Ions As ‘Second Messengers’

Abstract
The concept that the cyclic nucleotides are the bridge between a hormonal stimulus and the interior apparatus of the target cell seems to describe an “open loop” system lacking a feedback control to turn off the nucleotide once the cell has responded appropriately. But an interrelated nucleotide-cation system fills this conceptual gap, clarifying the role of the ions, especially Ca++, in mediating hormonal action.