Effects of caffeine on background potassium current in isolated guinea pig ventricular myocytes.

We investigated effects of caffeine on inward rectifier potassium current (Ik1) in voltage-clamped ventricular cells by slow ramp depolarization (15 mV/s). Caffeine 10 mM applied in the solution bathing the cells consistently reduced the slope of the current-voltage (I-V) relation over the range of -80 to -40 mV. This effect of caffeine was not prevented by loading cells with BAPTA (1,2-bis(2-aminophenoxy) ethane N,N,N',N'-tetra-acetic acid) to suppress contraction. In the absence of caffeine, reducing extracellular potassium from 5.4 to 2.7 mM caused the expected shift of the reversal potential for current in the negative direction and increased rectification. In low potassium, 10 mM caffeine continued to reduce the slope of the I-V relation. When 2 mM barium was applied to suppress Ik1, any effects of 10 mM caffeine were slight or absent. The observations are consistent with a blocking action of caffeine on Ik1 in guinea pig ventricular myocytes.