While risk assessment models attempt to predict human risk to toxicant exposure, in many cases these models cannot account for the wide variety of human responses. This review addresses several primary sources of heterogeneity that may affect individual responses to drug or toxicant exposure. Consideration was given to genetic polymorphisms, age-related factors during development and senescence, gender differences associated with hormonal function, and preexisting diseases influenced by toxicant exposure. These selected examples demonstrate the need for additional steps in risk assessment that are needed to more accurately predict human responses to toxicants and drugs.

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The goal of this study was to further investigate the role of endogenous APGWamide levels in imposex induction in snails. APGWamide is a common neurotransmittor/neuromodulator peptide found in many species of molluscs, and is often related to sex organ growth or reproductive behavior. Mud snails (Ilyanassa obsoleta) were collected from the Rachel Carson Estuarine Reserve near Beaufort, NC, and were dosed with the environmental contaminant tributyltin (TBT), testosterone (T), or solvent vehicle (EtOH/ saline) controls. Both TBT and T have been shown previously to induce female snails to grow penises (a condition termed imposex), and to increase male penis size. Male normalized penis length was correlated to endogenous APGWamide levels (as measured by Western blotting of whole animal homogenates, r(2) = 0.475), and control males had significantly higher APGWamide levels than control females. All TBT-treated animals, (male, female, and imposex) had levels of APGWamide similar to control males and significantly higher than control females. In testosterone treated animals, APGWamide levels were the same as controls and it is likely that testosterone interferes with a downstream signaling event to induce imposex. In addition, immunohistochemistry for APGWamide expression in abdominal areas was done on female, male and imposex snails collected from the wild. The pattern of APGWamide in imposex snails was similar to male snails, showing large patches of immuno-reactive areas in the top portion of the visceral mass. In female snails, no areas of cross-reactivity were found.