Acetylcholine-induced metabolic changes in the perfused rabbit mandibular salivary gland studied by 31P-NMR spectroscopy.

Changes in the content of high-energy phosphates, intracellular pH (pHi) and the ratio of MgATP to total ATP ([MgATP]/[ATP]t) resulting from continuous stimulation with acetylcholine (10(-9) to 10(-4) M) were measured by 31P-NMR spectroscopy in the isolated, perfused rabbit mandibular gland at 37 degrees C. With 10(-9) to 10(-7) M acetylcholine, no significant changes in these parameters were observed. On stimulation with 10(-6) M acetylcholine, the optimal concentration for sustained secretion, the content of ATP decreased by 28 +/- 10% (mean +/- S.E.; n = 8) of its control value. pHi decreased initially by approx. 0.05 pH unit, then showed an alkalinization of 0.09 +/- 0.02 pH unit (n = 8). With 10(-5) and 10(-4) M acetylcholine, changes in ATP and pHi were similar to those induced by 10(-6) M acetylcholine: the total content of high-energy phosphates remained at approx. 70% of the control value and no decrease in [MgATP]/[ATP]t was observed. As possible causes of the reduced secretory rate observed with higher concentrations of acetylcholine (10(-5) to 10(-3) M), we can exclude depletion of high-energy phosphates, inhibition of metabolism caused by intracellular acidosis, and inhibition of ATP usage caused by a decrease in MgATP availability.