Clues to how lead impairs growth, vision.

Clues to how lead impairs growth, vision

For at least 2,000 years, societies have recognized lead as a potent toxicant while remaining mystified as to how it poisons. But as more and more researchers address the question, an understanding is beginning to emerge. Now, in a pair of animal studies, scientists have identified important new clues to how the heavy metal retards growth and damages visual processing areas of the brain.

Researchers at the University of Cincinnati conducted a series of rat experiments probing what lies behind one of lead's most subtle effects: a stunting of children's growth (SN: 11/22/86, p. 333). A 1987 report on lead-intoxicated children hinted that the metal might shorten stature by suppressing the thyroid's production of growth hormone. However, even when the Cincinnati team supplied extra growth hormone, rats drinking water contaminated with high lead levels grew more slowly than companions on lead-free water. nor did stunting result from less efficient metabolism of food. When the researchers restrictec led-free rats to the same amount of food as the lead-treated rats chose to eat, both groups grew at the same rate.

It appears lead-intoxicated animals grow more slowly simply because they eat less, conclude Paul B. Hammond and his co-workers in the July TOXICOLOGY AND APPLIED PHARMACOLOGY. Whether they received the toxic metal in drinking water or through tiny controlled-release pumps implanted in their shoulder muscles, lead-exposed rats ate 11.5 to 16.4 percent less than lead-free counterparts when both groups could eat all they wanted. Follow-up work now suggests lead's effects on brain neurotransmitters or hormones probably account for at least some of the appetite suppression observed, says coauthor Robert L. Bornschein.

Lead also appears capable of disturbing the brain's processing of visual signals, according to a second report in the same journal. Researchers with Health and Welfare Canada, in Ottawa, administered lead daily to seven macaques, beginning in infancy. At age 6, the monkeys were killed and their visual system examined.

None showed evidence of damage in either the optic nerve, which receives visual stimuli from the retina, or the region where the optic nerve relays its messages to the brain. However, the three monkeys in the high-dose group-receiving 2,000 micrograms of lead per kilogram of body weight ([mu]g/kg) daily - showed a marked reduction in the volume of nerves within two areas of the visual cortex, compared with the monkeys in the very-low-dose group (receiving 25 [mu]g/kg lead daily). Both brain areas are important in interpreting visual stimuli.

"We don't know if the [observed volume reduction] reflects fewer neurons or just smaller ones. But either would be a toxic effect," says Deborah C. rice of the Ottawa team. Moreover, the two groups of monkeys had structurally different dendrites--tree-like structures at the ends of nerve cells, responsible for taking in messages from other nerve cells. rice says the rduced dendrite branching in the high-dose group suggests these monkeys "may not be able to take in as much information from surrounding cells."

EPA neurotoxicologist David A. Otto of Chapel Hill, N.C., says the Canadian results suggest lead may not affect the transmission of visual information so much as the brain's integration and processing of it. Neurotoxicologist Donald A. Fox agrees, but adds he has demonstrated in several species that the brain is not the only lead-sensitive organ in the visual system. "We've found lead is a selective poison for rods [light receptors] responsible for night and form vision," says Fox, of the University of Houston's College of Optometry. In his own study of lead-poisoned monkeys, which is scheduled to begin soon, Fox will look for rod changes and other visual-system effects.