Effects of Ca2+ removal and of tetraethylammonium on membrane currents induced by carbachol in isolated cells from the rat parotid gland

In freshly dispersed rat parotid acinar cells, 10 μM carbachol increased outward currents at 0 mV and also inward currents at −70 mV recorded with the whole-cell clamp method using patch pipettes containing 1 mM EGTA. When EGTA in the pipette was increased to 2.4 mM, carbachol increased only outward currents and a further increase of EGTA to 4 mM blocked the carbachol response. Effects of changes in external K+ and Cl− concentrations suggested that outward currents were carried by K+ and inward by Cl−. Effects of Ca2+ removal from the medium differed between experiments with 0 and 5 mM ATP in the patch pipettes. When pipettes contained no ATP, responses evoked by repeated applications of 10 μM carbachol (0.5–1 min) at 1.5–4 min intervals decreased only slowly after Ca2+ removal, outward currents being reduced to 90±6% and inward currents to 47±11% (n=6) in 10 min. On the other hand, when 5 mM ATP was included in the electrodes, Ca2+ removal abolished the carbachol responses in about 5 min (n=4). It was also found that tetraethylammonium (5 mM) strongly reduced both currents, by blocking muscarinic receptors, while Ba2+ (2.4 mM) inhibited only the outward K+ current.