Two Cell Volume Regulatory Systems in Thelimulusmyocardium: An Interaction of Ions and Quaternary Ammonium Compounds

The horseshoe crab Limulus polyphemus is extremely euryhaline. Previous studies have shown it surviving in salinities ranging from 6% to 200% sea water. Blood osmotic concentration is hyperregulated in low salinities, but above 65% sea water Limulus is an osmoconformer. Limulus regulates cell volume when exposed to low salinity, despite a small intracellular free amino acid pool. Instead, the quaternary ammonium compound glycine betaine is the major nitrogenous osmotic solute in Limulus heart tissue. However, volume regulation is complete before intracellular glycine betaine concentrations change. Isolated heart tissue exposed to low salinity shows no change in glycine betaine levels in 24 h though volume regulation occurs. During the initial phase of volume regulation intracellular Na+ and Cl- content in the isolated tissue decreases markedly with exposure to low salinity. Therefore, Limulus utilizes two osmotic solute types during cell volume regulation: Na+ and Cl- initially and glycine betaine later.