| Abstrakt: |
Exposure of adult crayfish Pacifastacus leniusculus to Artificial Freshwater (AFW) media containing 1.5 m and 0.15 mmol.l–1 total ammonia [Tamm; 0.1×acute lethal concentration (24 h LC50) and 0.01×24 h LC50] and adjusted to pH 6.5, pH 8.2 and pH 10.5 resulted in significant increases in haemolymph ammonia over a 24-h period. Ammonia accumulated most rapidly at pH 10.5. These media were chosen to expose animals to a range of different un-ionised ammonia (UIA) [NH3] and ionised ammonia [NH4+] concentrations. From comparisons of measured transepithelial potential differences (PDte) with calculated Nernst potentials (PDNH4+) for the known haemolymph-to-medium gradients of [NH4+], it was deduced that, in pH 8.2 and pH 6.5 AFW, NH4+ was not in thermodynamic equilibrium across the integument (presumably gill epithelium). In pH 10.5 AFW with 1.5 mmol.l–1 Tamm (predominantly NH3), the accumulation of ammonia in the haemolymph was in the NH4+ form due to haemolymph pH regulation by the crayfish in this alkaline external medium. Measured net fluxes of ammonia (Jammnet) were inwardly directed and maximal when [NH3] was the main component externally, but were also significant at pH 8.2 with high [NH4+] ([NH4+]:[NH3]≈20:1). Haemolymph Na+ depletion was significant and, over the 24-h exposure period, most rapid in high [NH3] medium but [Cl–] was unaffected. However, paradoxically, sodium uptake (measured JNain on immediate transfer to high Tamm medium) was not significantly inhibited when [NH3] was the predominant ammonia species. In 1.5 mmol.l–1 Tamm (mainly [NH4+]), VNain (the active component of JNain) was significantly inhibited, particularly at low external [Na+]. This inhibition could not be demonstrated as one of competition at an Na+/NH4+ apical gill exchange site. The resultant net efflux of sodium from the animal showed that the ability of the animals to balance sodium losses at low external [Na+] was severely affected. Longer exposure to pH 10.5 AFW with high [NH3] (12 h) resulted in significantly increased JNaout, while not significantly affecting JNain. Analysis of urinary Na+ losses showed that, while urinary flow rate and water reabsorption was most likely unaffected by ammonia exposure, final urine [Na+] was significantly elevated. The resulting urinary Na+ loss accounted for 63% of the increased JNaout in high [NH3] medium. [ABSTRACT FROM AUTHOR] |
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