Anion uptake and acid-base and ionic effects during isolated and combined exposure to hypercapnia and nitrite in the freshwater crayfish, Astacus astacus

Abstract

Nitrite influx into crayfish showed saturation kinetics, supporting a carrier-mediated uptake. Addition of 4,4′-diisothiocyanatostilbene-2,2′-disulfonate (DIDS: at 10⁻⁵, 10⁻⁴ and 10⁻³M) and bumetanide (at 10⁻⁵ M and 10⁻⁴ M) to the ambient water did not significantly affect nitrite influx. Rather than suggesting that neither Cl⁻/HCO₃ ⁻ exchange nor K⁺/Na⁺/2Cl⁻ cotransport were involved in the transport, this may reflect that the gill cuticle has a low permeability to the pharmacological agents, or that the sensitivity of the transport mechanism to the inhibitors is low. Nitrite accumulation in the haemolymph was significantly decreased during hypercapnic conditions compared to normocapnic conditions. This supports the idea that an acid–base regulatory decrease in Cl⁻(influx)/HCO₃ ⁻ (efflux) induced by hypercapnia should decrease NO₂ ⁻ uptake if NO₂ ⁻ and Cl⁻ share this uptake route. The respiratory acidosis caused by exposure to hypercapnia alone was partially compensated by HCO₃ ⁻ accumulation in the haemolymph. Combined exposure to hypercapnia and nitrite improved pH recovery, mainly by augmenting the [HCO₃ ⁻] increase, but also by decreasing haemolymph PCO₂. Exposure to nitrite in normocapnic water induced an initial increase in haemolymph [HCO₃ ⁻] and later also a decrease in PCO₂. Thus, the improved acid-base compensation during combined hypercapnia and nitrite exposure was an amplification of this nitriteinduced response. Haemolymph base excess rose much more than haemolymph [Ca], suggesting that transfer of acid-base equivalents between animal and water was more important than H⁺ buffering by exoskeletal CaCO₃ in mediating the increase in haemolymph [HCO₃ ⁻].