A method for measurement of low levels of guanosine 3′,5′-cylic monophosphate in blood plasma

A very sensitive method is described for the assay of guanosine 3',5'-cyclic monophosphate (cGMP) based on a competitive protein-binding technique. The procedure is simple to carry out, and does not require the purchase of expensive reagents. The binding-protein used is extracted from rat lungs and the bound and unbound cGMP are separated by ammonium sulphate precipitation. Although prior extraction of the cGMP is required, the time for this extra step is easily compensated for by the shortness of the assay time. The mean cGMP found in the blood plasma of normal young males and females is 9.9 ± 2.4 (S.D.) nmol/l (range 5.9–14.3 nmol/l) and 9.5 ± 2.1 (S.D.) nmol/l (range 6.9–12.7 nmol/l), respectively.