Cytochemical localization of guanylate cyclase in cardiac muscle

With the help of a cytochemical method based on the lead precipitation technique, the localization and distribution of guanylate cyclase in heart tissue from guinea pig and pigeon were studied. In heart muscle of both animals, the plasma membrane of the sarcolemma and the junctional SR are the precipitation sites of the reaction product. In guinea pigs the T-tubule membranes were also covered with precipitates. Presence of sodium nitroprusside strengthens the precipitates, indicating an activation of the enzyme; Mn2+ ions were essential for an optimal reaction. Quantitative measurements show that GMP-P(NH)P instead of GTP was accepted by the enzyme as substrate; after prefixation with 2% glutaraldehyde and incubation with lead ions, the enzyme activity was retained to about 30% of controls. Sodium nitroprusside greatly increases the retained enzyme activity.