Muscarinic receptor is coupled with a cation channel through a GTP‐binding protein in guinea‐pig chromaffin cells.

1. The ionic current evoked by muscarinic receptor agonists was investigated in dispersed chromaffin cells of the guinea‐pig adrenal medulla using the whole‐cell version of the patch‐clamp procedure. 2. Muscarine or oxotremorine (0.03‐10 microns) produced an inward current associated with an increase in current noise at a holding potential of ‐40 mV. The relationship between current and oxotremorine concentration fitted well to a rectangular hyperbole with an apparent dissociation constant (KA) of 0.23 micron. 3. The muscarinic antagonists pirenzepine (0.1 micronM) and AF‐DX 116 (0.3 microM) shifted the dose‐response curve to the right in a parallel manner. Dissociation constants (KB) for pirenzepine and AF‐DX 116 were estimated to be 50 and 70 nM, respectively. 4. The current‐voltage relation for the current induced by muscarine had a negative slope below ‐30 or ‐20 mV, and the current reversed its polarity at 0.4 +/‐ 0.8 mV (n = 4) in standard salt solution. Removal of Mg2+ or Ca2+ from the perfusate did not modify the muscarinic current‐voltage relationship. 5. When Na+ in the bath solution was replaced with Tris, the muscarinic current‐voltage relationship shifted to the left (the hyperpolarizing direction); the current reversed its polarity at ‐18.7 +/‐ 1.2 mV in a solution containing 72 mM‐Na+ (three cells) and at ‐57.5 +/‐ 2.7 mV in nominally Na(+)‐free solution (three cells). When Ca2+ was replaced by Mg2+, in Na(+)‐free solution, an inward current was not evoked by muscarinic stimulation. 6. Tetraethylammonium (TEA; 0.03‐3 mM) reduced the muscarinic current at ‐40 mV, and the KD value was 0.34 mM with a Hill coefficient of 1. Barium (4 mM) reduced the current to 0.69 +/‐ 0.06 of control (n = 3). 7. When the recording electrodes contained guanosine‐5'‐O‐(3‐thiophosphate) (GTP gamma S, 100 microM), an inward current developed at ‐55 mV during the first few minutes after breaking into the cell interior. This inward current was associated with an increase in noise and was not larger at ‐70 mV than at ‐55 mV; it reached a peak value about 3‐4 min after breaking into the cell interior and then declined over the next 2‐3 min. 8. Muscarinic agonists had no effect in those cells in which intracellular GTP gamma S had first evoked a transient inward current. The inward current caused by nicotine was unaffected by intracellular GTP gamma S.(ABSTRACT TRUNCATED AT 400 WORDS)