BARIUM-INDUCED BLOCKADE OF THE INWARD RECTIFIER IN CALF PURKINJE-FIBERS

The Ba-induced blockade of iK1 [K-dependent inward rectifier] in calf Purkinje fibers was studied in the low concentration range (3-1,000 .mu.M). The blockade induced by hyperpolarizations was time-dependent. The rate of blockade increased with the Ba concentration and the negative pulse amplitude. At Ba concentrations > 1 mM the time course of iK1 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 B was iK1. 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.