Muscarinic cholinergic receptor mediated inhibitory transduction of adenylate cyclase activity in subcellular fractions from rat heart: improved detection in sodium phosphate buffer.

Cholinergic inhibition of myocardial adenylate cyclase activity in cell-free fractions has been known for many years, although the reported degrees of inhibition have been rather modest (20-30%), notably in rat heart fractions. The present study conducted with rat heart subcellular fractions document following major findings: (1) Myocardial adenylate cyclase activity and notably its cholinergic inhibition in cell-free fractions are notoriously labile to storage at 4 degrees C whereas its stimulation by beta adrenergic receptor agonists or forskolin are reasonably well preserved during storage. (2) Among four buffers (Tris, glycylglycine, imidazole and sodium phosphate) examined, sodium phosphate buffer afforded the best preservation of cholinergic inhibitory response of adenylate cyclase. (3) The commonly used biochemical buffers, notably imidazole, exerted deleterious effect on the cholinergic inhibition of myocardial adenylate cyclase such that it was considerably attenuated or barely detectable; this explains, in part, the reported poor inhibition of myocardial enzyme by others. (4) Imidazole buffer, on the other hand, augmented beta adrenergic and forskolin stimulated adenylate cyclase activity. The likely significance of these findings is discussed from consideration that the observed differential influence of buffers results from differential actions on the interactions between the components (receptor/coupling G proteins/catalyst) comprising autonomic receptor coupled adenylate cyclase system in rat heart.