Regulation of cell volume and diffusion of intracellular water in salivary acinar cells

The regulation of acinar cell volume and the properties of intracellular water were investigated in perfused rat mandibular salivary glands by proton nuclear magnetic resonance (NMR) spectroscopy. Using an inversion-recovery pulse sequence, and an extracellular relaxation reagent (10 mM Gd-DTPA) to suppress the proton NMR signal from extracellular water, acinar cell volume (intracellular water content) in unstimulated glands was shown to depend upon Cl^- uptake by basolateral Na⁺-K⁺-2Cl^- cotransport. Muscarinic and β-adrenoceptor stimulation induced shrinkage and swelling respectively. In pulsed-field-gradient NMR experiments, the diffusion coefficient of intracellular water was found to be more than an order of magnitude smaller than that of extracellular water. Using this intrinsic difference in diffusivity between the two compartments, cell volume regulation was investigated in intact, perfused glands in the absence of relaxation reagents. Using both NMR techniques, acinar cells in perfused glands were observed to behave like simple osmometers in response to anisosmotic media, and did not show the volume regulatory responses described in dissociated acinar cells.