CALCIUM CURRENT INACTIVATION IN DENERVATED RAT SKELETAL-MUSCLE FIBERS

1. The inactivation of the calcium current (I(Ca)) was studied in single extensor digitorum longus muscle fibres of the rat. Denervation was performed by surgically removing 6-8 mm of the sciatic nerve at the sciatic notch. Electrical recordings were carried out using the double Vaseline-gap technique. Normal fibres were used as controls. 2. The time course of the onset of I(Ca) inactivation was studied with double pulse experiments. Denervation after 14 days slowed down the onset of the inactivation process. Two depolarizing pulses with variable interpulse potential were applied. The rate of recovery from I(Ca) inactivation was analysed with long interpulse intervals (9 to 1 s). The time constants for I(Ca) inactivation (tau(i)) at - 90 mV potential were 4.3 and 2.2 in normal and 14 day-denervated fibres respectively. 3. The onset of I(Ca) inactivation was studied with a double pulse protocol with variable duration of the first pulse with constant interval (120 ms) to the second pulse (300 ms). The plot of [I(Ca) (pulse 2)/I(Ca)(pulse 1)]-first pulse duration relationship was fitted with a single exponential equation. The inactivation time constant (Th) values for normal and denervated fibres were 428 and 619 ms, respectively. 4. The h(infinity)-V(m) relationship for denervated fibres was shifted toward more negative potentials and I(Ca) did not fully inactivate with large prepulses. The h(infinity)-V(m) relationship was fitted with a Boltzmann equation I/I(max) = 1 -{A/[1 + (exp((V(m1/2) - V(m)/k(h))]} where V(m) is the potential during the conditioning pulse and A is an amplitude factor. In normal fibres, V(m1/2) (mid-point) and k(h) (slope) values were - 28-7 mV and 7.6 mV, respectively. In 14-day-denervated fibres they were - 42.2 and 8.6 mV, respectively. 5. A temperature rise from 17 to 27-degrees-C greatly increased the inactivation rate of I(Ca). This effect was similar in control and denervated fibres. The temperature coefficient quotient (Q10) values for I(Ca) amplitude in normal and denervated fibres were 2.4 (n = 8) and 2.3 (n = 8), respectively. The Q10 values for the inactivation time constant (tau(h)) were 5.14 and 5-25, respectively. I(Ca) decay during 1 s pulses was fitted to a single exponential function in normal fibres at 17 and 27-degrees-C; the time constant values were tau(h) = 460 +/- 53 ms and tau(h) = 92 +/- 17 ms, respectively. The time constants of denervated fibres at both temperatures were tau(h) = 644 +/- 102 ms and 146 +/- 17 ms, respectively. 6. The kinetic changes in I(Ca) inactivation after denervation may be explained by alterations of the inactivation gate of the Ca2+ channels.