SigrafW is Windows‐compatible software developed using the Microsoft® Visual Basic Studio program that uses the simplified Hill equation for fitting kinetic data from allosteric and Michaelian enzymes. SigrafW uses a modified Fibonacci search to calculate maximal velocity (V), the Hill coefficient (n), and the enzyme‐substrate apparent dissociation constant (K). The estimation of V, K, and the sum of the squares of residuals is performed using a Wilkinson nonlinear regression at any Hill coefficient (n). In contrast to many currently available kinetic analysis programs, SigrafW shows several advantages for the determination of kinetic parameters of both hyperbolic and nonhyperbolic saturation curves. No initial estimates of the kinetic parameters are required, a measure of the goodness‐of‐the‐fit for each calculation performed is provided, the nonlinear regression used for calculations eliminates the statistical bias inherent in linear transformations, and the software can be used for enzyme kinetic simulations either for educational or research purposes.Persons interested in receiving a free copy of the software should contact Dr. F. A. Leone.

We evaluate osmotic and chloride (Cl^–) regulatory capability in the diadromous shrimp Macrobrachium amazonicum, and the accompanying alterations in hemolymph osmolality and [Cl^–], gill Na⁺/K⁺-ATPase activity, and expression of gill Na⁺/K⁺-ATPase α-subunit and V-ATPase B subunit mRNA during salinity (S) acclimation. We also characterize V-ATPase kinetics and the organization of transport-related membrane systems in the gill epithelium. Macrobrachium amazonicum strongly hyper-regulates hemolymph osmolality and [Cl^–] in freshwater and in salinities up to 25‰ S. During a 10-day acclimation period to 25‰ S, hemolymph became isosmotic and hypo-chloremic after 5 days, [Cl^–] alone remaining hyporegulated thereafter. Gill Na⁺/K⁺-ATPase α-subunit mRNA expression increased 6.5 times initial values after 1 h, then decreased to 3 to 4 times initial values by 24 h and to 1.5 times initial values after 10 days at 25‰ S. This increased expression was accompanied by a sharp decrease at 5 h then recovery of initial Na⁺/K⁺-ATPase activity within 24 h, declining again after 5 days, which suggests transient Cl^– secretion. V-ATPase B-subunit mRNA expression increased 1.5-fold within 1 h, then reduced sharply to 0.3 times initial values by 5 h, and remained unchanged for the remainder of the 10-day period. V-ATPase activity dropped sharply and was negligible after a 10-day acclimation period to 21‰ S, revealing a marked downregulation of ion uptake mechanisms. The gill epithelium consists of thick, apical pillar cell flanges, the perikarya of which are coupled to an intralamellar septum. These two cell types respectively exhibit extensive apical evaginations and deep membrane invaginations, both of which are associated with numerous mitochondria, characterizing an ion transporting epithelium. These changes in Na⁺/K⁺- and V-ATPase activities and in mRNA expression during salinity acclimation appear to underpin ion uptake and Cl^– secretion by the palaemonid shrimp gill.