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1 The eects of SK&F 96365 on cationic current evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgS (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-state I-V relationship shape which could be explained by assuming that increasing membrane positivity increased the anity of the blocker. The inhibition was similar for both carbachol-and GTPgSevoked 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 ®tted 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 713 mV. 5 Increasing the duration of voltage steps at 780 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.

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