Frequency-dependent excitability of "membrane" slow responses of Rabbit left atrial trabeculae in the presence of Ba2+ and high K+.

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Abstract
Small trabeculae of rabbit left atrium immersed in TKBa solution (Tyrode with 10 mM K+ and 1 mM Ba2+) were used to study frequency dependence of membrane slow response excitability at long cycle lengths (> 1 s). In TKBa, stimuli generate graded, low-amplitude (2-15 mV) subliminal responses of variable long duration (.ltoreq. 450 ms). A full all-or-none slow response is generated when a subliminal response depolarizes the membrane to about -35 mV. Subliminal response amplitude and rate of rise augment with stimulus intensity-duration product. For a fixed stimulus, the subliminal response is larger and faster at higher frequencies. Sudden changes in stimulus frequency or time course induce changes in subliminal response that take 4-8 cycles to attain steady state. For a fixed stimulus, slow response latency shortens progressively during the 1st few cycles after a sudden increase in frequency or when a rested preparation is excited (latency adaptation phenomenon, LAP). Slow response threshold stimulus requirements decrease during LAP (excitability hysteresis). The degree of excitability hysteresis is dependent on stimulation frequency and is more pronounced at higher frequencies. Frequency sensitivity of subliminal response (which causes frequency sensitivity of slow response excitability) is explained in terms of a transient state of enhancement set up by each stimulus. The enhanced state decays between stimuli with a half-time of .apprx. 4 s, thus allowing cumulative effects to become evident at rates > 0.1 Hz.