Several studies have shown an activation of adenylyl cyclase by the G protein-inactivating guanine nucleotides, GDP and its phosphate transfer-resistant analog, guanosine 5'- O-(2-thiodiphosphate; GDPbetaS). Here, we studied the mechanism underlying this unconventional activation. Adenylyl cyclase activity in sarcolemmal membranes from failing human ventricular myocardium, at a low Mg2+ concentration, decreased rapidly during incubation at 37 degrees C. This decrease in enzyme activity was paralleled by a rapid release of GDP from Gi proteins, amounting to 75% release of total Gi-bound GDP within 10 min at 37 degrees C. In contrast, no GDP release was observed at 4 degrees C, and adenylyl cyclase activity remained stable for up to 20 min at 4 degrees C. GDPbetaS did not alter the initial rates of cyclic AMP formation by the adenylyl cyclase, at either 37 degrees C or 4 degrees C, but almost fully prevented the decrease in enzyme activity occurring at 37 degrees C. Hence, after 10 min of incubation at 37 degrees C, adenylyl cyclase activity in the presence of GDPbetaS was increased by 150%, while no difference in activity was observed at 4 degrees C. Under conditions where adenylyl cyclase is uncoupled from regulation by the inhibitory Gi proteins, i.e., at high concentrations of Mg2+ or Mn2+, adenylyl cyclase activity remained stable even at 37 degrees C and GDPbetaS did not stimulate activity. In conclusion, Gi proteins in sarcolemmal membranes from failing human hearts rapidly release bound GDP. The data, furthermore, suggest that this process results in adenylyl cyclase inhibition by the empty but apparently active Gi proteins.