Tissue-Specific Regulation of GTP-Binding Protein and Muscarinic Acetylcholine Receptor Levels during Cardiac Development

A quantitative immunoblot assay was developed by using affinity-purified monospecific antibodies to quantitate levels of guanine nucleotide binding regulatory protein (G-protein) subunits in atria and ventricles during embryonic chicken cardiac development. The muscarinic acetylcholine receptor (mAChR) number was measured with [³H]quinuclidinyl benzilate. On day 10 of embryonic development (day 10E) there was no difference between the atrial and ventricular membrane concentrations of β-subunit, G₀α subunit, or mAChR. The level of Gia was found to be 44% greater in atria than in ventricles on day 10E. The atrial membrane concentration of β-subunit increased 80% between day 13E and 15E, G₀α increased 46% between day 10E and 15E, mAChR increased 61% between day 10E and 12E, and Gia decreased 34% between day 10E and 13E. The atrial levels of β-subunit, G₀α, Giα, and mAChR did not change further through day 20E. The ventricular membrane concentration of these proteins did not change between day 10E and 20E, except for that of G₀αa, which increased 47% between day 15E and 20E. The atrial specific increase in β-subunit correlated with a loss of GTP inhibition of basal adenylate cyclase activity. The difference in G_iα levels between atria and ventricles on day 10E correlated with a difference in carbachol sensitivity of atrial and ventricular basal adenylate cyclase activity. Thus, the levels of several components of the cholinergic neuroeffector pathway are regulated in a tissue-specific manner at a time that coincides with the onset of functional parasympathetic innervation of the embryonic chicken heart. These changes are associated with functional effects on the regulation of adenylate cyclase activity.