Cyclic nucleotide-gated channels in non-sensory organs.

Cyclic nucleotide-gated channels represent a class of ion channels activated directly by the binding of either cyclic-GMP or cyclic-AMP. They carry both mono and divalent cations, but select calcium over sodium. In the majority of the cases studied, binding of cyclic nucleotides to the channel results in the opening of the channel and the influx of calcium. As a consequence, cytosolic free calcium levels increase leading to the modifications of calcium-dependent processes. This represents and important link in the chain of events leading to the physiological response. Cyclic nucleotide-gated channels were discovered in sensory cell types, in the retina, and in olfactory cells, and were extensively studied in those cells. However, it is becoming increasingly evident that such channels are present not only in sensory systems, but in most, if not all, cell types where cyclic nucleotides play a role in signal transduction. A hypothesis is presented here which attributes physiological importance to these channels in non-sensory organs. Four examples of such channels in non-sensory cells are discussed in detail: those in the liver, in the heart, in the brain, and in the testis with the emphasis on the possible physiological roles that these channels might have in these organs.