Developmental differences in endplate response to P-type calcium channel blockade in the rat diaphragm

Endplate potentials (epps) were recorded intracellularly from single diaphragm fibers of newborn (7-10 days, n = 11) and older (24-30 days, n = 11) rats in the presence of 100 nM omega-agatoxin IVA, a P-type Ca2+ channel blocker. The muscle was stimulated via the phrenic nerve for 1 s at 40 Hz. In both age groups epp amplitude decreased with omega-agatoxin, however the decrease was greater in the older group (mean = 60% of control vs. 40% of control in the younger group). A larger number of fibers in the older group (84% vs. 54% in the young) showed a > or = 50% decrease in epp amplitude. These data suggest that although P-type Ca2+ channels are present in the immature presynaptic nerve terminals at the neuromuscular junction, functional maturation of these channels occurs with development. This may contribute to the susceptibility to neuromuscular transmission failure in the newborn diaphragm.