Competitive interactions among H, Cu, and Zn ions moderate aqueous uptake of Cu and Zn by an aquatic insect.

The absorption of aqueous copper (Cu) and zinc (Zn) by aquatic insects, a group widely used to assess water quality, is unresolved. This study examined interactions among Cu, Zn, and protons that potentially moderate Cu and Zn uptake by the acid-tolerant stonefly Zapada sp. Saturation uptake kinetics were imposed to identify competitive mechanisms. Decreasing pH reduced the maximum transport capacity, Jmax, in both metals, had little effect on the Cu dissociation constant, KD, and increased the Zn KD. Partial noncompetitive (Cu) and partial mixed competitive (Zn) inhibitor models most closely tracked the observed Cu and Zn influx rates across pH treatments. The estimated values for acid dissociation constants for the binary (proton-receptor) and ternary (proton-metal-receptor) complexes indicated the strong inhibitory effect of protons on Cu and Zn. In neutral pH water, Cu inhibited Zn influx, but Zn had little effect on Cu influx. The mechanism of Cu-Zn interaction was not identified. Results from separate Zn experiments suggested that the insect's developmental stage may affect the apparent Jmax. The study underscores some of the challenges of modeling metal bioaccumulation and informs future research directions.

Using biodynamic models to reconcile differences between laboratory toxicity tests and field biomonitoring with aquatic insects.

Aquatic insects often dominate lotic ecosystems, yet these organisms are under-represented in trace metal toxicity databases. Furthermore, toxicity data for aquatic insects do not appear to reflect their actual sensitivities to metals in nature, because the concentrations required to elicit toxicity in the laboratory are considerably higher than those found to impact insect communities in the field. New approaches are therefore needed to better understand how and why insects are differentially susceptible to metal exposures. Biodynamic modeling is a powerful tool for understanding interspecific differences in trace metal bioaccumulation. Because bioaccumulation alone does not necessarily correlate with toxicity, we combined biokinetic parameters associated with dissolved cadmium exposures with studies of the subcellular compartmentalization of accumulated Cd. This combination of physiological traits allowed us to make predictions of susceptibility differences to dissolved Cd in three aquatic insect taxa: Ephemerella excrucians, Rhithrogena morrisoni, and Rhyacophila sp. We compared these predictions with long-term field monitoring data and toxicity tests with closely related taxa: Ephemerella infrequens, Rhithrogena hageni, and Rhyacophila brunea. Kinetic parameters allowed us to estimate steady-state concentrations, the time required to reach steady state, and the concentrations of Cd projected to be in potentially toxic compartments for different species. Species-specific physiological traits identified using biodynamic models provided a means for better understanding why toxicity assays with insects have failed to provide meaningful estimates for metal concentrations that would be expected to be protective in nature.

Effects of denervation on laryngeal muscles: a canine model.

The purpose of this study was to chronologically evaluate the changes in functional and histomorphometry of denervated laryngeal muscles. In 14 adult mongrel dogs, a 2.5-cm segment of the right recurrent laryngeal nerve was excised. Videolaryngoscopy and electromyography were performed at 1, 2, 3, 4, 5, 6, and 9 months under intravenous sedation. The animals were then killed, and the laryngeal muscles were processed for histochemical reactions. The mean muscle fiber diameter, standard deviation, and muscle fiber type composition were determined. The findings indicate that, following recurrent laryngeal nerve sectioning, the canine intrinsic laryngeal muscles undergo denervation atrophy for approximately 3 months, after which reinnervation occurs. The source of reinnervation appears to be from regenerated nerve fibers of the sectioned recurrent laryngeal nerve. The nerve fibers nonselectively reinnervated the abductor and adductor muscles of the larynx.