Calcium action potentials and calcium currents in tonic muscle fibres of the frog (Rana pipiens).

Slow action potentials were evoked in cruralis tonic and twitch fibres of the frog after drastically reducing the Cl‐ and K+ conductances. Tonic fibres were identified by their electrical characteristics. They had an effective resistance (Reff) of 50 +/‐ 6 M omega (n = 27) and a membrane time constant (tau m) of 440 +/‐ 70 ms (n = 8). In twitch fibres Reff = 2.9 +/‐ 0.3 M omega (n = 16) and tau m = 50 +/‐ 4 ms (n = 6). In tonic fibres the slow action potential had a threshold of ‐50 to ‐60 mV and a peak amplitude of ‐10 mV. In twitch fibres the slow action potential had a threshold of ‐40 mV and reached a peak amplitude of +40 mV. The responses were blocked by the addition of Cd2+ (2 mM) or Co2+ (5 mM). These results strongly suggest that Ca2+ is the main carrier of current during the response. Using the three‐micro‐electrode voltage‐clamp technique a slow inward membrane current underlying the Ca2+ potential could be described in tonic muscle fibres. The slow inward current was mainly carried by Ca2+, since it was reduced when external Ca2+ concentration was lowered or when Cd2+ (2 mM) was added. Moreover, Ca2+ was the only cation in the solution that could carry inward current. It had a mean threshold of ‐60 mV, reached a maximum value at ca. 0 mV, ranged from 24 to 28 microA/cm2 and had a mean reversal potential of +35 mV. In about half of the examined tonic fibres inward current declined with time, only slowly. This can either be explained by there being less contamination by K+ outward current, or by the presence of two types of Ca2+ channels in the tonic fibre membrane.