Fetal canine cardiac Purkinje fibers: Electrophysiology and ultrastructure

We studied the ontogenesis of the transmembrane action potential and the ultrastructure of fetal canine Purkinje fibers. Fetal hearts were obtained from fetuses just after implantation to end gestation. Using standard microelectrode recording techniques, we found that action potential characteristics varied linearly over this period of development. Maximum diastolic potential (MDP) ranged from -65 to -85 mV; action potential amplitude (AMP) varied from 100 to 120 mV; maximum upstroke velocity (V̇(max)) increased from 200 to 550 V/s. Action potential duration measured to 50% repolarization (APD₅₀) increased from 15 to 156 ms while duration measured at full repolarization (APD₁₀₀) similarly increased from 75 to 236 ms. The relationship between external potassium concentration and membrane potential was equivalent across all stages of fetal development. Tetrodotoxin (TXX, 7.7 x 10^-7 to 1.6 x 10^-5 M) caused concentration-dependent decreases in AMP, V̇(max), and APD₅₀. Verapamil (1 x 10^-7 to 1 x 10^-5 M) decreased V̇(max) and APD₅₀ in a concentration-dependent manner. The effects of both TTX and verapamil were statistically equivalent across all stages of development (Purkinje fibers were not visually distinct at this time) were arranged as a tightly packed mosaic with a rounded shape, with a large amount of glycogen, small sparse mitochondria, and relatively large nuclei. Mitotic cells were observed frequently. Purkinje fibers when first identified grossly had fewer myofilaments than working myocardial cells and sarcomeres without M lines. By late gestation, intercalated disks appeared with an increase in surface areas; desmosomes occurred more frequently. Myofilaments are organized around Z bands into rudimentary sarcomeres that still lack M lines. These data indicate that, although the fetal canine Purkinje fiber undergoes marked developmental changes in ultrastructure, cellular electrophysiological changes are more subtle. The action potential has a qualitative appearance similar to those of the neonatal or adult fiber. At no time during fetal development could we find slow-response action potentials.