Ca²⁺-free medium enhances the magnitude of slow peak in compound action potential of frog sciatic nerve <i>in vitro</i>

Abstract

The present investigation was carried out to know the effect of Ca²⁺ on different peaks of compound act ion potential (CAP) representing the fibers having different conduction velocity. CAP was recorded from a thin bundle of nerve fibers obtained from desheathed frog sciatic nerve. Suction electrodes were used for stimulating and recording purposes. In Ca²⁺ free amphibian Ringer, two distinct peaks (Peak-I and Peak-II) were observed. The threshold, conduction velocity (CV), amplitude and duration of Peak-I were 0.32±0.02 V, 56±3.0 m/sec, 2.1 ±0.2 mV and 0.75±0.1 ms, respectively. The Peak-II exhibited ten times greater threshold, eight times slower CV, three times lower amplitude and four times greater duration as compared to Peak-I. Addition of 2 m<i>M </i>Ca²⁺ in the bathing medium did not alter CAP parameters of Peak-I excepting 25% reduction in CV. But, in Peak-II there was 70-75% reduction in area and amplitude. The concentration-attenuation relation of Peak-II to various concentrations of Ca²⁺ was nonlinear and 50% depression occurred at 0.35 m<i>M </i>of Ca²⁺. Washing with Ca²⁺-free solution with or without Mg²⁺ (2 m<i>M</i>)/verapamil (10 μ<i>M</i>) could not reverse the Ca²⁺- induced changes in Peak-II. Washing with Ca²⁺-free solution containing EDTA restored 70% of the response. The results indicate that Ca²⁺ differentially influence fast and slow conducting fibers as the activity of slow conducting fibers is greatly suppressed by external calcium.