to include reports showing that mitochondria in the cell can reduce nitrate from food and water to nitric oxide, Guillette says. Nitric oxide is a potent inhibitor of cytochrome P-450, a key group of enzymes that help synthesize steroids, Panesar adds.

“It appears that Guillette is seeing nitrate act as an endocrine disrupter and if his research is published it may intensify efforts to look at nitrate,” says Peter Weyer, associate director of the Center for Health Effects of Environmental Contamination at the University of Iowa. The nation’s waters are seriously contaminated with nitrate, and many Iowa streams report concentrations of 10–20 ppm nitrate-nitrogen, well over the level where Guillette is observing effects, he says. In fact, because Guillette’s preliminary work is finding an endocrine-disrupting effect of nitrate at levels around EPA’s nutrient criteria of 3.26 ppm nitrate-nitrogen, it may mean that nitrogen standards to protect wildlife and human health may have to be dropped even lower, adds Mary Skopec, research scientist with the Iowa Geological Survey.