We investigated the effect of pharmacological treatment with captopril, nitrendipine, and captopril plus nitrendipine on myocardial heterologous adenylyl cyclase desensitization and the underlying postreceptor defects in spontaneously hypertensive rats (SHR). In myocardial membranes from SHR, stimulation of adenylyl cyclase with guanylylimido-diphosphate (P < .001) and forskolin (P < .05) was significantly reduced, whereas no difference with forskolin was obtained in the presence of manganese chloride. Reconstitution of Gs alpha into Gs alpha-deficient S49 cyc- mouse lymphoma cells revealed no difference between SHR and control rats. In contrast, pertussis toxin labeling of Gi alpha was significantly increased in SHR. The reduction of adenylyl cyclase in SHR was abolished after pertussis toxin treatment of membranes. Treatment with captopril, nitrendipine, or both reduced Gi alpha and increased guanylylimidodiphosphate-stimulated adenylyl cyclase activity in SHR. In summary, heterologous adenylyl cyclase desensitization due to an increase of Gi alpha but in the presence of an unchanged activity of Gs alpha or the catalyst occurs in SHR. This alteration, which could contribute to the progression of contractile dysfunction by producing adrenergic subsensitivity, is sensitive to pharmacological treatment most likely because of a reduction of sympathetic activity.