Research Article: Increase in stable isotope ratios driven by metabolic alterations in amphipods exposed to the beta-blocker propranolol

Date Published: May 16, 2019

Publisher: Public Library of Science

Author(s): Caroline Ek, Andrius Garbaras, Zhenyang Yu, Hanna Oskarsson, Ann-Kristin Eriksson Wiklund, Linda Kumblad, Elena Gorokhova, Samantha E. M. Munroe.


Anthropogenic pressures, such as contaminant exposure, may affect stable isotope ratios in biota. These changes are driven by alterations in the nutrient allocation and metabolic pathways induced by specific stressors. In a controlled microcosm study with the amphipod Gammarus spp., we studied effects of the β-blocker propranolol on stable isotope signatures (δ15N and δ13C), elemental composition (%C and %N), and growth (protein content and body size) as well as biomarkers of oxidative status (antioxidant capacity, ORAC; lipid peroxidation, TBARS) and neurological activity (acetylcholinesterase, AChE). Based on the known effects of propranolol exposure on cellular functions, i.e., its mode of action (MOA), we expected to observe a lower scope for growth, accompanied by a decrease in protein deposition, oxidative processes and AChE inhibition, with a resulting increase in the isotopic signatures. The observed responses in growth, biochemical and elemental variables supported most of these predictions. In particular, an increase in %N was observed in the propranolol exposures, whereas both protein allocation and body size declined. Moreover, both ORAC and TBARS levels decreased with increasing propranolol concentration, with the decrease being more pronounced for TBARS, which indicates the prevalence of the antioxidative processes. These changes resulted in a significant increase of the δ15N and δ13C values in the propranolol-exposed animals compared to the control. These findings suggest that MOA of β-blockers may be used to predict sublethal effects in non-target species, including inhibited AChE activity, improved oxidative balance, and elevated stable isotope ratios. The latter also indicates that metabolism-driven responses to environmental contaminants can alter stable isotope signatures, which should be taken into account when interpreting trophic interactions in the food webs.

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In human physiology, the natural variations of the isotopic ratios of carbon, nitrogen and other major elements comprising biomass (δ15N, δ13C, δ18O, δ2H, δ44/40Ca, etc.) are attracting increasing attention since they offer a new means to study the imbalances linked to pathological conditions [1,2]. In non-human biology, however, the physiology of a consumer is rarely coupled to δ15N and δ13C values that are assumed to be a bare reflection of the diets’ isotope composition plus a discrimination factor (Δ15N and Δ13C, respectively). There is ample evidence that consumer Δ-values and the isotopic signatures may vary depending on various endogenous and environmental factors via their effects on metabolism and growth. These factors include variations in moulting status [3], food quantity [4,5] and quality [6], temperature [7,8], and contaminant exposure [9]. Therefore, to improve interpretation of stable isotope analysis (SIA) data in stress ecology and ecotoxicology, it is crucial to consider the physiological state in addition to the potential dietary sources of the consumer.

In animals, the majority of drugs either (i) mimic or inhibit normal physiological/biochemical processes; (ii) inhibit pathological processes; or (iii) inhibit vital processes of endo- or ectoparasites/microorganisms. Our study aimed to evaluate whether exposure to propranolol that belongs to the first group would cause predictable effects in crustaceans with regard to their stable isotope ratios and oxidative status. Based on the known targets and MOAs of propranolol, we expected to observe a lower scope for growth, improved oxidative balance, and AChE inhibition. As isotopic fractionation is a function of growth and metabolism [5], we expected to observe higher δ-values in the exposed animals. Most of the hypothesized effects (Table 1; Fig 1) were indeed observed, albeit only because of the magnitude of responses in the highest propranolol concentration (1058 μg L-1). The two exposure concentrations resulted in internal propranolol concentrations (3–6 μg g-1) that were within the range of the therapeutic concentrations reported in rabbit (~5 μg g-1) and human (0.47–11.67 μg g-1) brain, respectively [46]; thus, the exposure levels were consistent with the therapeutic doses for this drug.




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