Calcium transport in gill cells of Ucides cordatus, a mangrove crab living in variable salinity environments.

Crustaceans show discontinuous growth and have been used as a model system for studying cellular mechanisms of calcium transport, which is the main mineral found in their exoskeleton. Ucides cordatus, a mangrove crab, is naturally exposed to fluctuations in calcium and salinity. To study calcium transport in this species during isosmotic conditions, dissociated gill cells were marked with fluo-3 and intracellular Ca(2+) change was followed by adding extracellular Ca(2+) as CaCl2 (0, 0.1, 0.25, 0.50, 1.0 and 5mM), together with different inhibitors. For control gill cells, Ca(2+) transport followed Michaelis-Menten kinetics with Vmax=0.137±0.001 ∆Ca(2+)i (μM×22.10(4)cells(-1)×180s(-1); N=4; r(2)=0.99); Km=0.989±0.027mM. The use of different inhibitors for gill cells showed that amiloride (Na(+)/Ca(2+) exchange inhibitor) inhibited 80% of Ca(2+) transport in gill cells (Vmax). KB-R, an inhibitor of Ca influx in vertebrates, similarly caused a decrease in Ca(2+) transport and verapamil (Ca(2+) channel inhibitor) had no effect on Ca(2+) transport, while nifedipine (another Ca(2+) channel inhibitor) caused a 20% decrease in Ca(2+) affinity compared to control values. Ouabain, on the other hand, caused no change in Ca(2+) transport, while vanadate increased the concentration of intracellular calcium through inhibition of Ca(2+) efflux probably through the plasma membrane Ca(2+)-ATPase. Results show that transport kinetics for Ca(2+) in these crabs under isosmotic conditions is lower compared to a hyper-regulator freshwater crab Dilocarcinus pagei studied earlier using fluorescent Ca(2+) probes. These kinds of studies will help understanding the comparative mechanisms underlying the evolution of Ca transport in crabs living in different environments.