Ca2+ -free medium enhances the magnitude of slow peak in compound action potential of frog sciatic nerve in vitro.
The present investigation was carried out to know the effect of Ca2+ on different peaks of compound action 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 mM 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 mM of Ca2+. Washing with Ca2+ -free solution with or without Mg2+ (2 mM)/verapamil (10 microM) 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.
Mandal
Maloy B
MB
Department of Physiology, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221 005, India. maloy_mandal@yahoo.com
Deshpande
Shripad B
SB
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D003470:Culture Media; D004305:Dose-Response Relationship, Drug; D004492:Edetic Acid / Q000494:pharmacology; D004566:Electrodes; D004594:Electrophysiology / Q000379:methods*; D008274:Magnesium / Q000737:chemistry; D011898:Ranidae; D012584:Sciatic Nerve / Q000378:metabolism*; D013997:Time Factors
D003470:Culture Media; D004492:Edetic Acid; D008274:Magnesium; D002118:Calcium