Use dependence of sodium current inhibition by tetrodotoxin in rat cardiac muscle: influence of channel state

Tetrodoxin (TTX) is known to cause a voltage- and frequency-dependent inhibition of the rapid inward sodium current (INa) of cardiac muscle. This effect was studied by means of the loose-patch-clamp method on intact rat papillary muscle. The availability curve of the fast sodium system, determined by variation of the holding potential, is shifted in the presence of TTX (5.5 mumol x 1(-1] by 17 mV to more negative potentials. With clamp pulses of 5 ms duration to 0 mV, a frequency-dependent reduction of INa by TTX is found above 0.1 Hz that saturates at about 10 Hz. This frequency-dependent block was further analysed using trains of pulses (10 Hz) of various durations (minimum 50 microseconds), which allow TTX to equilibrate with channel states reached early during activation. The results show that more than 90% of the frequency-dependent block is attained with pulses of 1 ms duration. An analysis according to the guarded receptor hypothesis reveals that these results are well described by TTX binding to inactivated, activated and probably preactivated channel states.

[Arthrograph and operative findings (author's transl)]

Arthrograph and operative findings were compared retrospectively in 57 patients (53 examinations of the knee-,3 of the shoulder-, and 1 of the hip-joint). In medial disk lesions there were differences between arthrograph and operation in 19% (13,5 false positive and 5,5% false negative). In lateral disk lesions there were differences in 13% (5,5% false positive und 7,5% false negative). In a prospective series the results were far better. There were no differences for the shoulder and the hip-joint.