Ionic dependence of acetylcholine equilibrium potential of acinar cells in mouse submaxillary gland

Abstract

The ionic dependence of the acetylcholine equilibrium potential (E_ACh) of acinar cells in which acetylcholine (ACh) induced hyperpolarization under control conditions was investigated using intracellular micro-electrode recording in superfused segments of mouse submaxillary gland. For measurements of E_ACh two micro-electrodes were inserted into neighbouring communicating cells, direct current was passed through one of these electrodes and E_ACh was determined by plotting the relation between the size of the ACh potential and the resting potential. ACh was applied by micro-iontophoresis.

A complex potential change was induced by ACh when the membrane potential was set at high levels (−50∼ −80 mV). The appearance of complex responses dependend on the external [Na]. A severe reduction in external Na concentration abolished the appearance of complex responses, whereas alterations of external K concentration had no such effect. The results indicate that a depolarizing component separate from the hyperpolarizing component exists even in acinic in which ACh only evokes hyperpolarization under control conditions. Intracellular injection of TEA ions converted the ACh evoked potential change from hyperpolarization to depolarization in acini superfused with solutions containing Na in concentrations between 50 and 135 mM. However, the conversion was never obtained using solutions with low Na concentration (12.5 mM).

The mean E_ACh was −60 mV under normal conditions. E_ACh was made more negative (5 mV) by a reduction in external Na concentration from 135 to 12.5 mmol·l⁻¹. E_ACh was influenced by alterations of external K concentration, particularly when combined with reduction in external Na concentration. Alteration of K concentration from 2 to 20 mmol·l⁻¹ shifted E_ACh to more positive values by about 40 mV. E_ACh in acini treated with TEA was about −28 mV in control solution (Na: 135 mmol·l⁻¹) and −35 mV in a low Na concentration (50 mmol·l⁻¹).

Assuming that the response in submaxillary gland acinar cells to ACh under control condition is composed of two different kinds of potential changes (depolarization and hyperpolarization), the ionic basis of each of the potential changes and a possible explanation for the mechanism of ACh are discussed.