The myocardial β-receptor adenylate cyclase system was investigated in short-term streptozotocin-diabetic rats. Earlier reports of a decreased sensitivity of the myocardium to isoproterenol (ISO) in these animals were elucidated by measuring the in vivo production of cAMP after ISO. A substantial decrease was seen in diabetic animals compared with controls and starved animals, and thyroxine treatment, known to sensitize the myocardium to catecholamines, did not normalize the response. The desensitization was retained in a membrane fraction in such a way that ISO was unable to increase the cAMP production while stimulation via the nucleotide-binding protein (with NaF or GTP) leads to a normal cAMP response. As the β-adrenergic receptor number and affinity turned out to be identical in control and diabetic animals, a functional uncoupling of the myocardial β-receptor from productive adenylate cyclase activation seems thus to exist in experimental diabetes. It is unlikely that it has anything to do with the thyroid status of the animals, but the possibility of a catecholamine-induced desensitization cannot be excluded. The phenomenon is not universal as the β-receptor-adenylate cyclase system is normal in isolated spleen lymphocytes. Whether the described phenomenon obtained in an animal study has any relevance for the increased incidence of heart failure in human diabetes mellitus is not known at present.

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