Voltage-dependent inhibition of the muscarinic cationic current in guinea-pig ileal cells by SK&F 96365

1 The effects of SK&rF 96365 on cationic current evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTP gamma S (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal smooth muscle cells. 2 SK&F 96365 reversibly inhibited the muscarinic receptor cationic current in a concentration-, time- and voltage-dependent manner producing concomitant alteration of the steady-stale I-V relationship shape which could be explained by assuming that increasing membrane positivity increased the affinity of the blocker. The inhibition was similar for both carbachol- and GTP gamma S-evoked currents suggesting that the cationic channel rather than the muscarinic receptor was the primary site of the SK&F 96365 action. 3 Increased membrane positivity induced additional rapid inhibition of the cationic current by SK&F 96365 which was more slowly relieved during membrane repolarization. Both the inhibition and disinhibition time course could be well fitted by a single exponential function with the time constants decreasing with increasing positivity for the inhibition (e-fold per about 12 mV) and approximately linearly decreasing with increasing negativity for the disinhibition. 4 At a constant SK&F 96365 concentration, the degree of cationic current inhibition was a sigmoidal function of the membrane potential with a potential of half-maximal increase positive to about + 30 mV and a slope factor of about -13 mV. 5 Increasing the duration of voltage steps at -80 or at 80 mV, increased the percentage inhibition; the degree of inhibition was almost identical at both potentials providing evidence that the same cationic channel was responsible for the cationic current both at negative and at positive potentials. 6 It is concluded that the distinctive and unique mode of SK&F 96365 action on the: muscarinic receptor cationic channel is a valuable tool in future molecular biology studies of this channel.