The present study investigated whether reduced
adenylate cyclase activity and an increase in inhibitory
guanine nucleotide binding proteins (Gi alpha), which have been observed in the failing human heart, already occur in myocardial
hypertrophy before the stage of
heart failure. In membranes of hypertrophic hearts from rats with different forms of experimentally induced
hypertension without
heart failure (one-kidney, one
clip rats,
deoxycorticosterone-treated rats, and rats with reduced renal mass), basal as well as
isoprenaline-, 5'-guanylylimidodiphosphate-, and
forskolin-stimulated
adenylate cyclase activity was reduced. The activity of the catalyst was depressed in
deoxycorticosterone but unchanged in one-kidney, one
clip and reduced renal mass compared with controls. The number of
beta-adrenergic receptors was similar in all groups. Radioimmunological quantification of Gi alpha
proteins revealed an increase by 73% in one-kidney, one
clip, 67% in reduced renal mass, but only 20% in
deoxycorticosterone compared with
sham-operated, age-matched control rats. The increase of Gi alpha was accompanied by smaller changes of
pertussis toxin-induced [32P]ADP-ribosylation of a 40-kd
membrane protein. It is concluded that Gi alpha contributes to the reduced
adenylate cyclase activity in
cardiac hypertrophy in one-kidney, one
clip and reduced renal mass and to a smaller extent in
deoxycorticosterone. It is suggested that an enhanced expression of Gi alpha could occur not only in severe
heart failure but also in
cardiac hypertrophy and could, therefore, contribute to myocardial depression and progression of disease in
heart failure. In addition, Gi alpha might represent an important regulatory mechanism for cardiac
adenylate cyclase activity and thus, might play an important role in various
cardiac diseases.