In ovo neuromuscular stimulation alters the skeletal muscle phenotype of the chick

4-aminopyridine (4-AP) is a drug that blocks the potassium channels in neurons and stimulates the release of the neurotransmitter acetylcholine (ACh), enhancing its availability at the synaptic cleft. The effects of 4-AP induced neuromuscular stimulation on skeletal muscle formation and development were investigated in embryonic chicks. Fertile white Leghorn eggs were incubated at 37.5 degrees C and windowed on day three of incubation. On embryonic days (E) 10, 11, 12 and 13 half of the eggs were injected with 100 mu l of PBS buffer containing 0.2 mu g 4-AP and the control group was administered 100 mu l of PBS only. 4-AP treated (T) embryos showed at least a 10% increase in mean body mass relative to the controls (C) (P < 0.05) at ages E14, E15 and E16. Tibia and femur lengths in the 4-AP treated embryos were significantly greater than the controls at E15 and E16 ( P < 0.05). The 4-AP treated animals had a 36.8% greater number of myofibres than the control animals at E20. Nuclear number per cross sectional area in the M. Semitendinosus was significantly greater (P < 0.01) at E16 in the treated compared to the control embryos. The 4-AP treated group exhibited a greater percentage area of oxidative fibres in cross sections of M. Semitendinosus than the control group at E16 ( P < 0.01) and at E20 ( P < 0.05). It may be concluded from these results that 4-AP induced neuromuscular stimulation has a significant effect on skeletal muscle characteristics, leg bone length and overall body mass.