The whole-cell voltage-clamp technique was used to examine developmental changes in different types of ba currents through ca channels, Na^+ currents and inward rectifier K^+ current, I(K,inward), in single fibres acutely isolated from mouse flexor digitorum brevis muscles.
The muscle fibres isolated on day 1 postnatal (P1) showed transient, I(Ca,T), and sustained I(Ca,S)types of currents. The mean specific amplitude of I(Ca,T) was 2.7 A/F (-30 mV, 30 mM [Ba]o) at P1, decreased progressively afterwards, becoming undetectable by P17. The mean specific amplitude of I(Ca,S) increased from 6.9 A/F (+20 mV) at P1 to 27.7 A/F at P30. Denervating muscle fibres at P4, P8 or P17 did not prevent the disappearance of I(Ca,T). The increase of I(Ca,S) was suppressed by the denervation.
Geographutoxin II (GTX), a peptide toxin from the marine snail, distinguishes two distinct types of voltage sensitive Na currents; GTX-sensitive I(Na,s) and -insensitive currents, I(Na,i). Voltage-dependent activation
and inactivation of I(Na,i) occurred at membrane potentials 10 to 20 mV more negative than those for I(Na,s). I(Na,i) in fibres from mice older than P8 decayed more slowly than I(Na,s). However, in fibres from younger mice, The two currents decayed with similar speed. the mean specific conductance (g_<Na>) of I(Na,s) increased 15-fold in the first 20 days. The mean specific g_<Na> of I(Na,i) remained at a level of 0.15 mS/ F (5mM [Na]o) for the first 8 days, then decreased progressively, becoming undetectable by P16. In muscle fibres denervated at P12, g_<Na> for I(Na,i)increased afterwards, whereas an increase of the total g_<Na> was less than normal. By contrast, in fibres denervated at P4, the total g_<Na> increased more than normal until P12, and g_<Na> for I(Na,i) increased above the level seen at P0.
I(K,inward) was not observed in fibres of P0 and P4 (20 mM [K]o) and in cultured mouse myotubes (100 mM [K]o). However, instantaneous and time-dependent components of I(K,inward) were cleary seen in fibres of P12 and older. I(K,inward) reached its maximal level by P20. When muscle fibres were denervated at P4, the postnatal increase of the two components of I(K,inward) was mostly prevented. When muscle fibres were denervated on P20, I(K,inward) decreased to the level of P4-denervated fibres in 2 days after the operation.
Conclusions. I(Ca,T) and I(Na,i) disappear and I(K,inward) appears in the first few weeks after birth.I(Ca,S) and I(Na,s) increase during this period. The disappearing process of I(Na,i) and appearing process of I(K,inward) require the presence of innervation, whereas the disappearing process of I(Ca,T)does not. Less