Continuous measurements of tissue impedance during secretion in dog submandibular gland

The electrical impedance of the dog submandibular gland, as an indicator of changes in extracellular fluid (ECF) volume, was measured at 5 kHz, 500 kHz, and 5 MHz at intervals of 10 s during secretory stimulation, because the conductivity calculated from impedance at low frequencies reflected the ECF volume. The decrease in conductivity occurred in the first minute of stimulation. Its decrease was more marked during stimulation after circulatory arrest. Salivary secretion under intact circulation consists of two phases: an initial secretion occurring in the first minute of stimulation with a high secretory rate and a steady secretion continuing during stimulation at a constant rate. A decrease in conductivity occurred in the initial secretion. Within a few minutes following the cessation of stimulation, the conductivity increased to a level higher than the resting one with an intact circulation, while the blood flow remained several times higher than in the resting state. The conductivity of the gland slowly recovered to the prestimulation level over a 30-min period. The histological examination revealed that the main compartment of the ECF change was the interlobular space. Impedance and histological studies showed that the ECF volume of salivary glands changes dynamically during secretion. On the other hand, the cellular volume also increased in the initial secretion of an isolated gland. Its increase suggested that the fluid was transported to the lumen via a cellular pathway as well as a paracellular pathway in the initial secretion.