POTASSIUM CHANNEL MODULATION IN RAT PORTAL-VEIN BY ATP DEPLETION - A COMPARISON WITH THE EFFECTS OF LEVCROMAKALIM (BRL-38227)

1 The effects of levcromakalim and of adenosine 5'-triphosphate (ATP) depletion on membrane potential and ionic currents were studied in freshly-dispersed smooth muscle cells of rat portal vein by use of combined voltage- and current-clamp techniques. 2 Levcromakalim (1 muM) induced a glibenclamide-sensitive, non-inactivating K-current (I(KCO)) and simultaneously inhibited the slow, transient outward, delayed rectifier K-current (I(TO)). Levcromakalim also hyperpolarized the portal vein cells by approximately 20 mV. 3 Reduction of intracellular ATP by removal of glucose and carboxylic acids from the recording pipette and of glucose from the bath fluid, induced a slowly-developing, non-inactivating and glibenclamide-sensitive K-current (I(met)) within 60-300 s after breaking the membrane patch. I(met) reached peak amplitude after 300-900 s, remained at a plateau for 200-800 s and then slowly ran down. At the peak of I(met), the cells were hyperpolarized by approximately 20 mV and their input conductance was increased by 42%. 4 At the time of maximum development of I(met), the delayed rectifier current, I(TO), was reduced by 48%. 5 In the absence of glucose and carboxylic acids, addition of 1 muM free ATP to the recording pipette almost doubled the magnitude of I(met). At a holding potential of - 10 mV, I(met) was increased from 124 +/- 11 pA to 228 +/- 54 pA whereas the time-course of development and run-down of I(met) was unaffected. 6 During the development and after the run-down of I(met), levcromakalim (1 - 10 muM) failed to induce I(KCO). 7 Stationary fluctuation analysis of the current noise associated with I(met) revealed a unitary conductance of between 10-20 pS in a physiological potassium gradient. A second contaminating current with an underlying unitary conductance of approximately 150 pS remained after I(met), had run down. 8 It is concluded that I(KCO) induced by levcromakalim and I(met) are carried by the same population of relatively small conductance, glibenclamide-sensitive K-channels. The open state of these is increased by procedures designed to lower intracellular ATP concentrations. 9 The simultaneous inhibition of the delayed rectifier current (I(TO)) by both levcromakalim and during the development of I(met), is highly significant. It suggests that levcromakalim could modify the interaction of ATP with sites linked to more than one type of K-channel. This results in the opening of those channels which underlie I(KCO) (and which are normally inhibited by ATP binding) together with the modulation of phosphorylation-dependent channels such as those which underlie I(TO).