Paradoxical changes in excitability dependent on stimulation frequency in the papillary muscle of the guinea pig

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Abstract

It is known that in most cardiac tissues an increase in rate results in a decrease of excitability and, eventually, conduction block. We used microelectrode techniques to evaluate the rate and time dependence of excitation latency in 27 isolated guinea pig papillary muscles (GPPM). Latency was measured as the interval between the stimulus onset and action potential upstroke. When the intensity of current was just supra threshold, prolongation of the basic cycle length (BCL) from 300 to 1000 msec produced an increase in latency or failure of excitation. Such behavior was observed with extra- cellular as well as intracellular stimulation. Rate-dependent changes in latency were maximal during the rust 10-20 sec following the rate change and reached a steady-state in approximately 200 sec. Deceleration- induced decrease of excitability was attended by hyper- polarization, increase of action potential upstroke velocity (Vmax) and action potential amplitude, and decrease in membrane resistance. Our data suggest that paradoxical rate-related changes of excitability in GPPM are the result of changes in the passive membrane properties. Under conditions of depressed conductivity, this particular behavior may account for the occurrence of bradycardia-dependent block.