Effects of denervation on Ca2+ channels in slow skeletal muscle fibers of the frog

Effects of denervation on calcium channels in slow skeletal muscle fibers i n the frog (Rana pipiens) were studied using the three-microelectrode volta ge-clamp technique in intact fibers. Ca2+, Ba2+, and Sr2+ currents were all significantly reduced in amplitude during the first 2 weeks after denervat ion. After nerve section the selectivity sequence Ba congruent to Ca>Sr was changed to Ba>Sr>Ca and the values for relative ratio increased from 1.04 to 2.65 for Ba2+ and from 0.58 to 1.20 for Sr2+ (with respect to Ca2+). Bar ium current saturation was more obvious in denervated fibers than in non-de nervated fibers. The values obtained with the Michaelis-Menten type express ion, I = I-max/(1 + K-d/[Ba](e)) were K-d = 2.7 mM and I-max = 20 muA/cm(2) in fibers 2 weeks after nerve section compared with the values K-d= 4.4 mM and I-max = 60 muA/cm(2) obtained in non-denervated fibers. Additionally, the effects of two calcium channel blockers (cobalt and nifedipine) were gr eater by a factor of two in denervated fibers than in non-denervated fibers . Three weeks or so after nerve section, all the biophysical properties stu died began to show a tendency to recover toward the values obtained in non- denervated muscles (controls). These results suggest that calcium channels are modified or that there is a change in the types of calcium channels pre sent in frog slow skeletal muscle fibers after denervation. (C) 2001 Elsevi er Science B.V. All rights reserved.