POSTNATAL INDUCTION AND NEURAL REGULATION OF INWARD RECTIFIERS IN MOUSE SKELETAL-MUSCLE

The whole-cell voltage-clamp technique was used to examine developmental changes of inward rectifier currents in fibres of the flexor digitorum brevis muscle acutely isolated from mice on postnatal day 0 (P0) to P36. Neither a steady-state component (I(s-s)) nor a slowly activated component (I(rise)) of inward rectifier currents were observed in fibres of P0 and P4 mice. Both I(s-s) and I(rise) became apparent between days P8 and P16. The specific amplitudes of I(s-s) and I(rise) measured at a test-pulse potential of -100 mV at 20 mM extracellular K+ ([K+]o) increased to their respective plateau values of -68 +/- 10 and -15 +/- 7-mu-A/cm2 at P20. In fibres denervated on day P4 the developmental increase of I(s-s) was suppressed, its specific amplitude at P20 being one-tenth of that in the corresponding normal fibres. I(rise) did not appear in P4-denervated fibres throughout the development. In muscle fibres denervated at P16 or P20, the specific amplitudes of I(s-s) and I(rise) decreased, reaching the levels of P4-denervated fibres in 2-4 days after denervation. We conclude that I(s-s) and I(rise) develop within 3 weeks after birth, and suggest that innervation plays a key role in their induction.