Cloning and characterization of an olfactory cyclic nucleotide-gated channel expressed in mouse heart

Regulation of ionic currents in the heart is partly achieved by signaling cascades which alter intracellular levels of cyclic nucleotides. Changes in cyclic nucleotide levels can regulate channels either directly, like the direct binding of cAMP to the i(f) channel in pacemaker tissues, or indirectly through phosphorylation of channels by cAMP-dependent, or cGMP-dependent protein kinases. These types of regulation generally alter the voltage sensitivities of channels. A class of voltage-insensitive channels, first discovered in retinal rods and olfactory neurons, were recently identified in the heart. These channels are opened by the direct binding of cyclic nucleotides, providing a means of regulating ionic currents outside the influence of membrane voltage. Since different isoforms have different affinities for cAMP and cGMP, it is important to determine which isoforms are expressed in heart in order to predict their roles in heart function. We have cloned the olfactory channel from mouse heart, and find that although the message is very rare, Western blot analysis indicates the olfactory channel protein is stable in heart sarcolemma. Our data also suggest the olfactory channel protein forms homomeric channels in the heart since other isoforms or splice variants were not detected either by PCR amplification or by RNase protection. In addition, we have isolated and sequenced the mouse olfactory cyclic nucleotide-gated channel gene, and show the genomic organization is remarkably similar to that found in the human retinal channel gene. Part of this work was presented in abstract form.