Distribution of muscarinic acetylcholine receptors in amphibian cardiac muscle

Abstract

THE skeletal muscle-nerve junction and many interneuronal synapses are highly specialised and morphologically characterised by a direct apposition of pre- and postsynaptic elements separated by a cleft of no more than 50 nm, localised thickenings of the apposed membranes and an aggregation of small vesicles in the presynaptic process. At several such ‘focal’ synapses, neurotransmitter receptor molecules are restricted to patches of postsynaptic membrane corresponding to synaptic sites^1–3. For example, at the motor endplate of skeletal muscle, acetylcholine receptor packing density in the subsynaptic membrane is about 10³ times higher than in the extrasynaptic membrane several μm away^4–6. Many neuroeffector junctions such as those in autonomically innervated tissues, however, are much less specialised and lack synaptic membrane apposition and thickenings^7,8. In the heart for example, the terminal postganglionic autonomie axons consist of long chains of vesicle-packed varicosities and form a widely ramifying plexus over the muscle⁹. The varicosities sometimes lie in close apposition to the muscle but may often be several μm away from the nearest target. Even when pre- and postsynaptic membranes are closely apposed, morphological specialisation of the membranes is not normally seen^7,10–12. I was interested to determine whether receptor molecules for acetylcholine (ACh) in heart muscle are localised to the relatively infrequent sites of close apposition between nerve and muscle or are more generally distributed. This report demonstrates that muscarinic ACh receptors are uniformly distributed in cardiac muscle fibres in amphibian heart.