The developmental changes in receptors and ion channels in the chick heart could be summarized as follows. 1) The responsiveness to autonomic transmitters is present from an early stage of embryonic development before functional innervation takes place; innervation appears to modify myocardial responsiveness to transmitters. 2) ß-Adrenergic stimulation-induced positive chronotropy and inotropy is present throughout embryonic and post-hatch development. During the few days just before hatching, the inotropic sensitivity is transiently low to b-adrenergic stimulation and histamine. 3) Basal levels of ventricular tissue cAMP increased during embryonic development, which was followed by the appearance of direct muscarinic negative inotropic responses after hatching. 4) The functional role of the sarcoplasmic reticulum in myocardial contraction increases during embryonic development. 5) The resting membrane potential increases during early embryonic development due to an increase in K+ conductance. The action potential duration gradually increases during the embryonic period followed by a decrease just after hatching. Many of these features are common with mammalian animal species, but some are characteristic of the chick myocardium. Thus, the developing chick heart provides an interesting model for studies on the basic properties of the heart and their regulation.
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