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 Giα 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.
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