Barium-induced blockade of the inward rectifier in calf Purkinje fibres

The Ba-induced blockade of i_K1 in calf Purkinje fibres was studied in the low concentration range (3–1,000 M). The results showed that the blockade induced by hyperpolarizations was time-dependent, and that the rate of blockade increased with the Ba concentration and the negative pulse amplitude. At Ba concentrations higher than 1 mM the time course of i_K1 blockade was not separable from the capacity transient. At these concentrations, and in a limited voltage range, the Ba-sensitive instantaneous current displayed a marked inward rectification and behaved with external K as expected for a pure K⁺ current, suggesting that under these conditions the only current affected by Ba was i_K1. The Ba-sensitive current decreased with time during hyperpolarizations even when high external K was present, indicating that a voltage-dependent inactivation process could be at least in part responsible for the observed current decline.