Comparison of the acute toxicity of zygacine versus zygadenine.

Foothill death camas (Zigadenus paniculatus) is a common poisonous plant found throughout western North America. The toxic alkaloids in foothill death camas are zygadenine, esters of zygadenine, with zygacine, the 3-acetyl ester of zygadenine, often being the most abundant. Two additional esters of zygadenine that are found primarily in the floral parts of foothill death camas are 3-angeloylzygadenine and 3-veratroylzygadeine. Recent research has shown that very little zygacine is detected in the blood of animals dosed with zygacine. A recent investigation into the metabolism of zygacine demonstrated that zygacine is rapidly metabolized to zygadenine, demonstrating a clear first pass effect. The objective of this study was to determine if there is a difference in the acute toxicity of zygacine and zygadenine to mice and sheep. Additionally, two other esters of zygadenine, 3-angeloylzygadenine and 3-veratroylzygadenine, were evaluated for their acute toxicity in a mouse IV LD50 assay. All three esters of zygadenine tested were more toxic than zygadenine, with the following rank order of toxicity in the mouse IV LD50 assay: zygadenine-HCl (59.5 mg/kg) < zygacine-HCl (1.6 mg/kg) < angeloylzygadenine-HCl (1.0 mg/kg) < veratroylzygadenine-HCl (0.5 mg/kg). Similar to the results of the mouse experiments, zygacine-HCl was significantly more toxic than zygadenine-HCl in sheep dosed IV with pure compounds. Sheep dosed with 1.25 mg/kg zygacine-HCl showed severe clinical signs of poisoning. Whereas a dose of 12.5 mg/kg zygadenine-HCl was required to elicit a similar onset and severity of clinical signs. Overall, these data indicate that zygacine is more toxic than zygadenine when administered IV, when first pass metabolism is bypassed.

Identification of two death camas chemotypes within a population and evaluation of toxicity.

Foothill death camas (Zigadenus paniculatus) is a native, cool-season, bulbous perennial forb found throughout the western U.S. The toxins in death camas are steroidal alkaloids. Zygacine is often the most abundant alkaloid in death camas and is believed to be the primary toxic component. A population of death camas with plants consisting of two different chemical profiles (chemotypes) growing within the same location were identified. The objective of this study was to determine the percentage of a death camas population represented by each of the different chemotypes and to determine if there was a difference in toxicity between the two chemotypes. One third of the population sampled consisted of chemotype 1, while two-thirds of the population consisted of chemotype 2. The zygacine concentration of chemotype 1 was three times higher than chemotype 2. Chemotype 2 contained higher concentrations of several other steroidal alkaloids than chemotype 1. We hypothesized that chemotype 1, which consisted of higher concentrations of zygacine, would be more toxic than chemotype 2. The acute toxicity of each chemotype was determined in mice and sheep. In the mouse LD50 study, the acute toxicity of the chemotype 1 alkaloids (2.3 mg/kg BW) was different than the chemotype 2 alkaloids (3.2 mg/kg BW). However, in the sheep study there were no differences in the adverse effects between chemotypes. Based upon the results of this study, caution should be taken when livestock are grazing death camas, as both chemotypes of death camas appear to pose a similar risk to grazing livestock.

The Effect of Co-Administration of Death Camas (Zigadenus spp.) and Low Larkspur (Delphinium spp.) in Cattle.

In many rangeland settings, there is more than one potential poisonous plant. Two poisonous plants that are often found growing simultaneously in the same location in North American rangelands are death camas (Zigadenus spp.) and low larkspur (Delphinium spp.). The objective of this study was to determine if co-administration of death camas would exacerbate the toxicity of low larkspur in cattle. Cattle dosed with 2.0 g of death camas/kg BW showed slight frothing and lethargy, whereas cattle dosed with both death camas and low larkspur showed increased clinical signs of poisoning. Although qualitative differences in clinical signs of intoxication in cattle co-treated with death camas and low larkspur were observed, there were not any significant quantitative differences in heart rate or exercise-induced muscle fatigue. Co-treatment with death camas and low larkspur did not affect the serum zygacine kinetics, however, there was a difference in the larkspur alkaloid kinetics in the co-exposure group. Overall, the results from this study suggest that co-exposure to death camas and low larkspur is not significantly more toxic to cattle than exposure to the plants individually. The results from this study increase our knowledge and understanding regarding the acute toxicity of death camas and low larkspur in cattle.

Ecophysiology of Zigadenus nuttallii, a toxic spring ephemeral in a warm season grassland: effect of defoliation and fire.

Zigadenus nuttallii, a highly toxic spring ephemeral in tallgrass prairie, was studied in 1985 to ascertain: 1) several ecophysiological characteristics of the species, 2) seasonal patterns of biomass accumulation, and 3) its response to defoliation and fire. The maximum photosynthetic rate of Z. nuttallii measured in unburned prairie was 13.2 µmoles CO2 m-2 s-1 which occurred at 24-28° C and an incident quantum flux of 0.8-1.0 mmoles m-2 s-1. Maximum stomatal conductance measured was 5.4 mm s-1. Early in the season, belowground storage organs (bulbs) decreased in mass and supplied much of the energy for growth of leaves, even though CO2 uptake was possible. Buld mass did not increase until about 6 weeks after shoot emergence implying that, at this time, leaves had become a source rather than a sink for carbohydrates. The result of a single, severe defoliation event was a decrease in biomass of bulbs, leaves and reproductive structures in Z. nuttallii. Intrinsic compensatory mechanisms were not detected. In contrast, fire, which also defoliated plants, did not result in any biomass decrease at the end of the season. Improved post-fire microclimate and increased nutrient supply (extrinsic factors) may have contributed to higher photosynthetic rates and led to biomass compensation in burned prairie. These data support arguments that intrinsic compensatory mechanisms have evolved in response to chronic herbivory.

The effect of low larkspur (Delphinium spp.) co-administration on the acute toxicity of death camas (Zigadenus spp.) in sheep.

In most cases where livestock are poisoned by plants in a range setting, there is more than one potential poisonous plant in the same area. Two poisonous plants that are often found growing simultaneously in the same location are death camas (Zigadenus spp.) and low larkspur (Delphinium spp.). Sheep are known to be susceptible to death camas poisoning while they are thought to be resistant to larkspur. The objective of this study was to determine if co-administration of low larkspur would exacerbate the toxicity of death camas in sheep. A dose finding study was performed to find a dose of death camas that caused minimal clinical signs of poisoning. Sheep were observed for clinical signs of poisoning as well as changes in heart rate and muscle fatigue. Sheep dosed with 1.14 g of death camas per kg BW showed slight frothing and lethargy, whereas sheep dosed with death camas and low larkspur showed slightly more noticeable clinical signs of poisoning. Sheep dosed with only low larkspur, at 7.8 g/kg BW, showed no signs of poisoning. Although we observed a qualitative difference in clinical signs of intoxication in sheep co-treated with death camas and low larkspur we did not detect any quantitative differences in heart rate, exercise-induced muscle fatigue, or differences in serum zygacine kinetics. Consequently, the results from this study suggest that low larkspur does not affect the toxicity of death camas in sheep. The results from this study increase knowledge and understanding regarding the acute toxicity of death camas and low larkspur in sheep. As combined intoxications are most likely common, this information will be useful in further developing management recommendations for ranchers and in designing additional experiments to study the toxicity of death camas to other livestock species.