Changes in cardiac performance during hypoxic exposure in the grass shrimp Palaemonetes pugio.

In hearts of higher invertebrates as well as vertebrates, the work performed by the ventricle is a function of both rate and contractility. Decapod crustaceans experience a hypoxia-induced bradycardia that is thought to result in an overall reduction in cardiac work; however, this hypothesis has not yet been tested and is the primary purpose of this study. In the grass shrimp Palaemonetes pugio, cardiac pressure and area data were obtained simultaneously, and in vivo, under normoxic (20.2 kPa O(2)) and hypoxic (6.8 or 2.2 kPa O(2)) conditions and integrated to generate pressure-area (P-A) loops. The area enclosed by the P-A loop provides a measure of stroke work and, when multiplied by the heart rate, provides an estimate of both cardiac work and myocardial O(2) consumption. Changes in intra-cardiac pressure (dp/dt) are correlated to the isovolemic contraction phase and provide an indication of stroke work. At both levels of hypoxic exposure, intra-cardiac pressure, dp/dt, stroke work and cardiac work fell significantly. The significant decrease in intra-cardiac pressure provides the primary mechanism for the decrease in stroke work, and, when coupled with the hypoxia-induced bradycardia, it contributes to an overall fall in cardiac work. Compared with normoxic P-A loops, hypoxic P-A loops (at both levels of hypoxia) become curvilinear, indicating a fall in peripheral resistance (which might account for the reduction in intra-cardiac pressure), which would reduce both stroke work and cardiac work and ultimately would serve to reduce myocardial O(2) consumption. This is the most direct evidence to date indicating that the hypoxia-induced bradycardia observed in many decapod crustaceans reduces cardiac work and is therefore energetically favorable during acute exposure to conditions of low oxygen.

Changes in caridac output and hemolymph flow during hypoxic exposure in the gravid grass shrimp, Palaemonetes pugio.

The cardiovascular response of decapod crustaceans to hypoxic exposure is well documented; however, information is limited concerning the influence of reproductive state on cardiovascular demands during hypoxic exposure. Given the additional metabolic demand of reproduction, we investigated the cardiovascular adjustments employed by gravid grass shrimp Palaemonetes pugio to maintain oxygen delivery during hypoxic stress. Cardiac output values were elevated in gravid compared to nongravid grass shrimp. Gravid grass shrimp were exposed to hypoxia and the stroke volume, heart rate, cardiac output and hemolymph flow were determined using video-microscopy and dimensional analysis. Oxygen consumption rates were determined using respirometry. There where no changes in the cardiac output values of gravid females until reaching 6.8 kPa O2, with a significant redistribution of hemolymph flow at 13.7 kPa O2. Flow was significantly decreased to the anterior lateral arteries that supply the ovaries and hepatopancreas, the anterior aorta and the posterior aorta. The redistribution of hemolymph flow away from these vessels results in an enhanced hemolymph flow to the sternal artery that supplies the ventral segmental system, the gills, the buccal apparatus and the ventral nerve cord. The data suggest that during hypoxic stress, gravid females place a priority on survival.