INFLUENCES OF STIMULATION FREQUENCY AND TEMPERATURE ON INTERVAL-FORCERELATIONSHIPS IN GUINEA-PIG PAPILLARY-MUSCLES

Relationships between contractile force and the preceding and pre-prec eding stimulation intervals were studied in papillary muscles by inter posing variable test intervals during steady-state pacing. The strengt h of test contractions increased exponentially to a maximum as the pre ceding (test) interval was lengthened. Contractility decreased as an e xponential function of pre-preceding interval. At 37 degrees C, the ha lf times for these processes were unaffected by increasing the steady- state frequency from 1 to 3 Hz. At 27 degrees C, the force increase wi th preceding interval was accelerated and the decay with prepreceding interval was retarded as the stimulation frequency was increased from 0.33 to 2 Hz. The timecourses of force increase and decay were similar to each other during stimulation at an optimum frequency characterist ic for the temperature. Cooling from 37 to 27 degrees C prolonged the half times for force increase and decay by factors of 4.5 and 3 respec tively. The slope of the linear relationship between the force of the contraction pre-preceded by the test interval and the immediately subs equent contraction (recirculation fraction) was also halved. These res ults suggest that high stimulation frequency and low temperature uncou ples cellular processes underlying the interval dependence of cardiac contractility. The temperature sensitivities are consistent with these processes being enzymatic. The reduced recirculation fraction provide s a mechanism for the lowered threshold frequency for sustained mechan ical alternans at 27 degrees C.