Ammonia uptake and its effects on ionoregulation in the freshwater crayfish Pacifastacus leniusculus (Dana)

Exposure of adult crayfish Pacifastacus leniusculus to Artificial Freshwate r (AFW) media containing 1.5 m and 0.15 mmol.l(-1) total ammonia [T-amm; 0. 1xacute lethal concentration (24 h LC50) and 0.01x24 h LC50] and adjusted t o pH 6.5, pH 8.2 and pH 10.5 resulted in significant increases in haemolymp h 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-ionise d ammonia (UIA) [NH3] and ionised ammonia [NH4+] concentrations. From compa risons of measured transepithelial potential differences (PDte) with calcul ated Nernst potentials (PDNH4+) for the known haemolymph-to-medium gradient s 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 epitheliu m). In pH 10.5 AFW with 1.5 mmol.l(-1) T-amm (predominantly NH3), the accum ulation of ammonia in the haemolymph was in the NH4+ form due to haemolymph pH regulation by the crayfish in this alkaline external medium. Measured n et fluxes of ammonia (J(amm)(net)) 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]approximate to 20:1). Haemolymph Na+ deplet ion was significant and, over the 24-h exposure period, most rapid in high [NH3] medium but [CI] was unaffected. However, paradoxically, sodium uptake (measured J(Na)(in) on immediate transfer to high T-amm medium) was not si gnificantly inhibited when [NH3] was the predominant ammonia species. In 1. 5 mmol.1-1 Tamm (mainly [NH4+]), V-NA(in) (the active component of J(Na)(in )) was significantly inhibited, particularly at low external [Na+]. This in hibition could not be demonstrated as one of competition at an Na+/NH4+ api cal 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 exte rnal [Na+] was severely affected. Longer exposure to pH 10.5 AFW with high [NH3] (12 h) resulted in significantly increased J(Na)(out), while not sign ificantly affecting J(Na)(in). Analysis of urinary Na+ losses showed that, while urinary flow rate and water reabsorption was most likely unaffected b y ammonia exposure, final urine [Na+] was significantly elevated. The resul ting urinary Na+ loss accounted for 63% of the increased J(Na)(out) in high [NH3] medium.