CALCIUM PERMEABILITY INCREASE OF END-PLATE CHANNELS IN RAT MUSCLE DURING POSTNATAL-DEVELOPMENT

1. Patches of endplate membrane were isolated from rat flexor digitoru m brevis muscle at different postnatal stages to measure the time cour se of developmental changes in conductance, deactivation time constant and relative Ca2+ permeability of endplate channels. 2. The predomina nt channel conductance was 40 +/- 1 pS (n = 9) at postnatal day 9 (P9) or younger whereas it was 59 +/- 3 pS (n = 5) at P21 or in older musc le. The deactivation time constant of ensemble patch currents evoked b y brief ACh application, decreased from 8 +/- 3 ms (n = 45) at P5-9 to 2.3 +/- 0.3 ms (n = 5) in P21-28 muscle. 3. The relative Ca2+ permeab ility, measured by the shift of biionic (Ca2+/Cs+) reversal potential of ensemble patch currents upon the replacement of high [Cs+] by high [Ca2+] extracellular solution and with Cs+ as internal reference ion, increased during postnatal development. The biionic reversal potential shift changed from -21 +/- 1 mV (n = 8) at P5 to -8 +/- 1 mV (n = 10) in P15 or older muscle. 4. Recombinant gamma-AChR channels expressed in Xenopus laevis oocytes had a biionic (Ca2+/Cs+) reversal potential shift of -24.9 +/- 2 mV (n = 14) comparable to that of neonatal endpla te channels whereas the reversal potential shift for recombinant epsil on-AChR channels was -7.6 +/- 0.9 mV (n = 13), comparable to that of e ndplate channels in adult muscle. 5. It is concluded that an approxima tely 3-fold increase in Ca2+ current through endplate channels during postnatal development is caused by replacement of the fetal gamma-subu nit by the epsilon-subunit in juvenile and adult muscle.