The relationships between cyclic AMP, calcium and prostaglandins as second messengers.

There is considerable dissatisfaction with present second messenger hypotheses involving cyclic nucleotides and calcium. The recent findings that methyl xanthines and adenosine are prostaglandin antagonists casts doubt on much of the evidence in favour of the cyclic AMP hypothesis. There is evidence that allosteric sites may modify the binding of calcium and cyclic nucleotides to key cellular regulators and that a range of substances including steroids, adenosine and prostaglandins may occupy those sites. This concept introduces much needed flexibility into the second messenger concept, allows many experiments to be reinterpreted and has major implications throughout the biomedical sciences.