Two cDNAs encoding rabbit heart CI-channels (rahCIC-21~l and rabCIC-2c0 were isolated by a PCR cloning strategy. RabCIC-21~l is a novel cDNA consisting of 2998 bp and encoding the 822-amino acid protein, while rabCIC-2a is identical to previously reported CIC-2G. RabCIC-2/] is 68 amino acids truncated from NHz-terminus of rabCIC-2a, but aH 13 putative hydrophobic domains are conserved in rabCIC-2/3. Although rabCIC-2a was suggested to be activated by extracellular hypotonicity, expression of rabCIC-2/3 in Xenopus oocytes induced large Cl-currents even in the absence of extracellular hypotonicity. Induction of external hypotonicity did not further increase the amplitude of membrane currents. On the other hand, as similar to rabCIC-2a, rabCIC-21~ current was augmented by PKA activation. Thus, different RNA processing of the same gene appears to provide two highly homologous PKA-activated CIchannels with or without responsiveness to cell swelling in rabbit heart.