Direct measurement of K movement by³⁹K NMR in perfused rat mandibular salivary gland stimulated with acetylcholine

Abstract

The intracellular K content (K_in) of the isolated perfused rat mandibular gland was measured by³⁹K NMR spectroscopy at 25°C, using an inversion recovery technique based on the fact that the spin-lattice (T₁) relaxation of K_in is much faster than that of the extracellular K. K_in decreased by 30–34% of the resting level and reached a plateau level during secretion evoked by a sustained infusion of 1 μmol/l acetylcholine. Addition of 1 mmol/l ouabain decreased K_in by an additional 41% of the resting level. The K net flux to the blood and saliva was calculated from the K concentrations and flow rates of the effluent and the saliva. At an initial stage of secretion the gland lost K to the vascular side at a rate of 12.6±1.8 μmol/g-min (mean ± SEM,n=7). During sustained secretion, the gland took K up from the vascular side at a rate of 3.3±0.7 μmol/g-min (n=7), and the same amount of K was secreted into the saliva (4.7±1.1 at 5–10 min, 2.8±0.8 μmol/g-min at 20–30 min), resulting in no net K movement from the gland. Addition of 1 mmol/l ouabain stopped salivary secretion and caused a transient K release to the vascular side at a maximum rate of 12.8±1.1 μmol/g-min. Withdrawal of acetylcholine and ouabain induced K uptake from the vascular side (6.5±0.7 μmol/g-min) and the amount of K released was completely restored when K_in recovered completely. The ratio (0.38) of the acetylcholine-induced K loss (30 μmol/g) to the ouabain-induced total K loss (80 μmol/g) was very similar to the ratio (0.41–0.45) measured by³⁹K NMR. The present observations lead to the conclusion that the changes in³⁹L-NMR-visible intracellular K directly relate to K movement across both the basolateral and the luminal membranes of salivary epithelia.