CALCIUM-ACTIVATED POTASSIUM CHANNELS IN HUMAN PLATELETS

1. The effect of intracellular [Ca2+]([Ca2+](i)) on human platelet ion channels was studied using the nystatin whole-cell patch clamp recording technique. 2. Ionomycin-induced increases in [Ca2+](i) rapidly activated a voltage-independent K+-selective channel with a slope conductance of 30 pS in 154 mM K+ saline. The single-channel conductance decreased in proportion to the square root of the external K+ concentration such that the estimated conductance in 5 mM K+ was approximately 5 pS. 3. The peak current under conditions expected to increase [Ca2+](i) to micromolar levels indicated that each platelet possesses a small number (5-7) of 30 pS Ca2+-dependent K+ channels (K-Ca channels). 4. Spontaneous [Ca2+](i) spiking was observed in many patch-clamped platelets using fura-2 fluorescence measurements. Each Ca2+ spike triggered up to five K-Ca channels at any one time. K-Ca channels were not active at resting levels of [Ca2+](i). 5. The results suggest that platelet K-Ca channels are not active under resting conditions but conditions but may have an important role in determining the membrane potential during Ca2+ signalling.