[Analogy and differences in properties of soluble forms of guanylate cyclase of the heart and rat blood platelets].

Activity of guanylate cyclase in rat thrombocytes exceeded the enzymatic activity in heart tissue 3.2- and 6.6-fold, if Mn2+ and Mg2+ were used as cofactors, respectively. Dithiothreitol (DTT) at concentrations 2 x 10(-5) M-2 x 10(-2) M activated guanylate cyclase both in rat heart and thrombocytes, while 2 x 10(-3) M of DTT exhibited the maximal stimulating effect: 3-fold in heart tissue and 4.5-fold in thrombocytes. Only slight 2-fold activation of guanylate cyclase was observed in myocardium in presence of 1 x 10(-4) M nitroprusside, whereas this effect was distinctly augmented up to 26-fold after preincubation of the enzyme with 1 x 10(-4) M of nitroprusside within 45 min at 4 degrees in presence of 2 x 10(-4) M DTT. The stimulating effect of nitroprusside was increased up to 52-fold after addition of 3 micrograms hemoglobin into the sample. Nitroprusside did not show any stimulating effect on the guanylate cyclase activity in rat thrombocytes under experimental conditions used. Possible causes of the phenomenon observed are discussed.