Two types of extracellular action potentials recorded with narrow-tipped pipettes in skeletal muscle of frog, Rana temporaria.

Two types of muscle fibre action potentials (APs) were recorded using narrow-tipped extracellular pipettes in isolated sartorius muscles of frog, Rana temporaria. The waveform of type 1 responses (T1 AP, 75% of recordings) was biphasic, 'positive–negative.' The type 2 signals were tri-phasic, 'positive–negative–positive' (T2 AP, 18%). The type of AP was preserved for up to 1 h of collecting data from the given site on the muscle fibre. However, a re-positioning of the recording pipette by a few micrometres along the axis of the studied fibre resulted in a change of the type of AP in 73% of such attempts. In experiments with detubulated muscles, only T1 APs were observed. In experiments using 10 μmol l(−1) Ba2+ filled pipettes, the characteristics of the T1 waveform did not change, but the amplitude of the third phase of T2 APs increased progressively during continuous recording. In experiments with 100 μmol l(−1) Ba2+ filled pipettes, a decline and eventual inversion of the third phase of the T2 signal was observed. Amplitude and duration of the inverted third phase of T2 APs increased progressively with frequency of muscle stimulation. These results can be explained by suggesting that currents generated within the t-tubular system of muscle fibre form the third phase of extracellularly recorded APs. Measurement and analysis of T2 APs present a unique approach in determining the mechanisms controlling accumulation of t-tubular K+ and the role of these mechanisms in regulation of muscle contractility.