Cardiac Myocardial Hypertrophy and Altered Swimming Behavior in Xenopus laevis Embryos in Incrementally Increasing Hypergravity

Abstract

Every living organism on earth has developed and evolved in unit gravity (1G) conditions. It is likely that any deviation from earth’s standard gravity will influence development, particularly at early stages. Previous reports from this lab showed that total length is reduced and that ventricle size is increased during development at 7G. The objective of the present study was to investigate the effect of increasing levels of hypergravity on the development of the ventricular myocardial wall and the neuromuscular responsiveness of Xenopus laevis. At early gastrulation, embryos were placed in a centrifuge simulating 7G, 10G, 15G or 17G until they reached stage 45 (feeding tadpole stage, approximately 72 hours from initiation of gastrulation). Mortality was low and only the 17G exposure induced significant mortality. Immediately following centrifugation, the embryos were stimulated by touching with a probe to test neuromuscular responsiveness. With increasing G forces, responsiveness to this test was incrementally reduced. A quarter of the embryos were fixed and their body dimensions were measured. One group of remaining live embryos, those exposed to 7G, was maintained and swimming behavior was observed during daily 5-minute periods for 52 days. Overall, abnormal swimming behavior was found in 33% of 7G embryos and in 2% of controls during this time. These embryos were later subjected to an orientation-swimming test. Hypergravity-exposed embryos required, on average, one second longer (9% of time required) to become oriented. From the embryos exposed to increasing G levels and then fixed, we found that total length was reduced successively at increasing hypergravity levels, but the snout-vent proportion of total length increased. Stained sections of the fixed embryos revealed that the thickness of the ventricle wall was increased, especially the interior ridge component (trabeculae/papillary muscles). The data demonstrates that hypergravity has persistent effects on the development of the ventricular myocardial wall, neuromuscular responsiveness, and equilibrium organs.