ACTION OF CAFFEINE ON TRANSMEMBRANE POTASSIUM CURRENTS IN SINGLE SMOOTH-MUSCLE CELLS FROM GUINEA-PIG MESENTERIC-ARTERY

Whole-cell transmembrane outward currents evoked by Ca2+ release from caffeine-sensitive intracellular stores in isolated smooth muscle cells from guinea-pig mesenteric artery were investigated using patch-clamp method. Application of caffeine during voltage stepping from - 70 mV to 10 mV was followed by high-amplitude outward current consisting of fast and slow components whose rates of rise and fall differed markedly. Both components were carried by K+ ions and could be blocked by TEA with equal efficiency. Recovery of the fast component was found to be faster than that of the slow one and could be detected 50 s after caffeine removal. In contrast, the spontaneous transient outward currents (STOC) were restored in 1-2 s suggesting that Ca2+ release from Ca2+-sensitive stores is not related to STOCs activation. Caffeine was found to block the voltage sensitive transient K+ current, while TEA-sensitive Ca2+-activated K channels were resistant to caffeine. Our data support the suggestion that STOC's and both components of the caffeine-induced K+ current are carried through identical Ca2+-activated K channels that differ in location and the sources of activator Ca2+. STOCs and the fast component are carried through the same K channels located in the region of narrow gaps between the membrane and superficial sarcoplasmic reticulums (SR), but activator Ca2+ ions enter the gap from extracellular space in the first case or are released from SR in the second case. The slow component is activated by Ca2+ ions released from deep SR.