The ontogeny of the response of the avian embryo heart to autonomic neurotransmitters and to neurotransmitter-like drugs

Abstract

In the atria and the ventricles of the chick embryo heart, receptors for both autonomic neurotransmitters are present on cardiac muscle cells before the heart is innervated; the autonomous appearance of adrenergic and cholinergic receptors has also been observed in the hearts of mammals (Pappano, 1977, 1981), in blood vessels (Su et al., 1977) and in other target tissues of the peripheral nervous system (reviewed by Fambrough, 1976; Higgins, 1980; Higgins and Pappano, 1981a). Indeed, while there is a substantial amount of data available about the regulation of the number of receptors for various neurotransmitters during development, there is little information about either how a particular cell type comes to synthesize selectively certain types of receptors or how a receptor first becomes linked to its effector molecule(s). The chick embryo heart is probably not a favorable model in which to study the former problem because receptors for acetylcholine and norepinephrine are present early in embryogenesis when the mass of the heart is very small (Romanoff, 1960). The latter question may be amenable to study in this system since there is some evidence for physiologically silent muscarinic cholinergic receptors in chick cardiac muscle (Section 4.4). In addition, Lipshultz et al. (1981) have shown that, while chick cardiac muscle is unresponsive to epinephrine under certain conditions in vitro, it can begin to respond to this β-adrenergic agonist when it is grown in the presence of a high molecular weight fraction of chick embryo extract. These authors have suggested that this factor(s) may induce the synthesis of a membrane transducer molecule. The type of receptor which mediates the response to acetylcholine or to norepinephrine appears to remain constant during the development of the chick heart. While such a constancy of receptor type has been observed in some other systems, for example skeletal muscle (Fambrough, 1976), it does not appear to be a general phenomenon. There are synapses at which the postsynaptic responses to acetylcholine (Nunez et al., 1980) or to catecholamines (Rosen et al., 1977) are mediated by two different receptors and at which the relative importance of the two receptor types changes during development. The atria and ventricles of the chick embryo heart begin to respond to catecholamines early in embryogenesis and the responses of these tissues to β-adrenergic agonists appear to remain qualitatively the same during embryogenesis. In contrast, the evolution of the response of the chick embryo heart to muscarinic cholinergic agonists is quite complex. A response to acetylcholine is observed in pacemaker cells before it is observed in ventricular muscle. Furthermore, during development changes are observed in the qualities of the atrial response to cholinergic agonists and in the pathways mediating ventricular inhibition. There is also evidence that muscarinic agonists may act by at least two different mechanisms in the ventricle (cAMP-dependent and cAMP-independent mechanisms). While some ontogenetic changes in the agonist binding properties of muscarinic receptors have been reported, it seems possible that the developmental changes in the responses to acetylcholine occur distal to ligand binding sites. There are ontogenetic changes in the sensitivity of the chick embryo heart to catecholamines and to acetylcholine. The transient subsensitivity of the atria and the ventricles to catecholamines during the third week of incubation does not seem to be caused by the release of transmitter from adrenergic nerves. Experiments in which the cholinergic innervation of the embryonic heart has been interrupted have not been reported and so it is difficult to assess the role of the parasympathetic nervous system in regulating the sensitivity to acetylcholine; however, tissue culture studies indicate that increases in the sensitivity to acetylcholine can occur in the absence of both innervation and continuing hormonal influences. Thus, the data currently available would not seem to indicate that autonomic nerves regulate the sensitivity of the chick embryo to neurotransmitters. However, there are several systems in which trophic effects of autonomic nerves on their target tissues have been documented (Higgins and Pappano, 1981a). In some tissues, the trophic interactions may be quite complex; for example, Rush et al. (1982) have reported that sympathetic nerves regulate the number of muscarinic receptors in the chick expansor secundariorum muscle. Thus, negative results concerning the influences of autonomic nerves on cardiac muscle may only indicate the need for a more thorough analysis of the factors regulating the development of the heart.