Is there a role for cAMP and adenyl cyclase?

The validity of the hypothesis that cyclic AMP (cAMP) is the 2nd messenger for cell activation was reexamined. Some enzymological aspects of adenyl cyclase (AC) should cause concern: the lack of data on the stoichiometry of cAMP formation in mammalian systems and the unusual enzymatic properties, as well as lack of end-product inhibition and linearity. Furthermore, adenyl cyclase assays may lack precision and accuracy; this may depend on the organ studied. There are also problems of artefactual formation of cAMP during the work-up of cAMP extracts. Thus CrP, Pi, or ATP might influence this process and the actual measurement of cAMP, but solid data are apparently not available. Although a hormone-sensitive AC system has now been reconstituted from pure beta-adrenergic receptor, guanine nucleotide regulatory protein (Ns), and from bovine brain, the sensitivity to isoprenaline was very low and many questions remain--questions about the role of ions and Ns, in particular. The assumption that cAMP is the sole 2nd messenger is questioned since other nucleotides (AMP, ADP) and adenosine may change even more during hormone stimulation, and these compounds can also modulate protein kinase at concentrations often observed in vivo. Doubt over cAMP's role also stems from the observation that basal cAMP levels are sufficiently high to stimulate maximally protein kinase. Discrepancies between cAMP formation and lipolysis during isoprenaline or forskolin stimulation have been observed, and could indicate either compartmentalization of cAMP or alternatively disprove the cAMP hypothesis.(ABSTRACT TRUNCATED AT 250 WORDS)