Islet contents of cyclic 3',5'-guanosine monophosphate under conditions which affect the cyclic 3',5'-adenosine monophosphate.

The sensitivity of the radioimmunoassay for cGMP was considerably increased by previous 2'-O-succinylation of the nucleotide. The basal content of cGMP in beta-cell-rich pancreatic islets isolated from ob/ob-mice was similar to that of cAMP, i.e. about 3 mumoles per kg dry weight. Extra-cellular Ca2+ was a prerequisite for maintaining this amount of cGMP. The islet cGMP differed from cAMP in being only slightly enhanced or not affected at all when the islets were exposed to high concentrations of glucose, the sulphydryl reagents chloromercuribenzene-p-sulphonic acid and iodoacetamide, or the potent phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine. The data obtained suggest that the turnover rate for cGMP is much slower than that for cAMP in the pancreatic beta-cells. The interrelationships between the two cyclic nucleotides do not seem to fit into a simple pattern of antagonism.