Acetylcholine Sensitivity of Embryonic Ventricular Fibres

Abstract

IN adult hearts, only fibres of the sinoatrial and atrio-ventricular nodes are innervated. In adult hearts, innervated nodal fibres are very sensitive, while non-innervated fibres are relatively insensitive, to low concentrations of acetylcholine (ACh) and norepinephrine^1,2. Similarly, in innervated skeletal muscle, the end plate membrane is much more sensitive to ACh than the rest of the membrane³. The presence of the nerve terminal is in fact responsible for the low ACh sensitivity of the remainder of the fibre membrane, as with denervation, all of the skeletal muscle membrane becomes quite sensitive to ACh (ref. 4). Formation of a new end plate on a denervated muscle fibre is followed by decreased ACh sensitivity of the rest of the fibre membrane⁵. Furthermore, Diamond and Miledi observed that, in rat embryo, formation of an end plate is followed by decreased ACh sensitivity of the remainder of the skeletal muscle fibre membrane⁶. The following experiments were carried out to determine whether innervation of nodal fibres in embryonic heart is accompanied by decreased ACh sensitivity in non-innervated ventricular muscle.