Decrease in rat submandibular acinar cell volume during ACh stimulation

Changes in acinar cell volume were measured in the perfused submandibular gland of the rat at 23°C during salivary secretion induced by acetylcholine (ACh). Cellular volume was monitored by two methods: the impedance method and the morphometric method using video-enhanced contrast optical microscopy. Both measurements revealed a decrease in acinar cell volume in response to 1 μM ACh. Within the 1st min of stimulation, secretion increased to the initial maximum (initial secretion), and cell shrinkage occurred. During sustained stimulation, secretory rate and cell volume were maintained at the plateau level (steady secretion). The decrease in cell volume was 71.8 ± 2.9% of resting volume (means ± SE, n = 8) as measured by the impedance method and 76.1 ± 2.0% (n = 20) as measured by the morphometric method. With the removal of ACh, cell volume increased to 111.6 ± 2.7% (n = 8) of the prestimulation level as measured by the impedance method and 108.8 ± 1.5% (n = 20) as measured by the morphometric method, and then recovered to the prestimulation level slowly. The weight of the gland decreased significantly during stimulation. These findings proved that volume decrease occurred during stimulation. The measurement of cell volume gave the net fluid flux of the acinar cell compartment. The net fluid flux and the rate of salivary secretion gave an estimation of the fluid influx across the basolateral membrane. These findings suggest that a transcellular route for fluid secretion exists in the salivary gland.